NASA Contractor Report 3348

Solar Power Satellite

Offshore Rectenna Study

CONTRACT NAS8-33023

NOVEMBER 1980

NASA
NASA Contractor Report 3348

Solar Power Satellite

Offshore Rectenna Study

Rice University

Houston, Texas

Prepared for

Marshall Space Flight Center

under Contract NAS8-33023

NASA

National Aeronautics

and Space Administration

Scientific and Technical

Information Branch

1980
SOLAR POWER SATELLITE

OFFSHORE RECTENNA

STUDY

PERFORMED BY

RICE UNIVERSITY

BROWN AND ROOT DEVELOPMENT, INC.

ARTHUR D. LITTLE, INC.

FOR

THE MARSHALL SPACE FLIGHT CENTER

CONTRACT NAS8-33023

FINAL REPORT

MAY 1980

Rice University

Houston, Texas 77001

»eman

Study Director
The primary contributors to this report includ e

Rice University:

John W. Freeman

Wi11 i am L. Wilson , Jr.

Loren D. Lutes

Greg Greenwell

Martin Bordeaux

Herbert K. Beckman

Frank Fisher

Wayne Smith

Brown and Root Development Inc:

Don Hervey

Engi n Keyder

Gary Goldman

Peter Dove

Arthur D. Littl e Inc:

Peter E. Glaser

P hi lip K. Chapman
Table of Contents

Abstract 4

Executive Summary 5

1. Introduction and Background 6

2. Approach 7

2.1 Site Selection 7

2.2 Environmental Data 8

2.3 Icing Studies 12

2.3.1. The Monopole 12

2.3.2. Monopole Results 14

2.3.3. Dipol e Results 16

2.4 Rectenna Directional Sensitivity 18

2.5 Rectenna Design 19

3. Structural Design and Cost Estimates 26

(The Brown and Root Development Inc. Fina l Report)

4. Cost Comparisons 154

5. Secondary Uses, Design Requirements and Sea Defense 156

(The Arthur D. Littl e Fina l Report)

6. Summary of Results 219

7. Conclusions 223

Appendi x A.--Detailed Environmenta l Data 224
Abstract

Rice University , Brown and Root Development Inc., and Arthur

D. Littl e Inc. have jointly conducted a feasibilit y study of an

offshore rectenna servin g the Boston/New York area. We found

that an offshore rectenna is feasibl e and cost competitiv e with

land rectennas but that the type of rectenna which is suitable

for offshore use is quit e different from that specified in the

present reference system. We began by engineerin g the reference

system rectenna to the offshore location. When we estimated

costs for the resultin g system we found that the cost was prohibitivel y high . We then searched for modification s to the desig n

whic h would allo w significan t cost reduction. The result is a

non-ground plan e desig n whic h minimize s the weight and greatly

reduces the number of costly support towers. This preferred

desig n is an antenna array consistin g of individuall y encapsulated dipole s with reflectors or yagis supported on feed wires.

We find that such a 5 GW rectenna coul d be buil t at a 50 m water

depth site to withstand hurricane, winter storm and icin g condi -

tions for a one time cost of $5.7 billion . Subsequent units

would be about 1/3 less expensive . It is important to note that

the east coast site chosen for thi s study represents an extreme

case of severe environmental conditions . More benig n and more

shallo w water sites would result in substantiall y lower costs.

Secondary uses such as maricultur e appear practical with only

minor impact on the rectenna design. The potential advantages of

an offshore rectenna such as no land requirements, removal of

microwave radiation from populated areas and minima l impact on

t he local geopolitic s argue strongly that further investigatio n

of the offshore rectenna shoul d be vigorously pursued.
Executive Summary

The salien t results of this study may be summarized as

fol1ows :

1. An offshore rectenna is feasible along the central and

northeast coast at a first uni t cost of about $5.7

billion .

2. The environmental constraints are very severe and

dictate a fully encapsulated receivin g element.

3. The reference system ground plane design is not suitabl e

for offshore.

4. A non-ground plan e design is preferred.

5. Of four types of support towers studied, a pile d guyed

tower is the least expensive.

6. Secondary uses, in particula r mariculture and wave

energy extraction appear promising at a minimu m impact

to the rectenna design.

7. The preferred design offered here has not be optimized

for cost or efficiency.

8. The offshore rectenna offers signficant advantages and

should be investigate d further.
1. Introduction/Background

The Solar Power Satellite (SPS) concept involves the

conversion of solar energy to microwaves. The microwaves are

then beamed to earth usin g a hig h power phased array

transmitter. The microwave beam is intercepted and converted to

D. C. electricity at the surface of the earth using a large area

receiver and rectifier referred to as a rectenna. For the NASA

reference SPS design the total beam energy is delivery rate is 5

GW. In order to avoid a thermal overload on the ionosphere the

o

beam area at the earth must be approximately 100 km for the

anticipated frequency of 2.45 GHz.

A central question has been the availabilit y of rectenna

sites near major load centers. Ric e University [Blackburn and

Bavinger, satellit e power system white paper on mappin g of

exclusion areas for ' rectenna sites, DOE/NASA SA-13,1979]

performed a preliminary study to locate potential rectenna sites

withi n the U.S. A principa l findin g of this study was that, when

certain exclusion criteria were examined, there were no eligibl e

sites along the highly populated east coast. If it is assumed

that transmission of electrical power over long distances wil l

not be practical in the time frame when the SPS comes on line ,

this would exclude the east coast electrical load centers from

enjoying the benefits of the SPS.

A solution to this problem would be the locatio n of

rectennas offshore. In additio n to the solution of the lan d

availabilit y problem, an offshore rectenna would have the
following additiona l advantages:

1. The legal aspects of land acquisitio n are simplified .

2. Land clearin g and maintenance costs are eliminated .

3. The peripheral microwave radiation is removed from

populated areas.

4. The rectenna is "out of sight" from the general publi c

minimizin g regional politica l impact.

5. Various secondary uses such as aquaculture become

possible.

6. On-site hydrogen generation becomes a logica l

possibilit y makin g the SPS system a fuel source as well

as electrical source.

Counterbalancin g these advantages are the disadvantage s of

the more severe offshore weather environment, the more

complicate d construction logistic s and support tower increased

height.

2. Approach

2.1 Site Selection

Based on the idea that the SPS would better serve the U.S.

if an offshore rectenna could be buil t to service major load

centers on the east coast, six candidate sites were selected for

a rectenna. The criteria used for selection were as follows:

1. Capable of serving the New York and Boston Metropolitan

areas [approximately 320 km (200 miles) as an outer

limit].
2. Avoi d shippin g lanes.

3. Maximiz e distance from shore but do not exceed about 64

km (40 mi 1es) out.

4. Avoi d recreational boat traffic areas.

5. Avoi d heavy fishin g areas.

6. Avoid hazardous areas such as schoals or rip tides.

7. Stay on the continental shelf.

8. Avoid petroleum exploration areas.

9. Avoid waste disposal areas.

10. Level seabed.

Initiall y six candidate sites were examined. Based on the

above criteria, thi s lis t was narrowed to a singl e site (site

III). The general data for this site are as follows:

Location: 40°59' N, 70°44' W

Distance to N. Y.: 280 km

Distance to Boston: 121 km.

Distance to Martha's Vineyard: 40 km.

Seabed: coarse sand and scattered gravel

Water Depth: 50 m

Tida l Currents: about 1 km/hr.

Annua l Tides: 1.1 m

Figur e 2-1 is a map showin g the location of this site. An

important feature of thi s site is the uniform water depth which

does not vary by more then about 10 meters over the entire site.

Figure 2-2 gives the rectenna dimension s for this site.

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2.2 Environmental - Data

When the favored site was chosen, Rice began collectin g

environmenta l data on that site. The ful l collectio n of data on

thi s site is give n in appendi x A of this report.

A summary of the severe or worst case design data is as

fol1ows:

Storm Wi nds :

Extreme wind speeds: 67 m/sec (150 MPH) (sustained

hurricane storm wind 1 minute).

Winter storm windspeeds: 31.3 m/sec (70 MPH)

Three second gust velocity: 85 m/sec (188 MPH)

Storm Waves:

100 year recurrence maximu m wave height: 26.5 m (87.0

ft)

Significant storm wav e height: 13.6 m (44. 6 ft)storm

surge ti de : 1 meter

Icing:

Average monthly frequency of moderate superstructure

Icing : December, 12.5%; January, 22.5%; February, 15%.

Estimated icing , less than 1.3 cm

Snow:

Weight: 65 kg/m2

Based on years of experience as an industria l consultant.,

Professor Herb Beckman estimated that a conservative design would

allo w the reduction of the above 67m/sec (150 mph) 100 year storm

extreme wind speed to 49 m/sec (110 mph) and the maximum wave

11
height from 26.5 m (87.0 ft) to a 19.8 m (65 ft) non-breaking

wave. These lower values were their used throughout the study.

Icing is of particula r importance because of possible

efficiency loss to the rectenna when it occures. We were unabl e

to locate any quantitative estimates of the thickness of icing to

be expected, merely the probabilit y of "moderate super structure

icing". Potential Moderate Icing i-s defined as the simultaneous

combination of an air temperature less then -2°C and a wind speed

greater than 6.7 m/sec (15 mph). It seemed likel y that an icin g

thickness greater than several millimeter s could occur and

therefore an estimate of the efficiency loss due to ice buildu p

on the rectenna was necessary.

2.3 Icing Studies

Rice conducted a systematic study to determine the effects

of ice on the properties of a simple antenna. These tests also

determined the size and type of protective cover or radome

necessary to protect the antenna from severe icin g effects.

2.3.1 . The Monopole

The initia l tests were conducted with a quarter wavelength

monopole projecting through a ground plan e which was about 2

wavelengths in diameter. The antenna element was driven with a

50 ohm coaxial lin e connected to a Hewlett-Packard 8410 microwave

network analyzer.

The antenna was mounted in a microwave anechoic chamber with

dry ice to provide the subfreezing temperatures. Figure 2-3

shows the test setup.

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With the antenna mounted in the test chamber, the frequency

of the microwave source was adjusted for the resonant frequency

of the antenna, about 2.45 GHz. Resonance was determined by a

minimu m in the signal reflected from the antenna, usually not

more than 2%.

Distille d water, or salt water was then sprayed in a fine

mist over either the antenna element or ground plane or both and

allowed to freeze. Once the water was frozen completely, a

measurement was made of the reflection coefficient and the

thickness of the ice determined. The reflection coefficient is

the amplitud e of the reflected wave relative to that of the

incident wave. It is a good measure of the electrical mismatch

caused by the ice.

Two kinds of salt water were used. One was "Ringer's

solution" which is about 1/3 as salty as sea water - to simulat e

a salt water - rain water mix as would occur in a storm. The

other was simulated sea water which contains a very close approximatio n to the ion content of actual sea water.

Once the effect of ice and salt ice on the antenna

performance had been determined, a series of plexiglas s covers

were made for the active antenna element. These were placed over

the antenna and the tests repeated. Covers of several

thicknesses were used in an attempt to find the minimu m size

cover which would ameliorate the effects of ice.

2.3.2. Monopole Results

Figure 2-4 illustrates the measured reflection coefficient

vs ice thickness. It is seen that as the ice thickness build s up

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to about 0.005 m (5mm) the reflection coefficient approaches 0.5.

Another important result is that there was very littl e

change in the reflection coefficient as the water froze,

indicating rain water is as bad as ice.

Clearly a protective covering is required over the active

element.

Figure 2-5 illustrates the effect on the reflection

coefficient of various thickness covers. A 0.01 m (10 mm) radius

cover on the active element reduces the reflection coefficient to

0.1. A thicker cover yield s no significan t advantage.

We can summarize the monopole icin g test results as follows:

1. With no cover, the reflection coefficient asymptotically

approaches 0.5 at an ice thickness of about 0.005 m (5

mm).

2. 0.01 m (10 mm), radius cover on the active element

reduces the reflection coefficient to 0.1. Thicker

covers yield no significan t improvement.

3. Rainwater is a bad as ice.

2.3.3. The Dipole Results

Following the monopole icin g tests, there was some

uncertainty that the results would apply equally well to a dipol e

configuration. To verify this, a balanced feed dipol e antenna

was set up. Time and fiscal constraints did not permit a

complete test, but preliminary tests indicated :

1. Icing on the active element is as bad as for the

monopole.

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2. Ice on the ground plane is also a problem. In fact

0.002 m (2 mm) of ice on the ground plan e produces a

ref1ection . coefficient of 0.3.

There was no time to run tests of covers.

The overall conclusio n from the icin g studies is that an

insulatin g cover at least 0.01 m (10 mm) thick is necessary for

t he rectenna active elements and ground plane.

2.4. Rectenna Directional Sensitivity.

Possible suggestions for the rectenna support include d

semisubmersable or fully submersible floats, hence a tethered

floating structure. This would lead to pitch, roll and yaw

motion of the rectenna relativ e to the directio n to the

transmitter. (Here a yaw change is defined as an angula r offset

in the plane of the dipol e element axis and a pitc h change is an

angula r offset in the plan e orthogonal to the dipol e element

axis , -i.e., the fundamental direction is from the rectenna

lookin g directly backwards to the transmitter with the dipol e

active elements as aircraft wings.) It was therefore necessary

to have as a desig n inpu t to the support structure group the

allowabl e angula r discrepancy from perfect pointing .

Professor Wilson' s group from the Ric e Electrica l

Engineerin g Department undertook the calculatio n of loss of

rectenna efficiency with angula r misalignment . Misalignmen t in

t wo planes was studied ; misalignmen t in the plane includin g the

axi s of the diol e elements and misalignmen t in the plan e

orthogonal to the plan e includin g the dipole . The results for

18
the half-wav e dipole with ground plane are as follows :

Angle of

Mi sal i gnment

Yaw

Pitch

In the pian e of

the di pole axi s

Perpendi cula r

to Di pole axi s

i 5°

I22o

19°

29°

Power

Loss

,1% t5%

1%

5%

It was considered desirabl e to keep the power loss as close

as possibl e to 1%. If the recte'nna is designed with the dipol e

axes horizontal , rather than with a vertical component, the

rotation about the most sensitiv e angl e can be minimized . This

assumes that the structure allow s pitc h changes more readily than

yaw changes. Rol l changes should not effect the efficiency.

Accordingly the receiver panel is allowed to rotate up to 11° in

yaw or 19° in pitch . The 5° yaw angle necessary to achieve the

1% loss as indicate d above was considered unrealistic .

As seen in figure 2-2 the angl e of the receiver ground plane

relativ e to the local vertical must be 42-.6°. for the latitude of

the chosen site.

2.5 Rectenna Design

When the environmenta l constraints dictated hurricane

velocity winds, it became clear that wind loadin g on the rectenna

panels would be a significan t factor. For this reason, it was

decided early to employ the open rectenna which is part of the

19
reference system. [Satellite Power System Reference System

Report, DOE/ER-0023 1978]. This design consists of discrete

half-wave dipole s mounted above a steel mesh ground plane. The

diodes feed low-pass filters and Schottky by barrier diodes. The

rectenna panels form a series of serrated rows of panels with

each face perpendicular to the beam; the row long axis oriented

east-west.

There were two other reasons for selecting this basic

rectenna configuration:

1. Land rectennas of thi s design had already been costed

[Boeing Aerospace Co. SPS System Definition Study, Phase II,

Final Report, DI80-25461-1 , Rev A, Feb, 1980] and hence, once the

offshore study was complete, direct comparison would be possibl e

with land rectenna costs.

2. Don Hervey of Brown and Root Development Inc. had

already begun work on a submersible floatin g support structure

designed around the reference system rectenna.

The Brown and Root submersible float concept is illustrate d

in figure 3.1.1. Submerged float tanks which are anchored by

gravity anchors support towers which in turn support the receiver

diode panels. Because this system is floating , it is desirable

to have a non-rigid support for the diode panels, allowin g them

to remain pointed in the beam direction. To accomplish this, Don

Hervey of Brown and Root developed a double mass pendulu m design

which allows the panels one degree of rotational freedom. (see

figure 3.2.5). Considerable effort was spent optimizin g the

dimensions of this design.

20
At the midterm design review held at Rice November 6, 1979,

it was brought out that the submerged float system held numerous

problems not first appreciated. Brown and Root then began a

serious study of alternativ e support structures. The results of

thi s study are documented in section 2-6 structure design.

Four types of structures were evaluated. These were:

1. The submerged bouyant platform.

2. The pile d structure.

3. The gravity base structure.

4. The pile d guyed tower.

Of these, the pile d guyed tower (figure 5.2.1) was found to

be the least expensive, with the material and fabrication cost

alone approximately $400,000 per tower. For the doubl e mass

pendulu m panel reference system type rectenna 25,000 towers are

required. Peter Dove of Brown and Root Development, Inc.

determined the total rectenna cost, includin g installation , at

$36.6 billio n for a pile d guyed tower rectenna.

Since thi s total cost was considered prohibitivel y

expensive, cost sensitivit y calculation s were conducted on water

depth, type of soil and wind loads. The conclusion from these

cost calculation s was that the only way to substantiall y reduce

the cost was to reduce the number of towers required.

Furthermore, it was concluded that this could not be done without

greatly reducing the basic weight of the receiver panels

themselves.

Accordingly, Rice began a search for a new rectenna receiver

element concept whic h would greatly reduce both the dead weight

21
and the wind and snow load weight of the receiver panels. The

resultin g design, referred to as the clotheslin e or non-ground

plan e design is pictured in figures, 2-6, 2-7, and 2-8.

In this design, the ground plan e is eliminate d and replaced

by a reflecting passive dipol e element. Both the active element

a nd passiv e element, together with a harmonic wave trap and

rectifier may be printed an a P.C. board and encapsulated withi n

an oval of dielectri c foam as shown in figure 2-7. This foam

provides the required protection agains t icin g and rain. Power

is conducted away from the diode by horizontal rods which also

provide the mechanical support. A concept simila r to thi s but

without the foam encapsulant was investigate d by Ron Guttman and

Jose Barrego of Rensselaer Polytechnic Institute in connection

with a study of higher gai n rectennas for the Johnson Space

Center [Solar Power Satellit e Rectenna Design Study: Directional

Receivin g Elements and Paralle l Series Combinin g Analysis ,

Contract NAS9-15453 final report, 1978]. Numbers for the cost

per square meter used in our study were taken from the GuttmanBarrego report and doubled to accommodate the cost of foam and

P

other contingencies. This cost is $ 1 0 / m .

The mass estimate for the diode receivers is 2.5 kg/m^ ,not

includin g the support cables. This is about one tenth the dead

weight load of the ground plan e design. With the clotheslin e

concept, the structure is considered sufficiently open that no

snow load need be considered.

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Wit h this lower dead weight and snow load, the dipol e array

can be supported by a network of 0.0127m (0.5 in.) diameter

cables at 30.5 m (100 ft) spacing in a square criss-cross network

with 0.0762m (3 in.) diameter cables every 305 m (1000 ft).

Support towers are located every 305m (1000 ft) (See figure 2-

8). This permits the number of support towers to be reduced to

3000; down from 25,000 required for the ground plane

configuration.

The total cost for the first unit 5 G W rectenna is $5.7

billion , about 17% of that for the ground plan e configuration.

Subsequent rectennas are estimated by Brown and Root at $3.8

billion .

It is probable that if this clothesline concept were applie d

to a lan d rectenna considerabl e cost savin g could be realized

here as well. Moreover, as has been pointed out by Ala n Kotin

[private communication], the rectenna site selection criteria for

terrain conditions and land clearing could probably be relaxed

due to the fewer number of towers required.

Time and fiscal constraints did not permit an investigaito n

of the efficiency of the clotheslin e concept; however, the R. P.

I. study referenced above evaluated the printed circuit yagi for

efficiency. The clotheslin e desig n coul d easily be adopted to a

multipl e element yagi with very littl e cost impact.

3. Structural Design and Cost Estimates

A subcontract was let to Brown and Root Development Inc. to

26
perform a structural design and cost analysi s on the offshore

rectenna give n the site and design constraints specified by Ric e

University . The site location and environmenta l data were

determined early in the program; however, the receiver element

design remained flui d and the switch to the non-ground plan e

configuratio n was made late after cost estimates

from the reference system type antenna showed that thi s system

was inappropriat e for an offshore rectenna.

The Brown and Root Development Inc. subcontract final

report, which follows, give s the detail s of the structural design

and cost estimate.

27
OFFSHORE RECTENNA STUDY

FINAL REPORT

EF-0079

MARCH, 19GO

PREPARED BY:

BROWN & ROOT DEVELOPMENT, INC.

UNDER SUBCONTRACT NO. 437135A

TO

RICE UNIVERSITY

FOR NASA

I
FOREWORD

The Offshore Rectenna Structural Design Study was performed for Marshall Space

Flight Center under Subcontract No. 4371854 to provide conceptual offshore

rectenna designs and their preliminary evaluation. Rice University is the

prime contractor while Brown & Root Development, Inc. and Arthur D. Little,

Inc. are the subcontractors.

II
OFFSHORE RECTENNA

EXECUTIVE SUMMARY

TABLES & FIGURES

1. INTRODUCTION

1.1 OBJECTIVES

1.2 SCOPE

1.3 DEFINITION OF TERMS

2. DESIGN CRITERIA

2.1 SITE SELECTION AND ENVIRONMENTAL CONDITIONS

2.2 OPERATING REQUIREMENTS

3. CONCEPTUAL DESIGNS

3.1 SUPPORT SYSTEMS

3.1.1 STRUCTURAL CONFIGURATIONS

3.1.1.1 SUBMERGED BUOYANT PLATFORM

3.1.1.2 PILED STRUCTURE

3.1.1.3 PILED GUYED TOWER WITH PANELS.

3.1.1.4 GRAVITY STRUCTURE

3.1.1.5 PILED GUYED TOWER WITH IMAGE DIPOLES

3.1.2 MATERIALS ,. . .

3.1.3 FABRICATION, INSTALLATION AND MAINTENANCE

3.1.4 DESIGN EVALUATIONS

3.2 RECEIVER PANEL & TAUTLINE SYSTEMS

3.2.1 STRUCTURAL CONFIGURATIONS

3.2.2 MATERIALS

3.2.3 FABRICATION, INSTALLATION AND MAINTENANCE

III
OFFSHORE RECTENNA (Cont'd)

3.2.4 DESIGN EVALUATION

3.2.4.1 RECEIVER PANEL

3.2.4.2 TAUTLINE

4. PRELIMINARY COST ESTIMATES

4.1 COST VS. SUPPORT SYSTEM TYPE

4.2 COST VS. WATER DEPTH

4.3 COST VS. WIND LOAD

4.4 COST VS. SOIL CONDITIONS

4.5 TOTAL COST

5. PRIME SITE POINT DESIGN

5.1 STRUCTURAL CONFIGURATION

5.2 FABRICATION DEPLOYMENT AND INSTALLATION

5.3 MAINTENANCE

5.4 COSTS

6. PREFERRED DESIGN

6.1 STRUCTURAL CONFIGURATIONS

6.2 FABRICATION DEPLOYMENT AND INSTALLATION

6.3 MAINTENANCE

6.4 COSTS

7. SUMMARY

7.1 CONCLUSIONS

7.2 RECOMMENDATIONS

APPENDIX A SUPPORT SYSTEMS DESIGN CALCULATIONS

APPENDIX B RECEIVER PANEL AND TAUTLINE DESIGN CALCULATIONS

IV
EXECUTIVE SUMMARY

Preliminary designs and cost estimates for two types of offshore rectenna have

been developed during the present study. Considerations include fabrication,

deployment and installation schemes for the designs. Important findings

include:

. The baseline offshore rectenna design consisting of panel receivers

and guyed tower supports is feasible for an Atlantic coast site.

. Alternate conceptual designs (e.g. submerged buoyant platform,

piled structure, gravity structure) for supporting receiver panels

result in a costlier rectenna.

. Changing the receiver configuration from panels to image dipoles

greatly reduce costs in materials, support systems and installation.
LIST OF FIGURES

FIGURE NO. " TITLE PAGE NO.

2.1.1 Primary Site Location 9

2.1.2 Wave Spectrum 11

3.0.1 Wave Height vs. Tower (Wave + Wind Loads) 13

3.0.2 Soil Conditions vs. Tower Base 14

3.1.1 Submerged Buoyand Platforms 17

3.1.2 Piled Jacket Type Offshore Platform 19

3.1.3 Piled Guyed Tower Structure with Panel

Receivers 21

3.1.4 Gravity Platforms 23

3.1.5 Piled Guyed Tower Structure with Dipole

Receivers 25

3.2.1 Single Mass Pendulum Design 32

3.2.2 Single Mass Pendulum Model" 33

3.2.3 Above M.W.L. Single Mass Pendulum System 34

3.2.4 Submerged Single Mass Pendulum System 35

3.2.5 Double Mass Pendulum Design 36

3.2.6 Double Mass Pendulum Model 37

3.2.7 Receiver Panel Configurations 39

3.2.8 Tower Pivot Point Location Tautline Design 41

3.2.9 Receiver Panel Structural Configuration 49

3.2.10 (a) Dynamic Condition-Mass vs. Pendulum Arm

Length 52

3.2.10 (b) Dynamic Condition - Period vs. Mass Ratio 52

3.2.11 Pivot Point Location Criteria 54

3.2.12 Pivot Point Location 55

3.2.13 Single Panel Tautline Models 56

4.2.1 Cost vs. Water Depth for Piled Guyed Tower

with Panels 60

5.1.1 Prime Site Guyed Tower Structure 65

5.2.1 Side Elevation of Jack-up Units Showing

Installation of Columns and Panels in Channels 76

5.2.2 Possible Layout of Typical Staging Port 77

VI
LIST OF FIGURES (Continued)

FIGURE NO. TITLE PAGE NO.

5.2.3 Site Elevation of Column Deployment Jack-up

Showing Dynamically Positioned Column

Transporting Vessel Discharging Columns 78

5.2.4 Equipment and Arrangements for Deploying

and Piling Columns 79

5.2.5 Plan of Column Deployment Jack-up Barge

Showing Column Transporting Vessel 80

5.2.6 A Jack-up Panel Barge in Position Between

Columns Prior to Jacking Out 87

5.2.7 Jack-up Panel Barge - Jacked Clear of Water

and Deck Raised So That Panel Line Can Be on

Connected and Tensioned

5.2.8 Jack-up Panel Barge - Cargo Deck Lowered 89

5.2.9 Jack-up Panel Barge - Jacked Back Into

Water and Legs Raised Off Seabed in

Preparation for Moving to Next Pair of

Columns 90

5.2.10 Jack-up Barges Setting Panel Sections in

Consecutive Columns 91

5.2.11 Installation of Panels Over Columns 92

6.1.1 Platform and Taut-Wire Support Arrangement

for Flexible Non-Ground Plane Dipoles 101

6.1.2 Configuration of Support Towers and

Perimeter Taut Line Cables in Preferred

Design 102

6.1.3 Typical Non-Ground Plane Flexible Dipole

Arrangement 103

6.1.4 Encapsulated Non-Ground Plane Diode, and

Detailing Electronics and Supporting

Arrangement 104

6.1.5 Section of 100' By 100' Non-Ground Plane

Dipole Microwave Receiving Arrangement 105

6.2.1 Dipole Network Installation 115

6.2.2 Dipole Network Installation on Tautlines 116

V II
LIST OF TABLES

TABLE NO. TITLE PAGE NO.

3.1.1 Support Systems 16

3.2.1 Panel Structural Weight ' 50

4.1 Preliminary Costs For Supporting Systems 58

4.2 Pile Stresses versus Uind Loads 61

4.3 Cost versus Soil Conditions For a Gravity

Structure . 62

4.4 Total Costs For Piled Guyed Tower

Structures 63

5.1 Cost Summary For Prime Site Point Design 94

5.2 Itemized Costs For Prime Site Point Design 94

6.1 Cost Summary For Preferred Design 111

6.2 Itemized Costs for Preferred Design 112

VIII
1. INTRODUCTION

The Solar Power Satellite System (SPS) is an energy conversion system;

receiving solar energy, converting it to microwaves, which in turn

becomes electrical energy to be sent through utilities power grids. The

solar energy is collected by a 5 to 10 gigawatt (GW) satellite in

(geostationary) orbit about the earth using solar cells (or perhaps

mirrors or photoklystrons) typically made of silicon or gallium

arsenide. The geostationary orbit permits the satellite to receive

continuous sunlight except for brief eclipses at the equinoxes. The

microwaves are generated using klystrors, amplitrons, or solid state

devices. The energy transmission beam from the satellite to earth is a

phase controlled microwave beam at 2.45 GHz (frequency selected for

maximum transparency of the atmosphere). The microwaves are received on

earth by a rectifying antenna termed a rectenna, which is an array of

dipole elements connected to rectifier and filter circuits. The

rectenna encompasses an oval which is approximately 10.00 kilometers by

14.77 kilometers (based on prime site location). The dipole rectifiers

convert the microwaves to direct current to interphase with existing

ground based utility grids.

One main reason behind siting a rectenna offshore is the difficulty in

finding an acceptable site on land near large power users. Rice

University performed a study in an effort to screen rectenna sites in

the 48 contiguous states. Using various levels of exclusion criteria it

was concluded that less than 11% of the geographic area of the 48

states was not excluded as possible rectenna sites. There were no

eligible sites along the east coast. Since electricity cannot be
transmitted economically over long distance, an SPS with a land based

rectenna would not serve the electrical needs of this region (based on

the exclusion variables in the Rice Study).

There are other reasons for considering siting a rectenna offshore. An

offshore site can be. located closer to many power load centers than a

site on land. It would tend to minimize the environmental and political

impact of the microwave beam in the vicinity of the rectenna. Since the

utilities would not have to use the right of eminent domain, the lead

time to obtain the land for the rectenna could be substantially

reduced. With no clearing problems site development costs would be

minimal.

The primary problem with an offshore site is the increased

environmental loadings on the rectenna structure. Land based rectennas

do not have to deal with wave and current forces and high velocity

offshore winds. There are significant differences in potential economic

design concepts. A land based rectenna can have frequent supports to

the ground while an offshore design must use supports sparingly because

materials and installation costs for such supports to the seabed (even

in moderate water depths) are large. For offshore construction an

efficient installation plan is necessary (requiring minimizing offshore

construction). Offshore construction is especially susceptible to down

time due to bad weather conditions and to the high expense of offshore

equipment. These add design and construction problems which must be

overcome to produce a rectenna which is competitive in cost to land

based versions.

- 2 -
During the course of the study various panel, tautline, and support

systems were developed and costed. Detailed structural analyses were

not performed due to the conceptual nature of the study and the many

designs evaluated. Cost analyses were performed on all concepts to

determine which design to investigate further.
1.1 OBJECTIVES

This study was performed to evaluate the feasibility of constructing a

rectenna structure offshore. A specific site was chosen by Rice

University and environmental information from that site was used in the

analysis. Environmental criteria were then varied and a parametric

study was performed to investigate cost drivers. Rectenna

specifications were developed by Rice University and changed (from a

rigid receiver panel to a flexible, non-ground plane receiver network)

during the latter stages of the study. BARD I costed all concepts in an

effort to get a cost competitive offshore rectenna design.
1.2 SCOPE

This study is of conceptual nature only. The emphasis of the study was

on systems design and costing. Design evaluations were performed first

to establish technical feasibility of any given design and then to the

point of developing material requirements and installation and

fabrication scenario in order to get a complete cost estimate.

- 5 -
1.3 DEFINITION OF TERMS

The following terms from the text are defined as follows:

. Prime Site - The site chosen by Rice (with BARDI consultation) to

conduce a specific, point design, conceptual offshore rectenna study

(see further details in Section 2.1).

. Support Systems - structures which support the rectenna microwave

receiver elements (panels) and tautline systems above the sea surface.

. Receiver Panel - an array of diode on a groundplane, supported by an

arrangement of rigid panel sections encompassing a 20 x 40 meter area

at a 47.40 angle with the horizontal.

. Image Dipole Receiver - networks of dipoles encapsulated in synthetic

material, supported on small steel cables forming a "web-like" array.

. Tautline - a pretensioned line that suspends the receiver elements

between support structures.

. Guys - lines which act as horizontal restraints by connecting the top

of a tower to the ocean floor (e.g. permanent seabed anchors, adjacent

tower footings, etc.).

. Piled Guyed Tower (PGT) - support structures which are piled into the

seabed with guys attached to the tower tops and the base of adjacent

towers to give additional support.

. PGT Rows - lines of towers as placed at the offshore field site.

. PGT Channels - rows of space between towers running in the direction

(east-west) of the receiver panels and tautline.

. Staging Port - the area on land for the collection of components prior

to final assembly (as required), loading and dispatch to field site.

. Field site - the location for placement of the offshore rectenna.

- 6 -
"Purpose-built" Equipment - equipment which has been specifically

designed and built for the task to be undertaken.

Jack-up units - equipment which has the capability of lifting itself

out of the water using self-elevating support legs.

Semi-submersible units - equipment which has the capability of

increasing or decreasing its draft by ballasting or de-ballasting

procedures.

Linear Winches - winches which are capable of leaving or laying out

wire cable in a straight line without spooling the cable on a drum or

reel.

- 7 -
2. DESIGN CRITERIA

Design criteria development was a joint effort between Rice University

and BARDI. Both rectenna specifications and environmental conditions

were reformulated during the study in order to obtain a cost effective

rectenna structure, while maintaining a conservative design approach.

2.1 SITE SELECTION AND ENVIRONMENTAL CONDITIONS

The factors considered by Rice in the site selection study include:

close proximity to New York City and Boston, environmental conditions,

and avoidance of shipping lanes, conmercial fishing areas and migratory

fowl flight paths. Due to the unavoidable harsh weather conditions in

the area, a different site would be a more cost effective choice. The

site chosen, as shown in Figure 2.1.1 is 40° - 59N latitude and 70

-44W longitude (175 miles from New York City and 75 miles from Boston).

It is considered the prime site for this study. Rice and BARDI

collected environmental data from the area and developed a design

premise. Subsequent changes have altered the environmental loadings,

reducing them to more realistic values for that region. The (hurricane)

design wind velocity was reduced from 150 to 110 mph (240-176 km/hr)

for the 100 year storm. The maximum wave height from 87.0 (26.5 m) feet

breaking wave to a 64.8 feet (19.8 m) non-breaking wave. These new

values were provided by industry consultant and Rice University

Professor Dr. Herb Beckman, and are considered to be conservative

values. The following are the critical prime site environmental

criteria:

- 8 -
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- 9 - .\
Water Depth - 162.0' (49.4m)

Storm Wave Height - 64.8' (19.8m)

Wave Spectrum - Figure 2.1.2 (greater than 25 sec. structure

natural frequency will minimize response to high energy area -

based on 87 ft. (26.5m) wave height

Storn Wind: Hurricane - 110 mph (176 km/hr)

Winter Storm: 70 mph (112 km/hr)

Snow Load: 13.6 psf (.65 kpa)

Ice Load: 2.9 psf (0.12 kpa)

2.2 OPERATING REQUIREMENTS

The operating requirements (developed by Rice University) include the

following for the prime site design:

. The receiver panel is a rigid structure.

. The receiver panel is tilted at 47.4° to the horizontal and

cannot rotate more than lio aiong the row nor more than 19°

about the row axis.

. Microwave reception ability must remain high during storm conditions.

. Materials in the microwave path must not interfere with microwave

reception.

3. CONCEPTUAL DESIGNS

Conceptual designs were developed considering either site specific or

generalized data. Thus, point designs were developed for the prime site

but critical parameters were varied to provide cost versus parameter

information (e.g. cost versus water depth).

- 10 -
(03S -
Varying design parameters significantly impacts the support structure

economics. Wave heights and thus loads change with water depth and

dictate the support structure height. This height effects material

requirements and establishes magnitude of the overturning moment which

bends the structure due to wind as shown in Figure 3.0.1. The size of

the base of the supporting structure is determined by the soil

conditions as illustrated in Figure 3.0.2. The soil strength controls

an important trade off between material requirements and installation

cost. A gravity structure is easily installed without the expense and

time necessary for piling, yet poor soil conditions create the need for

extensive bases and thus increased structural materials.

Receiver panel and taut line designs change little for the different

environmental loads considered. Wind, snow, ice and dead loads

determine the weight to be supported by each panel. However, the

conceptual design is dependent upon dynamics and efficient force

distribution to the support structures.

3.1 SUPPORT SYSTEMS

The supporting system for panels with a reflecting ground plane should

safely carry the weight of the panels while the structure is subjected

to 100 year wind and wave loads. An alternate design condition is the

panel weight plus the winter storm snow, wind and wave loads. The 100

year storn is a hurricane which cannot occur in conjuction with the

snow load.

- 12 -
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- 14 -
3.1.1 Structural Configurations

BARDI considered five different support systems in the study.

Four preliminary designs were developed using the conditions of

the prime site (162 ft. - 49 m - water depth) as design

criteria. The support systems considered are shown in Table

3.1.1.

3.1.1.1 Submerged Buoyant Platform

The submerged buoyant platform design employs buoyancy

tanks to support tower strucures. Two panels can be

supported by the taut line between towers. The towers,

supported by buoyancy tanks can be anchored with

cables to either dead weight anchors or to piled

anchors in the seabed as shown in Figure 3.1.1.

Since the tower structure does not extend to the

mudline, the resulting shear and overturning moments

in the tower are reduced. Further reduction of the

tower member sizes can be achieved by connecting the

top of the tower to anchors with additional cables as

shown in Figure 3.1.1.

- 15 -
TABLE 3.1.1 SUPPORT SYSTEMS

SUPPORT CONFIGURATION

SUBMERGED BUOYANT

PILED STRUCTURE

PILED GUY TOWER

GRAVITY STRUCTURE

PILED GUYED TOWERS

RECEIVER TYPE

PANELS

PANELS

PANELS

PANELS

IMAGE DIPOLES

- 16 -
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- 17 -
Preliminary studies indicate that five tanks 8 feet in

diameter are needed to support the tower and the

panels. In the case of using guys at the top of the

tower, the tower may consist of a single pipe of 36

inches (91.4 cm.) in diameter.

•3.1.1.2 Piled Platforms

A design employing a standard, jacket type offshore

platforms forms a basis for comparison of other

systems. When a jacket structure is installed, piles

are driven through the legs to fix the structure to

the sea bed. The jacket transfers the wind and wave

loads and the panel weight to the piles as shown in

Figure 3.1.2. The jackets can be spaced in rows, 96.9

feet (29.5 m.) apart and 530 feet (161 m.) apart along

the taut line. Each tower is designed to carry the

load of four panels.

Preliminary sizing of four legged fixed platforms

yields 48 inch (122 cm.) diameter legs with 46 inch

(117 cm.) diameter piles and 24 inch (61 cm.) diameter

braces, (penetrating 100 feet (30.5 m.) below the

mudline).

3.1.1.3 Piled Guyed Tower Supporting Panels

A piled guyed tower system uses the guys to resist

part of the lateral loads. Thus a lighter structure is

obtained.

- 18 -
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- 19 -
Each guyed tower in the BARDI design concept supported

four panels. The tower is supported on four piles

driven through the tower leg. Wind loads are resisted

by guy cables which extend from the top of one tower

to the bases of the neighboring towers as shown in

Figure 3.1.3.

The guy cables are prestressed to 750 kips (3333 kn)

to keep them in tension under all loading conditions.

The piles are designed to withstand the pull-out

loads coming from the cables which are partly

compensated by the weights acting on the platform.

Preliminary designs were done for water depths of 162

feet (49 m.) 101.5 feet (31 m.) and 75 feet (23 m.).

162 foot (49 m.) water depth structure was checked

against different load conditions with proprietary

BARDI computer program.

Preliminary sizing of the Guyed Tower Structure

results in 26 inch (66 cm.) diameter legs with a 0.5

inch (1.3 cm.) wall thickness. The horizontal and

diagonal bracing are 16 inches (41 cm.) in diameter

with a one inch wall thickness and 112 feet (34 m.)

penetration.

- 20 -
>-

- 21 -
For the prime site and for the design considered in

this study, the guyed tower design is the most

economical support system for the panel-rectenna

configuration. Improved bottom soil conditions and/or

sheltered water could render gravity platforms more

economical.

3.1.1.4 Gravity Platforms

Gravity platforms are considered because of the ease

and thus cost of installation and their suitability

for shallow water.

Initially a heavy platform was considered to resist

all wind and wave loads. The platform base transfered

the loads and overturning moments to the soil. For the

given site (162 feet - 49 m - water depth) the

combination of wind and wave forces resulted in an

expensive, heavy platform.

To eliminate the large base, a guyed gravity platform

was considered as shown in Figure 3.1.4. Guy wires

connected to the top of platform, transfer the wind

forces directly to the base and thereby eliminate the

need for heavy legs and bracing of the platform.

- 22 -
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The base is a rectangular box (95 ft. x 50 ft. x 5

ft.) (19 m. x 15 m. x 1.5 m.) to accommodate the

larger wind loads in the direction perpendicular to

the panel lengths. Structurally the base works like a

box girder with 1/2 inch -1.2 cm. - plates for the

flanges and webs.

Preliminary sizing of the gravity platform results in

26 inch (66 cm.) diameter legs and 16 inch (41 cm.)

diameter braces. The guy cables are connected to the

base of the neighboring platform. The high strength

steel guy cables which are pre-tensioned to 750 kips

(333 KN) are 4 inches (10 cm.) in diameter.

3.1.1.5 Piled Guyed Tower With Image Dipoles

The high cost of the four support systems discused in

Sections 3.1.1.1 through 3.1.1.5 led to the search for

a new microwave receiver configuration with fewer

supporting platforms.

In this system instead of using fixed panels, image

dipoles supported by wires are use to collect

microwaves as shown in Figure 3.1.5. A net of cables

stretching over an area of 1000 feet (305 m.) by 1000

feet (305 m.) is supported by platforms at the

corners. The outer cables are pretensioned every 2000

(610 m.) feet. The towers resist the weight of the

- 24 -
- 25 -
supporting wires, the wave forces and the components

of the tension ing force in the cables.

The towers will be guyed as in the guyed tower

configuration. The piles will resist the dynamic

uplift forces coming from the cables (the static

forces are self equilibrating), the vertical component

of cable forces at the top and the overturning moments

due to waves.

The supporting net for the dipoles consists of 3 inch

diameter cable taut lines which join the four towers

at the corners of a 1000 feet (305 m.) by 1000 feet

(305 m.) module. The taut line will support 1/4 inch

diameter cables which are laid across. At every 100

feet (30.5 m.) the taut line will need to be

pretensioned to approximately 250 kips (111 kn).

Similarly, 1/4 inch (0.64 cm.) cable will need

tensioning of approximately 1 to 1.5 kips (4-5 kN).

Preliminary sizing of the net is based on the

assumption that the flexible diode panels will be

designed so that no snow will accumulate on panels to

cause significant loads.

- 26 -
3.1.2 Materials

In all support systems considered, platforms or towers will be

made of A-36 type of steel. A-36 steel will also be used for

buoyancy tanks and piles.

The guy cables for the towers will be made of high strength

steel wires with a minimum breaking strength of 200 kips per

square inch (1.4 kN/mm2). Similar strength steel cables will

be used for the net support system of the flexible image dipole

receivers.

3.1.3 Fabrication Installation and Maintenance

Fabrication, installation and maintenance for all the support

systems considered are within the present state of the art.

Installation of gravity platforms will be easier than the

installation at guyed towers or piled structures since time for

driving the piles is eliminated.

The deployment, installation and maintenance of a guyed tower

structure is discussed in detail in Sections 5.2, 5.3, 6.2, and

6.3.

3.1.4 Design Evaluations

The submerged buoyant platform is the lightest of the support

system configurations considered. In fact, when guyed at the

top, it can consist of a single pipe section. The weights acting

- 27 -
on the platform require the use of large buoyancy tanks which

are nearly twice as expensive to fabricate as jacket structures.

Thus, the resulting total cost of a submerged buoyant platform

is higher than for a guyed tower structure.

The dynamic behavior of such a structure warrants a model study

since little information and experience is available at present.

The practically of such a flexible system should be investigaged

before substantial further effort is expended.

At the prime site, the total height of the piled structure is

280 feet (85.3 m.). A piled structure which resist the wind

loads which

 gact at the top and the current and wave forces along

its length, result in the heaviest tower structure among all the

systems studied. The high cost of fabrication is partly

compensated by the ease in installation of the piled structure

since there are no guy cables.

A guyed tower is a lightweight structure. Since wind loads are

resisted by pretensioned guys, the tower is designed to

withstand the weights and the wave loads only. Thus, slenderer

sections are used in the tower. For example, tower jacket leg

size is 26 inches (66 cm.) in diameter with 0.5 inches (1.2 cm.)

wall thickness which compares with a jacket leg of 60 inches

(152 cm.) in diameter with a one (1.0) inch (2.5 cm.) wall

thickness in the case of the piled structure. The main

disadvantages of a piled guyed tower structure are the following:

- 28 -
- The use of guy cables and their tensioning present additional

installation work.

- The guy cables will restrict the access to the towers.

- Information with respect to the life cycle costs of underwater

cables is limited.

The piled guyed tower configuration as a support system for

panels or image dipoles is the least costly of the systems.

A gravity structure is the simplest to install of the support

systems considered. There are no piles to be driven. Thus, on

the average two days of installation work per platform are

eliminated.

The main disadvantage of a gravity structure is the large size

of the base. For the prime site, a gravity structure of 280 feet

(85 m.) height requires a base which is 130 feet by 50 feet (40

m. x 15 m.). This is greater than the 96.9 feet (29.5 m.)

spacing between the rows of platforms. The use of tensioned

guyed wires reduces the base dimensions to 90 feet by 50 feet

(27 m. by 15 m.), but even then the resulting fabrication costs

for the base are comparatively uneconomic.

Gravity platforms may be competitive for shallower water in the

50 feet to 75 feet (15 to 23 m.) water depth range. Smaller

bases may be obtained with concrete bases. Deployment and

installation of such a structure will be more costly than for a

steel base platform..

- 29 -
For the prime site, the guyed tower configuration is the

preferred support system design.

3.2 RECEIVER PANEL AND TAUTLINE SYSTEMS

3.2.1 Structural Configurations

Receiver panel and tautline systems were developed to maintain

structural integrity during severe environmental conditions. The

following essential criteria were considered in formulating

these configurations:

. Static load support (for wind, snow, etc.)

. Dynamic response (with wave frequencies).

Panel isolation from support structure movement.

Once these essential criteria were met, other parameters such as

ease of fabrication and installation and cost were considered.

The receiver panel is a stiff plane supporting the diode and

groundplane network. It is suspended between the support

structures by a tautline system. To optimize microwave reception

efficiency, the receiver panels were designed to encompass as

large an active area as possible (reducing edge loses) and

tilted 47.40 (in the prime site design) in order for the

receiving elements to be normal to the microwave beam. The

reception area contains 98,175 panels in the 10 kilometer by

14.77 kilometer reception area.
In one conceptual design the receiver panel is stiffly attached

to a single mass pendulum, as illustrated in Figure 3.2.1. The

pendulum serves as both a resistant mass to the environmental

loads and as a dynamicallly tunable mechanism (as Illustrated in

Figure 3.2.2) to avoid resonance in the range of the-wave

periods. Two major problems exist with this configuration. The

pendulum mass must be significantly larger than the panel mass

(or the panel mass must essentially be at the pivot point) which

results in increased material requirements including the

pendulum mass and the extra structural material required to

support its weight. The greater problem with this configuration

is the required length of the pendulum arm which is necessary to

attain a 25 second natural period (as described in Section 1.1).

Using the period equation for small oscillations of a simple

pendulum, T = 2 (L/g)l/2, the required length of the pendulum

arm is 508.8 feet (155.4 m.). This is too long to practically

design an above water mass as shown in Figure 3.2.3. For a deep

water location it may be possible to obtain the required length

by submerging the pendulum mass as shown in Figure 3.2.4. In

this configuration the wave and current forces on the arm and

mass will cause motion and stress in the panels.

A second conceptual design configuration is the double mass

pendulum concept which is illustrated in Figure 3.2.5. Both the

dynamic and static models are illustrated in Figure 3.2.6.

Static environment and dead loads are balanced considering the

ratios between upper and lower panel areas and masses. The

- 31 -
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- 32 -
ENVIRONMENTAL LOAD

INDUCED MOMENT (Mo)

FIGURE 3.2.2 SINGLE MASS PENDULUM MODEL

- 33 -
DIODES AND

GROUND PLANE

TAUT LINE

FIGURE 3.2.3 ABOVE M.W.L. SINGLE MASS PENDULUM SYSTEM

- 34 -
-TAUT LINE

DIODES AND GROUND

PLANE

FIGURE 3.2.4 SUBMERGED SINGLE MASS PENDULUM SYSTEM

- 35 -
- 36 -
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- 37 -
dynamic model is a double mass pendulum. By adjusting the top

and bottom panel mass, the pivot point, and the mass length

ratios, the desired natura periods can be attained.

Within this concept various configuration possibilities exist.

Figure 3.2.7 illustrates three of these possibilities. The first

two (a and b) represent the extreme situations which are felt to

encompass all others. The first configuration with the panels

vertically aligned has good mass and area distribution about the

pivot point. This results in less rotational sensitivity to

eccentric loads on the panels. A drawback in this design is the

large height required between panels which increases the support

tower height and its material requirements. The second

configuration with the panels horizontally aligned has

contrasting properties to the first. There is minimal height

between the panels (none) but there is poor mass and area

distribution about the pivot point (i.e. the increase in panel

moment arm about the pivot point makes the system sensitive to

loads inducing rotation). The third configuration is a

compromise between the first two with a significant installation

advantage. The top and bottom panel edges are aligned with the

microwave beam path which eliminates alignment concerns in the

direction of the panels along a row (alignment normal to the

panels must still be in this acceptable tolerances). This eases

installation providing substructural savings on installation

costs. Of the three designs, the third configuration

(illustrated in Figure 3.2.7c) is considered to have the

- 38 -
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greatest potential as far as performance and costs are concerned

and its design is further evaluated in Section 3.2.3.

A prestressed tautline system is the only cost effective way

found to support the panel arrangements considered because of

the large span between towers. Two primary tautline designs were

investigated. The principle difference in the tautline designs

(Figures 3.2.8 and 3.2.5) is the pivot point location.

Figure 3.2.5 shows the pivot point centrally located above the

lower panel. The panels are connected by cables to the main

tautline. This cable is in the microwave path. Thus it is

desirable to have a cable material which will not interfere

significantly with microwave reception. From a structural and

deployment standpoint, the central pivot point allows for large

distances between towers (which is critical for reducing costs)

because the cable alone supports the entire (tower to tower)

span.

Each panel connects directly to the tautline and supports only

the panel span. The deflection of the main tautline (which

influences tower clearance and dynamics), the installalion

practicalities and costs are the primary limiting factors of

this concept.

Figure 3.2.8 illustrates the tower pivot point location concept

for a single panel span. This arrangement uses a composite panel

- 40 -
- 41 -
tautline support system between towers to eliminate the need for

a cable system in the path of the microwaves. This reduces the

material cost of the tautline dramatically (since steel may be

used instead on an aramid fiber material). The dynamics of the

double mass pendulum with the pivot point on the towers are more

troublesome since the pendulum period changes with the movement

of the support towers. This side to side movement of the tower

(caused by waves) causes vertical motion of the panels due to

varying tension in the tautlines. In turn, the distance from the

center of mass of the panels to the pivot point (at the tower

tops) changes causing the natural period of the pendulum to

vary. This increases the dependency of the dynamic response of

the panel system on the environment. A second and perhaps more

significant problem is the panel span distance. The concept

works easily for a single span (40 meters) between towers with a

slight increase in structural material. When two or four panel

spans are considered, the increase in panel material is

substantial and uneconomical (e.g. an inefficient span truss).

For these reasons the center pivot point design was chosen to be

evaluated in greater detail.

3.2.2 Materials

Materials for various panel and tautline components were

selected with consideration given to their specific structural

function, performance in the offshore environment, availability

in usable form in great quantities, and unit cost. The unit

weight of the material will be considered only if a net cost

- 42 -
saving is realized. Environmental loadings such as wind and snow

govern structural design, not material weight (e.g. the towers

.can quite easily support vertical loads but horizontal forces

resulting from winds and wave cause bending in the structure and

large uneven soils loadings). There is no justification for

increasing panel and tautline material costs in order to

decrease structural dead weight. Structural materials considered

for the panels include fiberglass, aluminum, and steel. Future

studies should again examine possible uses of fiberglass as

structural members in the rectenna design, but considering

present availability, fabrication techniques, and at sea

experience, steel is the preferred material.

The member sizes required are large due to the unsupported

length of the panels. To reduce the panel size increases costs

significantly. Fiberglass efficiency as a compression member is

limited, which considering of the load reverals makes fiberglass

unsuitable for use as truss members.

Aluminum has performed well in the marine environment when it is

above the splash zone. One area of cost savings exists in the

area of corrosion protection. All aluminum alloys are protected

from corrosion by a thin, dense, inert film of aluminum oxide.

Thus, the painting of aluminum is unnecessary. Corrosion of

aluminum is a problem when other metals are used in combination

with it (e.g. steel washers and nuts used to connect aluminum

parts). This corrosion is twofold in that due to the contact of

- 43 -
the dissimilar metals, crevice corrosion (due to stress) and

electrical corrosion (due to the comparitive electrical

properties of the attached metals) will eat away the aluminum

locally. Corrosion resulting from this type of situation can be

extensive, especially if steel or copper bearing metals are

involved, and can lead to failure. Attention to details can

readily overcome problems of this nature. The dissimilar metal

couple should be avoided by using all aluminum materials or by

placing only passive materials such as stress corrosion

resistant stainless steels in contact with aluminum. The crevice

occurance can be avoided through careful design use of

non-wickup gaskets, and use of resilient sealants.

By using aluminum alloys for primary structural elements, a

weight savings of at least 50 percent can be realized over

steel. Yet the overriding problems with aluminum are its cost in

dollars and energy consumption (in production) and unavilability

of high strength alloys in quantity (if the need exists in the

design). As weight again does not seem to be critical, aluminum

will be considered used only for the ground plane and not for

structural members in the support structure.

Steel is the preferred structural material to use in the

fabrication of the panels. Steel is susceptable to corrosion in

the marine environment, but a protective coating of the

structure in the splash coupled with a state-of-the-art cathodic

protection system can achieve the desired 30 year design life.

- 44 -
During design, attention paid to details and elimination of

areas of possible water collection and ponding will aid in

corrosion protection. Steel is a heavy material (490 pcf or 7849

kg/m3), yet it can easily be managed during installation with

moderate present day lift capacities. Steel availability is good

with a proven offshore applications history. The present design

employs A36 steel but the weight can be reduced using various

high strength steels (e.g. if 50 ksi - 344 N/mm2 steel at a

10% cost increase is used more than 13% of steel by weight is

saved).

The preferred tautline material is steel cable for cost and

termination schemes presently available. This may not be

acceptable in areas exposed to the microwaves. In such cases,

other materials such as polyethylene, nylon and kevlar which are

less affected by the microwaves but are substantially more

expensive may be used.

Kevlar (an aramid fiber) can withstand the high tensile stress

in the line with the Tow deflection required (necessary for

tower clearance and avoiding dynamic resonance with sea waves).

Kevlar 29 (or Kevlar 49 which has a higher modulus) meets the

strength and elongation requirements and has been tested for

retention of its original properties when exposed to the marine

environment, ultra-violet rays, and fluctuating loads (fatigue

and creep overtime) and approaches the properties of steel in

some cases. Technology is proceeding in developing suitable

- 45 -
terminations for Kevlar cable but presently, abrasion resistance

(which is the basis for many steel cable terminations) is the

limiting factor. Kevlar is expensive (approximately $300 per

foot for the taut line strength required), even when projecting

reduced costs when manufacturing huge quantities.

3.2.3 Fabrication, Installation and Maintenance

Fabrication, installation and maintenance of receiver panel and

tautline systems is discussed in Sections 5.2 and 5.3.

3.2.4 Design Evaluation

The prime site receiver panel and tautline design were evaluated

in this section. Costs and support systems designs are based on

this rec.iever panel and tautline configuration. Refer to

Appendix B for all calculations concerning receiver panel and

tautline design.

3.2.4.1 Receiver Panel

Figure 3.2.5 illustrates the point design evaluated

for the prime site. The design winds are as follows:

Hurricane wind velocity = 110 mph (49.2 m/sec).

Winter storm wind velocity = 70 mph (31.3 m/sec).

The design assumptions include:

. Panel will resist normal and tangential wind forces

(due to panel makeup of small tubular members).

. Wind will act uniformly over panel area.

. Wind velocity remains constant with elevation.

- 46 -
. A 57% reduction in wind force, according to the DNV

code, will exist on interior panels due to the

shielding effects of the exterior panels (pertains

to tautline and tower design only).

. The panels are 65% opaque to winds during hurricane

conditions ad 100% opaque (iced over) during the

design winter storm.

. One panel area is 65.6 by 131.2 feet (20 by 40 m).

The design snow and ice loads are as follows:

Snow load = 13.6 psf (65.9 kg/m2).

Ice load = 2.4 psf (11.6 kg/m2).

Snow and ice loads are assumed to be uniformly

distributed over the entire panel (snow drifts are not

considered).

The design dead load of the receiving elements is:

Ground plane and diode dead load =4.6 psi

(22.3 kg/m2).

Panel structural members were sized considering

critical static loadings. The following assumptions

were used in sizing members:

. Members are of A36 or A572 GR.50 steel

- 47 -
. Dynamic mass ratios and the pivot point location

are not taken into account (panel period tuning

does not effect panel structural make-up

appreciably).

. Installation and transportation loads were not

considered critical for the conceptual designs

performed but would have to be considered in a

final design.

The members were sized using conservative assumptions

without taking into account the panel structure acting

as a system to redistribute stresses. Figure 3.2.9 and

Table 3.2.1 show the final results of this analysis.

For all analyses a 90,000 pound (400 KN) panel weight

was assumed. As shown in Table 3.2.1, a weight of

114,211 pounds (508 KN) was in fact attained after the

member analysis was performed. This weight difference

is not critical because of the conservatism employed

in the analysis (if a computer analysis of the

structure were performed the structural weight would

probably reduce to nearly 90000 pounds - 400 kN) and

the general insensitivity of design concerning panel

weight.

PANEL DYNAMIC ANALYSIS

The panel configuration, consisting of one or two

panels per tower, will act dynamically as a double

- 48 -
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- 50 -
mass pendulum. For a larger panel to tower ratio a

more detailed examination of the tautline slope effect

on the panel (integral to its performance as a

pendulum) pivot point location should be undertaken. A

natural period of at least 25 seconds must- be attained

in the design of this system in order to minimize

movement induced by wave action. Only the first mode

of vibration has been considered. Assumptions employed

in the panel dynamic analysis are:

. All mass is lumped at the center of mass of the

panels.

. Panels are rigidly connected to the pivot point.

Pivot point is free to rotate with no damping.

The natural period of the receiver panel structure

dictates the distance between top and bottom panel

parts, the pivot point location, and the panel mass

ratios (top to bottom). Figure 3.2.10a illustrates the

relationship between the pivot point to panel center

of mass distance and the panel mass ratios. The choice

of 1.03 as the LB/LT and MB/MT ratios gives a design

and fabrication tolerance allowance on either side,

while retaining a reasonable distance between top and

bottom panels. It will be possible to actually tune

the panel response after fabrication, if necessary, by

adjusting the panel masses. Figure 3.2.10b illustrates

this mass versus period relationships.

- 51 -
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- 52 -
The pivot point location is critical to static

equlibrium and to the dynamic response. It is a

function of the upper and lower panel ratios of area,

mass, and distance to panel center of mass as shown in

Figure 3.2.11. The panels must be in static

equilibrium under the assumed loadings and the dynamic

response must be within acceptable bounds of reception

performance and materials limitations. Area loadings

(wind, snow, and ice) are applied at the geometric and

center of panels. While balancing these loads, the

panel masses (or dead weight) must be balanced yet the

center of mass can be adjusted to act at any necessary

location on the panel. This will be done by adjusting

the structural steel location (i.e. top and bottom

panels structural member layout will not be identical)

or by adding small masses (e.g. concrete weights). In

taking all these variables into account, Figure 3.2.12

shows the pivot point location in the design being

considered.

3.2.4.2 Tautline .

The static and dynamic models for the single panel

span tautline are shown in Figure 3.2.13. Tautline

tensile strength, deflection, and dynamic response

requirements were determined. It was concluded that a

9 inch (0.23 m) diameter Kevlar cable would be

required. This "includes a factor of safety of 4 which

- 53 -
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- 55 -
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is typical of offshore practices. A 1,000,000 pounds

(4448 KM) prestress load would be needed to keep the

deflection within the allowed tolerance necessary to

clear the support structure during winter conditions

(snow and ice loads). Dynamically, the tautline has a

low enough period (0.69 seconds) so as not to resonate

with wave induced oscillations.

When considering multiple panel spans the relationship

between panel and tautline stiffness becomes critical.

In order for the tautline to accept loads it must

deflect. The panel would start to take the loads if

the tautline is constrained from deflecting by it. In

order to minimize the chance of this occuring, the

panels could be built in the deflected shape of the

loaded tautline. When snow and ice loads are applied,

both panels and tautline must be considered as a

single system and analyzed, but such an

analysis is beyond the scope of this study.

- 57 -
4.

4.1

PRELIMINARY COST ESTIf-'ATES

Preliminary cost estimates versus type of support systems are

surMiarizec! in Table 4.1. In the panel receiver configuration there are

100,000 panels to support. A submerged buoyant platform carries two

panels end the other platform types are designed to support four panels

each. The total material and fabrication costs in Table 4.1 do not

include the costs of the panels or .cables.

Table 4.1

Preliminary Costs For Supporting Systems

Supporting System

SulM.'.ercju'd Buoy^t •

Platform

Piled Structure

Piled Guyed To'.-sr

with Panels

Gravity Structure

Piled Guyed Tcv-'er

with Image Dipolas

Material Fabrication

Cost per Platform

$

496,000

1,300,000

348,000

564,100

200,000

Total Material

Fabrication

Cost for all

Platform:, $x!09

24.8

32.5

8.7

14.10

0.6

- 58 -
4.2 Cost Versus Water Depth

The 162 feet water depth at the prime site was considered as a

potential cost driver of support systems. The effect of water depth was

studied on the piled guyed tower with panels design. Figure 4.2.1 shows

the variation in cost per guyed tower structure for water depths of

162, 150, 100 and 75 feet (49 m., 45 m., 31m., 23 m.).

There is a linear relationship between the cost and the water depth.

This is due to the use of guy cables in the system. Guy cables, by

greatly reducing the lateral load effects on the structure, change the

normal exponential increase in cost due to water depth, to a linear one.

4.3 Cost Versus Wind Load

The effect of wind load variation was investigated for the piled guyed

tower design. Since all wind forces are transferred by guy cables to

the piles of the neighboring platforms, the jacket structure is not

significantly effected by wind load variation. The jacket is subject to

the vertical component of the cable forces. These combined with panel

weights, dead weights and overturning moment reactions due to waves

determine the design axial load on the jacket. The variation of cable

forces does not significantly alter the total axial forcer

- 59 -
$400,000 •

300,000 -

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$200,000

$ 100,000

1

50 100 150 200

WATER DEPTH

FIGURE 4.2-1 COST VS WATER DEPTH FOR PILED GUYED

TOWER WITH PANELS

- 60 -
When the pretensioning force In cables are reduced along with the wind

forces, the pile loading changes. The pull-out load in the piles may

change from 754 kips (3427 kN) to 130 kips (591 kN) corresponding to a

75* reduction in wind loads. This does not affect pile design

appreciably since in this case the design load for the piles become the

winter storm compressive loads. Variation in pile pull-out 'loads and

pile stresses corresponding to reductions in wind and pretensioning

loads is presented in Table 4.2. It may thus be concluded that

reduction in wind load does not effect the cost of a piled guyed tower

to any significant degree. For a fixed platform or a gravity platform,

however, variations in wind loading may effect costs considerably.

Table 4.2

Pile Stresses Versus Wind Loads

Total Wind Load

(kips) (kN)

238 (1082)

212 ( 964)

184 ( 836)

156 ( 709)

Pretensioning Force

(kips) (kN)

750 (3409)

562.5 (2557)

375 (1705)

265 (1205)

Pull-out Load

(kips) (kN)

754 (3427)

522 (2373)

287 (1305)

130 ( 591)

Stresses in Piles

(ksi) (N/mm2)

24.77 (174.5)

21.56 (151.9)

18.30 (128.9)

16.10 (113.4)

- 61 -
4.4 Cost Versus Soil Conditions

In all the piled systems considered (Sections 3.1.1.2, 3.1.1.3, and

3.1.1.5) the cost of piling constitutes only 7-10% of the total

material and fabrication costs. Thus, variations in the soil conditions

which will effect the piling can not influence their costs to any

significant degree.

Gravity structures on the other hand are greatly affected by soil

conditions. The size of the gravity base is dictated by soil bearing

capacity. Table 4.3 gives the cost of a guyed gravity structure for two

different types of soil. The size of the base is reduced by 1/2 for

favorable soil conditions which corresponds to a 17% reduction in costs.

Table 4.3

Cost Versus Soil Conditions for a Gravity Structure

Type of Soil

Weak Soil

°b = 0.55

(^ = 2.69 t/m2)

Medium Strength Soil

0'. _ -j ^Q kip/ft2

(°b = 5.38 Vm2)

Cost per

Structure

565,000

470,000

The relationship between the base size and soil conditions is not

linear. The total weight of the structure and overturning moments are

additional significant parameter - 62 -
4.5 Total Costs

Cost estimates including all materials, fabrication, deployment and

installation were made for piled guyed tower designs. Details for

costing the piled guyed tower with panels and the tower .with image

dipoles are given in Sections 5.4 and 6.4. Total costs for these

configurations are summarized in Table 4.4. It should be noted that the

panel receiver configuration involves 25,000 towers, whereas image

dipole receivers require a total of only 3000 supporting structures.

Table 4.4

Total Costs for Piled Guyed Tower Structures

Structure Type

Piled Guyed Tower

Receiver Type

Panel

Image Dipoles

Total Cost in fcxlO9

36.30

5.69

5. PRIME SITE POINT DESIGN

5.1 Structural Configuration

The piled guyed tower as a support system is discussed in Section 3.1.1.3.

The analysis of the guyed tower for the prime site was made for two major

loading conditions; the 100 year hurricane loading and the winter storm

loading. A BARDI proprietary computer program was used in the analysis and

design check. Maximum bending stresses in the tower were under 25,000 psi

(176 N/mm2) which is below the allowable stresses for A=36 steel for 100

year storm conditions.

- 63 -
The weakest soil conditions at the site were assumed to be the

prevailing soils properties. This meant that the piles had little

pull-out resistance for the first 55 feet (16.8 m) of penetration. The

piles designed for 112 feet (34.1 m) penetration were required.

Computer analysis revealed that the structure behaved as predicted in

the preliminary design. The largest deflection was 6.2 inches (15.7 cm)

occur ing at 27.5 feet (8.37 m) below the mean sea level. Thus the. tower

was acting as a beam supported by piles at one end and guy cables at

the other. Figure 5.1.1 illustrates the prime site piled guyed tower

structure in detail.

5.2 Fabrication Installation and Deployment

5.2.1 Support Systems

The following sequence describes the assumed order of

activities, for the rectenna subsystems.

. Initial fabrication

. Transportation to staging port

. Final assembly at staging port

. Deployment to field site

. Installation at field site

The study includes consideration of the following components:

. Support pile guyed towers

. Permanent and temporary guy lines

. Pilings

. Equipment required for transportation and deployment

- 64 -
SECTION A-A

S

\

16** 0.500" W.T.

26** 0.500" W.T.

to

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00

24'Vx 1.00 "W.T.

PILES

-162'

(-49.4m.)

(6.1m.)

FIGURE 5.1 .1 PRIME SITE GUYED TOWER STRUCTURE

- 65 -
5.2.1.1 Initial Fabrication

The United States offers ample suitable sites for the

fabrication of the Piled Support Towers. Fabrication

will not involve problems in view of the simple design

and material specifications. Transportation problems

for tower components are eased by using none longer

than 50 feet (15 m). Suitable joining arrangements can

ensure effective interfacing of the tower components

at the staging port.

Guy lines are most likely to be constructed of wire

cable. Typical construction was considered to be:

. 6 strands with 37 individual wires per strand

. Independent wire rope center core (IWRC)

. High strength galvanized steel (180 ksi - 1268

N/mm2)

The permanent guys attaching the columns to the seafloor, or outside ends of each row or at any junction

in the overall pattern, will be 4 inches (10 cm) in

diameter. The permanent guys attaching the adjacent

column top to piled feet in the channel lines will be

3 inches (7.5 cm) in diameter. The temporary guys,

where required, wil l be 4 inches (10 cm) in diameter.

All guys will be fabricated in predesigned

predetermined lengths with special reinforced hard

eyes, machine spliced in each end to facilitate

connection. Wires of the above mentioned construction

and diameter are presently only available as special

order items and are very expensive. Only a few

manufacturers are able to meet these specifications.

- 66 -
Pre-tensioning of guy lines prior to installation wil l

ensure that design tensions are obtained without

further tensioning.

Due to the high vertical loadings to be imposed on guy

line anchors, piled moorings are used for the prime

site point design. Other possibilities include

displacement anchors and a combination pile/fluke type

anchor which is presently under development. Careful

consideration must be given to deployment methods for

any permanent guy anchors due to the large quantity to

be employed.

The piles for the support towers are 24 inches (61 cm)

in diameter. Fabrication is common state-of-the-art

and many steel manufacturers can meet the overall

requirements of the project.

In the point design BARDI has required that all.links,

swivels and manual tensioning devices (e.g. turn

buckles) have a safety factor of five to one. This is

common practice within the offshore industry and

fabrication will involve forged alloy steel

construction. Many fabricators of this equipment can

comply. The tensioning devices are of simple

construction to facilitate tensioning operations

- 67 -
after an extended period of service. One preferred

tensioning concept is a hydraulic cylinder arrangement

that locks in several positions to bleed off pressure

after pretensioning. By reconnecting the hydraulic

cylinder arrangement, retensioning is accomplished.

The tensioning device will be big enough to allow for

tolerances in the guy line manufacturing to ensure

that design tensions are achieved.

5,2.1.2 Transportation to Staging Port

Several ports along the coast in close proximity to the

field site of the rectenna will be assigned as staging

ports for the collection, final assembly, loading and

dispatch of the components to the field site.

Due to the relatively large areas required for the

handling and assembly of the components at the staging

port, it will be necessary to develop areas

specifically for the task. The overall planning wil l

include provision for continuous production of

components by manufacturers, to meet field installation requirements. Overstorage of large dimension

components at staging port areas must be avoided. In

this way storage area requirements (where space is

probably at a premium) can be minimized. Sufficient

storage margin will be maintained to allow for a

nominal shut down period in the field while fabrication

and transportation of components is continued.

- 68 -
Transportation of components to staging port can be

anticipated as follows:

. Tower sections - by coastal or inland barges. By

truck or rail with some development of

transportation and arterial road and rail systems.

. Other items - by rail or truck with no extension

within the state of the art. Guy lines wil l be

stowed on specially designed re-usable storage

reels. A 40 foot (12.2 m) maximum pile segment

length would ease transportation problems but 60

foot (18.3 m) segments are transportable.

5.2.1.3 Staging Port Assembly of Support Towers

The staging ports near the rectenna site wil l provide

convenient locations for the final assembly of the

components of the support towers thus alleviating

transportation difficulties. A staging port, equipped

with a network of overhead gantry cranes can handle

the large components of the towers. Final assembly

wil l include welding the large components together and

installing anodes or other anti-corrosion measures.

Special cranes can lift the towers onto the

transportation vessels. Proper design of the staging

area wil l assure a smooth flow of the components to

the rectenna site. Warehouses can protect small

components prior to assembly or transportation to the

- 69 -
rectenna. Figure 5.2.2 illustrates a possible layout

of a staging port. Roads, railroads and access from

the sea will afford the necessary transportation to

the staging area to maintain the supply of components

and required materials.

5.2.1.4 Deployment at Field Site

Purpose built vessels can be used to carry the rectenna

components from the staging port to the field site. The

size of the components and the need for minimizing the

installation in the field wil l dictate the requirement

for special vessels. These vessels should have open,

clean decks for facilitating unloading operations at

field site. Such a vessel could nominally carry ten

complete towers at a time. Figure 5.2.3 shows one such

vessel unloading at the jack-up tower installation

barge. Design criteria for these vessels will include

bow and stern thrusters, and fixed propulsion in order

to facilitate maneuvering during loading and unloading

operations in the limited spaces between towers.

Another alternative includes the use of special

vessels with the propulsion and living section

separate from the cargo section. Thus, procurement of

fewer "power" sections is necessary and the turn

around time for the cargo sections is minimized.

- 70 -
The "power" sections can work on other tasks

(returning empty cargo sections) while unloading or

loading operations proceed. Soviet ocean and river

timber trade employs a development of this kind of

equipment.

A third option is to use regular "oil field" type

supply vessels, equipped with thrusters for versatile

maneuvering and with open decks for facilitating

loading and unloading operations. They could

accomplish the transportation of other components

(e.g. guys, swivels, connecting links) to field site.

5.2.1.5 Field Installation of Support Systems

The installation of the towers at field site provides

certain difficulties. There are listed as follows:

. Requirements for high accuracy during tower

positioning

. Proximity of tower spacing

. Installation of a high volume of structures

. Consideration of overall time and costs, -

In our preliminary selection of the best method for

installation of the prime site point design, all of

the above points were considered by BARDI.

Environmental conditions at the prime site are a

difficulty which must be taken into account. Wind, sea

- 71 -
height and storm frequency are hinderances which will

cause delays during deployment and installation.

Conventional state-of-the-art equipment can operate in

hostile areas of the world on a limited basis. For

instance in the North Sea the maximum available

working time on an annual average is no more than 30%

of the time. The Prime Site will provide hostile

weather conditions. Working limitations and increased

time and costs will impact any final selection of a

rectenna location.

For the purpose of this study, the primary criterion

for evaluation of potential deployment and installation

methods and equipment (including purpose built

equipment) was that it must meet the above listed

points in a practical manner. The criterion was not

that it solve the problems caused by environmental

conditions.

The installation plans for the structure were

developed considering the following equipment:

. Conventional barges

. Conventional semi-submersibles

. Conventional jack-up units

. Purpose built or converted barges

. Purpose built semi-submersibles

. Purpose built jack-up units

- 72 -
Although some conventional equipment might be

successfully employed to install the rectenna

components, their high costs particularly due to

anticipated long installation times preclude

recommending them for this work. Availability of

sufficient conventional units to complete the task in

a timely, cost effective manner is unlikely. Daily

hire costs for such equipment is high so the capital

expenditure for the design and fabrication of

specially built units to meet the listed criteria is

not only justified, but a requirement for economic

project feasibility.

In the determination of the type of unit to be used

for tower installation at the prime site, a purpose

built jack-up barge was selected for the following

reasons:

. Jack-up operations are feasible at the prime site

(water depth approx. 150 feet - 45.7m.)

. Positioning on site with thrusters, tug assistance

and accoustic positioning equipment, is feasible to

within the small tolerances required.

. Proximity of towers at prime site prohibits the use

of conventional moorings during installation.

- 73 -
. Absence of dynamic forces in the barge in its

elevated condition and the use of overhead gantry

cranes, both facilitates the positioning and piling

of towers at both ends of the barge.

. Movement from one tower row to the next row can be

facilitated by raising legs off seabed and moving

the barge with thrusters and tugs.

. On site unloading of towers from supply vessels is

possible.

Moored semi-submersibles, although having certain

motion advantages over floating jack-up units (but not

jacked-up barges), require complex mooring systems to

eliminate high excursions. Tower positioning requires

minimal excursion of the deployment platform and

mooring of a semi-submersible would be difficult

within the complex maze of towers.

The patented "Slo-Rol" system can increase the

operational capabilities of jack-up barges. Testing to

date on this system (both model and full size)

indicates that the capabilities of jack-up units when

operating in marginal sea conditions, will

substantially increase (by about 100%).

It is estimated that one jack-up barge can install and

pile two towers per day (one at each end of the barge)

once initial problems are ironed out. Thus, the

following deductions are made:

- 74 -
. 20 barges can install 25,000 towers in

approximately 1.7 years (no weather down time)

. Allowing for problems installation may take 2 years.

. When accounting for the anticipated hostile weather

conditions at the prime site of this study actual

installation may take a minimum of 4 years.

Figures 5.2.1, 5.2.3, 5.2.4 and 5.2.5 show details of

installation of towers at a field site.

While the jack-up barge is accomplishing the tower

installation, it can be used to make the connection of

guy lines between adjacent columns. Divers and diving

support facilities wil l set any required underwater

guy connections but where feasible, such connections

will be made prior to subsea placement of components

since divers and their support facilities are rather

expensive. Both jack-up barges and diving tender

vessels will be leased on a long term basis. Piling

vessels or barges will set and pile permanent guy

anchors and should be leased on a long term basis.

Hydraulically operated linear winches will tension all

guys as required. Because of the high cost of long

term usage, these winches should be purchased.

- 75 -
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- 79 -

5.2.2 Receiver Panel and Taut Line System

For the purpose of this study, the following sequence is

employed in the formulation of plans:

Initial fabrication

. Transportation to staging port

. Final assembly at staging port

. Deployment to field site

. Installation at field site

The components of the receiver panel and the taught lines which

wil l be considered in the following sections include steel truss

members, aluminium ground planes, steel or Kevlar taut tension

lines, and the equipment required for transportation and

deployment.

5.2.2.1 Initial Fabrication

Many steel prefabrication yards and workshops

throughout the United States are capable of producing

steel truss members on short notice and at competitive

prices. Components of the trusses consist of tubular

and box girder members manufactured from A36 or A572

steel. Discussions relating to the use of steel over

other materials are included in Section 3.2.2.

Aluminium groundplane screens will not provide

problems in fabrication. Many companies which

manufacture a high volume of aluminium products are

- 81 -
capable of meeting design and delivery specifications

at competitive prices. The design must include

effective corrosion prevention measures.

Design and manufacturers capabilities will govern the

selection of tension line materials which in turn will

determine the appropriate installation methods. Kevlar

is still in the development stage and its costs are"

high. Kevlar does not have good abrasion resistance so

handling it is currently difficult and uncertain.

During the next ten years, development may be expected

to take place which will reduce costs and handling

difficulties. The use of wire rope evokes problems

with mi crowave reception if the lines pass above a

section of the receiver panel (as in the case of the

point design). Cost factors may warrant the use of

wire over kevlar. Present cost comparisons of lines

strong enough to carry the design loads shows wire

cable with a substantial advantage:

. Kevlar (9 inch -23 cm diameter) is quoted at

$300/foot. 2500 miles (4500 km) are required at a

total approximate cost of $4.5 x 10^.

. Wire (5 inch -12.7 cm diameter) is estimated at

$90/foot. The total cost of wire is $1.35 x 10q.

BARDI recommends further development of this aspect of

the rectenna project.

- 82 -
5.2.2.2 Transportation to Staging Port

Referring to Section 5.2.1.2 in which the question of

staging ports is discussed, the same criteria will

apply for the collection, final assembly, loading and

dispatch of the components of the panel and taut }ine

systems. Transportation of components to the staging

port which wil l ensure continuous loading and dispatch

to the field site, is accomplished by road, rail or

sea. Special transporters will overcome difficulties

in the shipment of the trusses.

5.2.2.3 Final Assembly at Staging Port

The selected staging ports will include suitable areas

for the assembly of the panel sections. Completion of

assembly at the staging port is carried out because:

. Large dimensions of completed panel units

(approximately 131' x 65' x 28' -40 m x 20 m x 8.5

m) inhibit their transportation by road or rail to

the staging port.

. Since the completed panel units are relatively

fragile, handling (lifting, loading, and eventually

installing) wil l be somewhat complex and should be

avoided when possible.

A network of overhead gantry cranes will facilitate

the movement and loading of panel units onto barges.

Figure 5.2.2 shows a possible plan lay out of a

staging port area and the assembly and loading of

panel units.

- 83 -
5.2.2.4 Field Deployment and Installation

The selection of special purpose built jack-up barges

for the deployment and installation of panel sections

at field site is made for the following reasons:

. A one time lift of panel units onto a barge will

allow final joining and securing of completed units

onboard.

. The complete accessible panel units will permit the

taut line sections to be threaded and secured to

swivel unions on the barge, leaving the end

connection eyes clear for easy connection to

tensioning wires.

. Use of the barge for transportation of panel units

and the jacking units for pinning and raising the

barge will facilitate the positioning of the

complete panel sections between towers.

. Dynamic forces caused by vessel excursion are

eliminated.

. Panel unit taut line connections can be made

without lifting or moving operations.

. Tensioning of taut line and lowering of the barge

will allow weight of units to be taken by the taut

line in a controlled manner.

Immediate return of barge to staging port will

permit access by the next panel barge.

- 84 -
. Such a work pattern for the barges will provide a

relatively continuous production of the panel unit

installation after the towers have been installed.

The limitations of jack-up barge operations in

marginal sea conditions, as discussed in Section

5.2.1.5 applies to jack-up panel installation barges.

The patented "Slo-Rol" system specifically designed

for installation on such barges can minimize down time

due to weather condition.

It is estimated that one jack-up panel barge could

install a set of panels between one pair of towers in

six hours. The same barge can return to port (six

hours), load next panel section (six hours) and return

to site in a total of 18 hours. Forty barges can

install the total estimated 24,000 panel sets in under

two years.

The setting of panel sections over the tops of towers

provides problems with lifting and access. Helicopters

provide a possible solution. A converted conventional

semi-submersible drilling unit could suffice as a

storage unit and flight base for panel sections and

helicopters. Supply vessels could provide transport to

satisfy requirements for continuous installation

operations of tower top panels from a staging port. It

- 85 -
is estimated that one helicopter could install 12

panels a day (during daylight hours only). Therefore 5

helicopters could install 24,000 panel sections in

less than two years. Figures 5.2.6 through 5.2.11 show

installation steps of panel sections.

5.3 Maintenance

Design criteria for the offshore rectenna will permit the operation of

the system over a 30 year life span with minimal maintenance. Cathodic

protection schemes must include consideration of sea currents, salinity

and sea temperature to provide the electrical current density to meet

the design requirements. Aluminium ground screens must incorporate

effective measures for the prevention of electrolytic action. Anticorrosion measures for piled towers will include special coatings

especially in the splash zone. Careful monitoring of corrosion through

instrumentation will permit maximum life span of the components.

Certain components will require replacement in less than the 30 year

design life. Wire cable which is presently available wil l not provide

service for longer than 10 years. Large diameter cable corrosion

factors in different environmental conditions over long periods are not

fully understood and require further investigation. (Activity in this

area should be undergoing rapid development in the next few years as

the offshore oil industry pursues the development of guy lines for

guyed towers for deep water applications.) Galvanization and special

sheathings under development might retard corrosion and increase cable

- 86 -
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- 88 -
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life span but these items may prove to be uneconomical. Periodic

change-out of cable is therefore presently envisaged. Accomplishment of

taut line and guy wire cable change-out could involve the use of the

following equipment:

. Diving support capability (for underwater guy lines)

Installation of temporary guys to support loads

. Use of jack-up panel installation barges to support panels

. Auxiliary vessel support

Kevlar taut lines (if used) may require replacement due to chafing

damage. Complete change-out of permanent guy anchors and support towers

could involve the use of special maintenance equipment including:

. Diving support capability

. Use of jack-up piling and tower installation barges

. Installation of temporary anchors and guys to support loads

. Auxiliary vessel support

Maintenance of the offshore rectenna will require the formation of a

program which will include the following main points:

. What equipment will be used for the program and how can it be

obtained.

. What personnel will be required for the maintenance program

. Inspection and maintenance frequency

. Post casualty contingency plans

. Overall budgeting

5.4 Costs

Prime site point design costs are summarized in Table 5.1. Cost details

are itemized in Table 5.2.

- 93 -
Table 5.1

Cost Summary for Prime Site Point Design

Design Subsystem Cost in $x!09

Piled tower and support systems

Panel and taut wire systems

Miscellaneous

Combined total point design for

prime site

11.6

23.6

1.3

36.5

Table 5.2

Itemized Costs for Prime Site Point Design

Design Subsystem Description Cost in

IxlO9

Piled tower

and support

systems

Material, labor and

fabrication costs

($3.2x!05/tower x 25,000 towers)

Material fabrication of

jack-up barges for

tower installation

($5.0xl07/barge- x 20 barges)

8.0

1.0

- 94 -
Table 5.2 (Continued)

Itemized Costs for Prime Site Point Design

Design Subsystem Description Cost in

Piled tower

and support

systems

Material, Fabrication of

miscellaneous hardware

(guys, connecting links,

linear winches, guy anchors

or pilings)

Leasing of diving, piling

vessels and associated .

equipment

($5.0xl04/day x730 days x 10

vessels)

Operation of jack-up barges

setting towers including piling,

mobilization and fuel

($6.8xl04day x 730 days x 20 barges)

1.2

0.4

1.0

Total for piled tower and support systems 11.60

- 95 -
Table 5.2 (Continued)

Itemized Costs for Prime Site Point Design

Design Subsystem Description Cost in

Panel and

taut wire

systems

Panel and

taut wire

systems

Panels, material, labor and

fabrication costs

($1.4x!OVpanel x 107,000 panels

Material, fabrication of jack-up

barges for panel installation

($5.0xl07/barge x 40 barges)

Operation of jack-up barges

installing panel sections including

fuel costs

($6.8xl04/day x 730 days x 40 barges)

15.0

2.0

2.0

Tower top panel installation (using

helicopters)

-semi submersible barge rental

$3.0xl04/day x 730 days

-two supply vessels

$1.0xl04/day x 730 days x 2 vessels

-helicopters

$2.0xl03/hr. x 43,800 hrs x 5 helicopters

0.1

- 96 -
Table 5.2 (Continued)

Itemized Costs for Prime Site Point Design

Design Subsystem Description Cost in

$x!09

Miscellaneous

Miscellaneous

Taut lines

-9 in. (23 cm) diameter Kevlar

$300/ft x 15xl06 ft.

-or-

-5 in. (12.7 cm) diameter wire cable

$80/ft x 15xl06

Total for panel and taut wire systems

Auxiliary supply vessels/tugs

for supplying and moving jack-up

barges

($1.0xl04/day x 730 days x 20 vessels)

Staging port alteration costs

including installation of gantry

cranes

- 3 ports -

4.5

23.6

0.2

1.0

- 97 -
Table 5.2 (Continued)

Itemized Costs for Prime Site Point Design

Design Subsystem Description Cost in

Miscellaneous

Total

Staging port operational costs

inclusive of labor

($2.0xlQ4/day x 730 days x 3 ports)

Transportation of components

and other miscellaneous costs

Total Miscellaneous Costs

0.04

0.1

1.3

36.5

- 98 -
6. PREFERRED DESIGN

6.1 Structural Configuration

The preferred design configuration was conceptualized after the study

on the prime site point design indicated that problems exist. These

problems include:

4

. Complicated and lenghty deployment procedures

. High overall costs

Consideration was therefore given to a configuration that is:

. Efficient in its generation of electrical power

. Light enough to reduce the volume of supporting towers

. Relatively durable

. Comparatively inexpensive to manufacture

The preferred design involves the use of light weight dipoles

encapsulated in synthetic material to form a module measuring 8 cm by 8

cm. Each dipole uses support rods around a horizontal axis which act

both as a means to hold the module in position at the correct angle and

as a means of conducting generated electrical power to the main artery

of support cables.

A network of the dipoles measuring 100 feet (30.5m) by 100 feet (30.5m)

(or as deemed to be the most efficient from a cost and deployment

viewpoint) forms a unit. These units of dipoles are supported by a

network of criss crossed wires which are attached to perimeter wires.

These wires are supported between a 1000 (305m) feet square of towers.

- 99 -
The support towers have a similar configuration as used in the prime

site point design and as described in Section 5.1. The reduced design

loads permit smaller steel sections to be used. At 3000 foot (915 m)

intervals, the tower configuration is broken and a 100 foot (30.5 m)

wide channel is introduced. This channel permits extra guying from top

to base of adjacent towers to be installed, "thus providing the

necessary tower support for the network. The channels also permit

access. This is illustrated in Figure 6.1.1 and Figure 6.1.2.

Figures 6.1.3, 6.1.4, and 6.1.5 show the configuration of the dipoles

and how they are supported. The use of permanent guys and piled guy

anchors on the perimeter of the rectenna will give additional support

to the tower pilings.

6.2 Fabrication Deployment and Installation

For the purpose of this study, the following sequence is used for the

components of the preferred design:

. Initial fabrication

. Transportation and assembly at staging port

. Collection and assembly at staging port

. Deployment to field

. Installation at field site

- 100 -

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1000' ,

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^->_ ^TOWERS DOUBLE SPACED AND GUYED

FROM TOP TO FOOTING.

FIGURE 6.1.2 CONFIGURATION OF SUPPORT TOWERS AND

PERIMETER TAUT LINE CABLES IN PREFERRED

DESIGN.

- 102 -
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- 104 -
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- 105 -
The following components are considered:

. Piled support towers

. Permanent and temporary guy wires

. Guy wire anchors

. Pilings

. Taut wire lines

.. Encapsulated dipole networks

. Equipment required for transportation and deployment

6.2.1 Initial Fabrication

Reference is made to Section 5.2.1.1 in which fabrication of the

piled support towers for the prime site design is discussed.

Design loadings for the preferred design will allow smaller

dimensions of the tower components. The towers need only be 200

feet (61m) long instead of 300 feet (91.5m), but 50 foot (15.3m)

long components would ease transportation to staging port.

The preferred design concept allows guy lines to be of similar

construction, materials and dimensions as for the prime site

point design. It is discussed in Section 5.2.1.1.

.The design loadings will permit pilings of 20 inches (51 cm) in

diameter to be used for securing support towers. Fabrication

provides no problems.. Pilings produced in 40 foot (12.2 m)

lengths would ease transportation.

- 106 -
Wire cables can be used exclusively in the taut line network.

The perimeter lines are of similar construction to guy lines.

They will be 4 inches (10 cm) in diameter while the network of

criss-cross lines only 1/2 inch (1.2 cm) in diameter. The

perimeter lines need not be longer than 1000 feet (305 m) which

provides no problems with fabrication. The use of galvanization

in the fabrication of the taut line network would probably deter

corrosion and prolong service life.

The fabrication of the encapsulated dipole networks wil l provide

the greatest problem in the fabrication of the preferred design.

The scheme allows networks of these dipoles (measuring 100 feet

by 100 feet -30.5m by 30.5m) to be supported between the network

of taut lines. The dipoles will incorporate a central axis which

will act as support and means for conduction of electrical

current to the network which in turn wil l conduct the generated

power of all dipoles to collection points. Small diameter wires

which support the dipoles can also serve as conductors. Small

clamps wil l attach the central axis of the dipoles to the

support wires, ensuring that the 45° reception angle of the

dipole is maintained. The fabrication and encapsulation of the

electronic components of the dipoles must be planned with the

following considerations:

. Low unit fabrication costs

. Low unit encapsulation costs

. Speed in performing fabrication

- 107 -
Purpose built machines for encapsulating the dipoles will

connect the central axis to the wire networks. Units packaged in

100 foot by 100 foot (30 m by 30 m) sections for transportation

to field site are envisaged. Although it is considered that the

methodology and technology for achieving the above mentioned

functions is well within the state-of-the-art, further research

and development into the details of materials and fabrication

methods will enable accurate costing and production planning to

be formulated.

The remarks as included in Section 5.2.1.1 of the prime site

point design will apply to the miscellaneous components of the

system. The requirement for lower pretensioning in perimeter

taut lines will effect the size and power requirements of linear

winches thus reducing costs (from those of the point design).

Manually operated "come along" tensioning devices (as employed

for tensioning the smaller criss-cross wires) wil l be easy and

inexpensive to manufacture.

6.2.2 Component Transportation and Assembly

The assignment of staging ports in the proximity of the field

site wil l be necessary for the preferred design (as with the

prime site point design). The transportation and assembly of

components at staging ports will follow similar methods to those

described in Section 5.2 with the following main differences:

- 108 -
. Support towers of smaller dimensions will require smaller

areas for assembly

. The elimination of the large, unwieldy panel section will

reduce the overall assembly area requirements and the need

for the costly network of overhead gantry cranes. This in

turn will reduce overall staging port development and

operating costs.

. The smaller dimensions of the major components will facilitate

transportation and deployment as well as reducing costs.

Encapsulated dipole networks, carefully stowed in re-usable wire

taut line and guy line reels, are expected to further reduce

costs (from those of the point design).

6.2.3 Deployment and Installation at Field Site

The overall design changes of the preferred design over the prime

site point design will minimize the requirements for purpose

built transportation vessels to field site. Conventionally

designed supply vessels will meet the main requirements, although

tower transportation will be facilitated by using vessels as

described in Section 5.2.1.4.

For installation of the preferred design at field site, the use

of semi-submersible and jack-up units is possible. Jack-up units

can set and pile towers at both ends in the channels between the

1000 foot by 1000 foot (305m by 305m) tower configurations. This

is described in detail in Section 5.2.1.5 and shown in Figures

5.2.3 and 5.2.4. The requirement for less tower installation and

reduction in tower dimensions will reduce the overall dimensions

and therefore building costs of the jack-up units.

- 109 -
The use of semi-submersible units for the installation of

encapsulated dipole networks is possible for the following

reasons:

. Increased tower spacing will allow barge moorings to be

employed

. Reduced weights in components will permit components to be

carried and deployed without stability problems (which are

common on semi-submersible units)

. Reduced height of towers will allow access to taut line

network from underneath without the necessity of raising the

unit

. Configuration of encapsulated dipole networks will allow

installation without completely eliminating excursions.

The advantages of using semi-submersibles for the installation

of the dipole networks are:

. Easier and faster deployment. Spread moorings will permit an

installation unit to move as required while setting the

dipoles. Work boats can reset anchors as required (similarly

to a conventional pipe leg barge) thus allowing the

continuous installation of networks over the rectenna as the

towers and taut lines are installed.

It may be possible to convert existing semi-submersible

drilling units to fulfill the .proposed installation tasks. If

the availability of units does not permit the conversion,

then specially built units could be converted for other tasks

(such as crane barges, drilling units, etc.) or used for

other rectennas after installation is completed.

- 110 -
Figure 6.2.1 and 6.2.2 show how the encapsulated dipole networks

are installed using semi-submersible units

6.3 Maintenance Preferred Design

Section 5.3 covers maintenance of towers and miscellaneous items. The

question of maintenance of the network of dipoles is pertinent. The

system design will allow for a percentage of failure throughout the

network. When this percentage is exceeded, changeout of sections of the

dipole network can be accomplished using the semi-submersible

installation barges. Planning for a maintenance program therefore will

include:

. Equipment for maintenance

. Personnel requirements

. Inspection and maintenance frequency

. Component replacement

. Overall budget

6.4 Costs

Total costs for preferred design is summarized in Table 6.1. Costs are

itemized in Table 6.2.

Table 6.1

Cost Summary for Preferred Design

Design Subsystem

Q

Cost in $xlO

Piled tower and support systems; 2.1

Dipole network and taut lines

Miscellaneous

Total

2.8

0.7

5.6

- Ill -
Table 6.2

Itemized Costs for Preferred Design

Design Subsystem Description Cost in

Piled tower Towers, material labor and

and ( fabrication

support systems ! ($2.0xloVtower x 3000 towers)

Material fabrication of jack-up

j barges for tower installation

($5.0 x 107/barge x 10 barges)

Material and fabrication of miscellaneous

hardware:

- guys ($3.0xl08)

- connecting links ($1.0xl05)

- linear winches (20) ($4.0xl06)

- Guy anchors or pilings ($3.0x10')

Leasing of diving, piling vessels

and operating equipment

($5.0xloVday x 730 days x 5 vessels)

Operation of jack-up barges

setting towers including piling

mobilization and fuel

($6.8xl04/day x 730 days x 10 barges)

0.6

0.5

0.3

0.2

Total for piledJggSggjgd support systems

- 112 -

0.5

2.1
Table 6.2 (continued)

Itemized Costs for Preferred Design

Design Subsystem Description Cost in

Dipole network

and taut line

system

Dipole network: material labor and

fabrication cost

($10/m2x900m2/module x 10000 modules)

Material fabrication of semi-submersible

barges for dipole network installation

cost

($5.0xl07/barge x 20 barges)

Operation of semi-submersible

barges installing dipole network

($5.0xl04/day x 730 days x 20 barges)

0.9

1.0

0.7

I Tautlines: 3 in. (7.5 cm)

diameter perimeter lines

($30/ft x 4x106 ft.)

1/2 in. (1.2 cm) diameter criss-cross lines!

($5/ft. x 16x106 ft.)

Total for Dipole network and tautline

system

0.2

2.8

- 113 -
Table 6.2 (Continued)

Itemized Costs for Preferred Design

Design Subsystems

Miscellaneous

Description

Auxiliary supply vessel/tugs for

supplying and moving units

($1.0xl04/day x 730 days x 15 vessels)

Staging port alteration costs

3 ports

Staging port operational costs

inclusive of labor

($1.5xl04/day x 730 days x 3 ports)

Transportation of components

and other miscellaneous costs

Cost in

_$x!09

0.1

0.5

0.03

0.1

Total for miscellaneous costs 0.7

- 114 -
PILED TOWER

GUYLINE

NETWORK OF ENCAPSULATED

DIPOLES BEING PAID OUT

FROM CONTAINER AS

BARGE MOVES

SEMI-SUBMERSIBLE BARGE

IN BALLASTED CONDITION

FIGURE 6.2.1 DIPOLE NETWORK INSTALLATION

- 115 -

BARGE MOVED IN DIRECTION

INDICATED BY HEAVING IN

MOORINGS ONE END AND

SLACKING OUT MOORINGS

OTHER END
ILED

GUYED

SUPPORT

TOWER

•3 DIAMETER

PERIMETER LINE

MOORINGS REDEPLOYED BY.

WORK VESSELS AS

REQUIRED

NETWORK OF

OIAPOLES BEING DEPLOYED

AS BARGE

MOVES

CRISS-CROSS '2"

DIAMETER WIRE

TAUTLINES

SEMI- SUBMERSIBLE

BARGE IN BALLASTED

CONDITION

BARGE MOVING

DIRECTION

INDICATED BY

DEPLOYMENT Al

HANDLING OF

MOORINGS

FIGURE 6.2.2 DIPOLE NETWORK INSTALLATION ON TAUTLINES

N
7. SUMMARY

7.1 Conclusions:

- Constructing an offshore rectenna to convert solar energy from space

to electrical energy on earth is feasible in view of today's

technology and costs.

- For the considered prime site, the point design had the following

disadvantages:

Difficult transportation

Difficult installation

High costs (Total cost = $36.9 x 109)

High susceptibility to weather down time

- For the considered prime site, a preferred design using image dipole

receiving is selected for the following reasons:

Relative ease in construction and transportation

Realistic methods for deployment and installation

Drastic cost savings over other methods (Total Cost =

$5.6xl09)

Less area for snow and ice to form build-ups.

- Among the support systems considered, piled guyed tower structures

--. are the most economical in design, construction and installation for

the prime site.

7.2 Recommendations:

- Change of site for shallower water depth will help to further reduce

costs.

- 117 -
Significant downtime for weather can be expected with all methods of

installation and deployment at the prime site. Changing the site to

a more benign weather area will minimize down time and reduce

installation and deployment costs.

In the design of image dipole receiver networks, it must be ensured

that no heavy build-up of snow and ice will occur. This will be

accomplished by diode network component spacing. This will be true

for any areas subject to snow whether on land or at sea.

- 118 -
"Page missing from available version"
4. Cost Comparisons

The latest estimate for the cost of a land rectenna is $2578

millio n [Boeing Aerospace Co., Solar Power Satellit e System

Definitio n Study, Phase II Fina l Report. Volume I, Rev. A,

February, 1980]. The estimate from our study is $5700 millio n

for the first offshore rectenna at the candidate site. Brown and

Root estimates that this wil l drop by 33% to $3800 millio n after

one time costs have been incurred. These includ e the purchase of

custom equipment necessary for the fabrication and installation .

The costs of the two types of rectenna are not directly

comparable for the following reasons:

1. Considerable attention has been given to the abilit y of

an offshore rectenna to withstand severe weather,

includin g icing . The preferred design is a fully

weatherproof system. We suspect simila r weather

protection wil l have to be incorporated in the land

rectenna as wel1.

2. The offshore rectenna is sited at about 41° N latitud e

and has a N-S axis 14.77 km for a total area of 116 km2'

about 15% greater than the reference system rectenna.

3. The preferred offshore rectenna design was conceived

late in the study and has not been fully optimized for

cost. Also, efficiency data on the antenna is not yet

avai1abl e .

154
4. The present offshore rectenna cost estimate does not

includ e transmission to shore or power pool interface

equi pment.

5. We believ e there may be considerable cost saving

potential in adopting the clothelin e concept to a land

rectenna.

For additiona l comparison, Collin s [Feasibility of Siting

SPS Rectennas over the Sea, Spa. Sol . Powr. Rev. .1 , 133-144,

1980] has done a rough parametric analysis estimate of several

types of offshore rectennas. Collin s estimates that a floating

rectenna could be buil t for about $6000 million . We feel,

however, that he has underestimated installatio n costs, which of

course is possible with a parametric analysis.

155
5. Secondary Uses, Design Requirements, and Sea Defense

A subcontract was let to Arthur D. Little , Inc. to

investigat e various secondary uses of our offshore rectenna and

to specify the design requirements and constraints connected with

these. During the variou s design reviews Arthur D. Little

personnel also became interested in the problems of protecting

the rectenna against wave and wind damage. They also

investigated this area.

The Arthur D. Littl e final report constitutes this section

of this report.

156
..ANCILLARY USES AND DESIGN REQUIREMENTS

FOR AN OFF-SHORE RECTENNA

by

Philip K. Chapman Peter E. Glaser

Sara E. Bysshe Charles B. Cooper

Gerard D. Downing

and

William Sargent Kenneth A. Youngstrom

(Francis B. Sargent Productions, Inc.) (Manomet Bird Observatory)

Subcontract No. 437185B

March 1980

FinalR eport

Prepared by

Arthur D. Little, Inc.

Cambridge, Massachusetts 02140

Prepared for

Rice University Space Solar Power Research Program

Houston, Texas 77058

83570

Arthur D Little Inc

157
TABLE OF CONTENTS

List of Figures

List of Tables

CHAPTER I: POTENTIAL SECONDARY USES OF AN OFF-SHORE RECTENNA ISLAND

1. Introduction

2. Fishery Uses of an Off-Shore Rectenna Island

3. Ancillary Energy-Related Uses of the Rectenna Island

4. Industrial Uses of the Rectenna Island

5. Conclusions and Recommendations

CHAPTER II: SEABIRDS AND THE RECTENNA ISLAND

1. Introduction

2. Marine Bird Species to be Expected at the Rectenna Site

3. Marine Bird Interaction with Vessels

4. General Considerations

CHAPTER III: SITE SELECTION AND DESIGN IMPLICATIONS

1. Introduction: Siting Criteria for Off-Shore Rectennas

2. Sea Defense

3. Aerodynamic Design

4. Conceptual Designs for an Off-Shore Rectenna

5. Conclusions

158
LIST OF FIGURES

Figure No. Page

I Potential Mariculture Systems at the Rectenna Island 168

II • Salmonid Grow-Out Facility 177

III The Horizonatal-Billboard Rectenna 213

LIST OF TABLES

Fable No. Page

I Productivity of U.S. Mariculture 170

II Mariculture Criteria 181

III Cost Factors for Industrial Uses 189

IV Relative Abundance of Inshore Bird Species at . 195

Georges Bank

V Relative Abundance of Off-Shore Bird Species at 196

Georges Bank

VI Size Data for Marine Birds 198

159
Chapter I: Potential Secondary Uses

Of An Off-Shore Rectenna Island

1. Introduction

There is generally a strong correlation between population density

and electric-power demand, so that in regions which might make best

use of the power supplied by the Solar Power Satellite (SPS) difficulties

in assembling sufficiently large tracts of land for installation of

rectennas may be encountered. For example, significant problems may

be encountered in finding acceptable rectenna sites in the Northeast and

Mid-Atlantic states, where about half the U.S. population live on 14%

of the land area.

The options available for supplying SPS power to the most populous

regions of the nation include the following:

1. Accepting the impact of rectennas, in terms of displacing the present

population and restricting uses of the required tracts of land. This is

of course what must be done when major hyclropower facilities are built,

so there is precedent for this approach. The land requirements for hydropower are generally much larger than for the SPS — for example, powerproducing dams controlled by the Tennessee Valley Authority (TVA) cover,

on the average, about 30 times as much land area, per kilowatt of installed capacity, as would a rectenna.

160
2. Utilizing long transmission lines (>800 miles) to bring in power

from regions where rectenna sites are more readily available (mostly

west of the Mississippi). Unless underground transmission lines

become practical, the land pre-empted for rights of way may be quite

comparable in area to rectennas of equivalent capacity, and land use

problems may be equally severe.

3. Building rectennas in rough terrain, in mountainous or swampy

areas, or in areas now heavily forested, all of which impose additional

costs. Moreover, it is highly desirable to preserve many of these

areas, not now intensively utilized, in as close to their natural state

as possible.

4. Designing rectennas as desirable facilities, incorporating multiple

land uses. For example, it may be possible to utilize the waste heat

from a rectenna to delay the onset of frost for crops grown beneath it;

or the rectenna structure might be integrated with greenhouses, making

large-scale greenhouse agriculture economically feasible.

5. Building rectennas off-shore. This option may be of particular

interest because most populous areas are relatively close to coasts

(Atlantic Ocean, Gulf of Mexico, Pacific Ocean, or the Great Lakes),

and because land-use requirements will be greatly relaxed. In several

areas of the world (e.g., Japan and perhaps Europe), off-shore rectennas

may be essential to the utilization of the SPS, because of the unavailability of on-shore sites.

161
If appropriately designed, an off-shore rectenna island could

support a variety of ancillary uses in addition to its principal function.

Such secondary uses may be important, not only because the revenues

from them could offset to a limited degree the increased construction

and operational costs which an off-shore site might involve, but because

they could improve the acceptability of the structure to other interests

such as the fisheries industry which might be impacted by it.

The primary purpose of the present study was to examine briefly

a number of ancillary uses of a rectenna island, with emphasis on

estimating the costs and benefits involved and on suggesting design requirements or features of the structure to facilitate such uses. In addition,

a single ecological issue was considered: the probable effects of the

structure on seabirds, and vice versa.

2. Fishery Uses of the Rectenna Island

2.1 Overview of the U.S. Fishing Industry

In order to provide a context for possible fishery uses of the

rectenna island, and to allow estimates of its probable impacts, it is

useful to review briefly the present magnitude of U.S. fisheries.

Total catches from all areas by U.S. commercial fishermen, together

with catches by foreign fishermen in the U.S. Fishery Conservation Zone

*

(FCZ) , amounted to 4.6 million metric tons (MT) in 1978, up 11% from

1977. This total excludes the weight of mollusk shells and estimated

catches by recreational fishermen. The increase was due to a moderate

increase in U.S. landings and a slight increase in the foreign catch.

*

The area of the "200-mile limit".

162
In 1978 the foreign catch of fish (excluding tunas) and shellfish

in the U.S. FCZ was about 1.8 million MT, up 3% from 1977. The FCZ

off Alaska was by far the most important, accounting for 91% of the

total. The Pacific zone accounted for 6%, with only 3% from the Atlantic

zone. About 97% of the foreign catch was finfish, of which 62% was

Alaskan pollock.

Landings by U.S. commercial fishermen at domestic ports were a

record 2.8 million MT (round weight); they were valued at $1.9 billion,

also a record. Thus for all species, the simple average value was

$0.69/kg, round weight. The increased quantity was due in large part

to an increase in landings of menhaden, used for fish meal and other

industrial purposes. This upsurge in the U.S. landings, together with

a reported downturn in the Norwegian catch, probably will put the United

States into fourth place in world landings in 1978, behind Japan, the

U.S.S.R. and mainland China.

Commercial landings of edible species in 1978 were 1.5 million MT,

valued at a record $1.7 billion ($1.17/kg), an increase of 10% in.quantity

and 23% in value over 1977. This was the largest catch of edible fish

and shellfish since 1951. The principal reason for the increase was

higher landings of tuna, salmon, cod and other groundfish, crabs and oysters.

Landings of shrimp and clams declined. The price index compiled by the

National Marine Fisheries Service for edible fish stood at 384.4 in

1978 (1967 = 100), up 12% from 1977.

163
As would be. expected, records were also established in the

foreign trade aspect of U.S. fisheries. The total value of U.S. imports

of edible and non-edible fishery products was $3.1 billion, up 18% from

the previous year. Edible imports were 1.1 million MT, valued at $2.3

billion ($2.11/kg). Total exports were valued at $906 million, a 74%

increase from 1977. Edible exports totalled 200,000 MT, up 35%, and

were valued at $832 million ($4.10/kg), up 76%.

The U.S. per capita consumption of fishery products in 1978 was

also a record, 6.1 kg of edible meat per person, an increase of 4% over

1977.

2.1.1 Species of Potential Interest: Cod, Haddock, Halibut and Lobster

Total U.S. trawl landings of the principal North Atlantic groundfish

species in 1978 were 170,000 MT, up 12%, valued at $109.1 million

($0.64/kg), up 26%. Domestic landings of cod were 39,000 MT, with an

average value of $0.55/kg. The value of U.S.-produced cod fillets was

$2.91/kg, manufacturers' level, an increase of 15%.

Domestic landings of haddock were 18,000 MT, with an average value

of $0.70/kg, a 39% increase in quantity over 1977 and the highest since

1969. The values of U.S.-produced haddock fillets was $3.33/kg, manufacturers' level, up 9%.

The U.S. fishery for cod and haddock (as well as yellowtail flounder)

has been under a Fishery Management Plan (FMP) since March, 1977, which

has involved quarterly quotas, allocations by vessel size, etc., in

.response to a rapid influx of vessels into this fishery and strong market

demand for catches.
The U.S. hr.libut fishery showed 1978 landings of 8000 MI, with an

average value of $2.31/kg. The Atlantic fishery accounted for only 93

MT, with a value of $3.13/kg. Halibut steaks were valued at $5.75/kg,

an increase of 10% at the manufacturers' level.

Finally, U.S. lobster landings in 1978 amounted to 15,600 MT with

an average value of $4.14/kg.

2.2 Use of the Rectenna Island as an Artificial Reef (Fish Habitat)

The rectenna island, especially if it is a bottom-mounted design,

may be expected to provide a habitat which will attract many pelagic

and reef-dwelling fish species, generating recreational as well as

commercial fishing possibilities. On a much smaller scale, improvement

in fishing has often been noted in the vicinity of other off-shore

structures, such as oil-drilling platforms. Some enhancement of the

natural fishery may be obtained in the waters around the rectenna, but

the structure covers such a large area that the majority of the increased

fish population would generally be found within its borders. To maximize

the productivity of this use, the underside of the rectenna should thus

be far enough above mean water level and the support masts far enough

apart to permit fishing vessels to operate beneath it. Shielding must be

provided to prevent exposure of the crews of such vessels to unacceptable

levels of microwave radiation either as part of the rectenna structure or

as a design feature of the vessels themselves. Since trawling is likely

to be more productive than line-fishing, the underwater structure of the

support masts should preferably be designed to avoid snagging nets.

165
To some extent, this may conflict with maximizing the attractiveness

of the island to desirable fish species by designing the structure

to provide refuge against predatory species. Design to create an

effective fish habitat may increase drag on the island due to tides or

currents, as would the encouragement of marine growth to provide food.

In order to maintain a healthy fish population, care must be taken to

avoid leaching toxic chemicals from the rectenna; in particular, this

could impose restrictions on the use of anti-fouling paints. Finally,

an increased fish population would naturally attract seabirds to the island.

It is difficult to estimate the fish catch which might be expected

from this simple use of the rectenna island, but it is likely to be minor

compared to some of the more complex fishery systems discussed in the

following sections, which involve more direct intervention in control

of the fish population or more efficient catching techniques. The

productivity is also likely to be quite strongly site-dependent, being

higher in southern waters where reef-breeding species are more common.

2.2.1 Conclusion

Limited improvement of fishing around the rectenna could be achieved

with little design impact other than enhancement of fish habitat features

of the peripheral part of the system. To give access to substantially

increased fish populations under the rectenna would involve significant

design impacts, which are probably not worthwhile unless the required

features can be provided in connection with a more profitable ancillary

use (see below).

166
2.3 Fish Weir/Fish Trap

A suitable net suspended from the rectenna, around part of the

periphery or across an internal diameter, could be used as a fish weir

to direct local or migratory fish into a fish trap, as shown in Fig. I (a).

To be effective in harvesting pelagic fishes, a depth of about four meters

would suffice. The size of the rectenna island would permit construction

of a fish weir which i ould be very extensive by current standards. If

the site were in an area (e.g., offshore between Cape Cod and the Carolinas)

regularly traversed by migratory fishes, the catch produced might be

quite comparable to that presently obtained by conventional harvesting

techniques such as trawling. The fish traps attached to the weir could

also be constructed of netting, closed at the bottom to allow harvesting

by hauling up the net; however, careful management would be required to

assure release unharmed of species not immediately required or of fish

below a desired size, since otherwise serious depletion of natural fish

stocks coi'ld occur.

A fish weir would be a relatively minor addition to the rectenna

and would have a significant design impact only in restricted areas, to

permit manipulation of the fish traps and packing of the catch for transportation ashore. It might be desirable to establish a fish processing

plant on the rectenna island, to allow maximum freshness of the product

and to minimize transportation requirements, but this too would incur

only a localized impact on the rectenna design. The underwater netting

would cause a slight increase in the drag on the structure, and some

restrictions on leached chemicals might also be required. Seabirds

would be attracted to the site, but would tend to concentrate near the

167
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fish traps and/or processing plant; it would be possible to design

these areas to deny birds access, reducing the incentive to congregate there

2.3.1 Conclusion

A fish weir/fish trap would have a modest design impact but could

substantially improve harvesting in the natural fishery, and may therefore

be very cost-effective. The .catch could exceed considerably that presently

obtained by conventional methods in the general area of the rectenna site.

However, this use is likely to be regarded as undesirable competition by

the local fishing industry, especially as it would be a large-scale

operation with presumably little opportunity for small entrepreneurs.

This would be especially true for species covered by a FMP: if the total

catch is regulated to preserve stocks, then fishing is a zero-sum game,

in which the catch at the rectenna island would reduce the harvest

allowed to existing fishermen.

2.4 Mariculture

The rectenna island could provide the basic structure for a large

facility which has many aspects in common with other proposed mariculture

systems, although the scale of the island is considerably larger than is

common in such proposals. The magnitude of the mariculture system which

could be integrated with the rectenna is likely to be limited, not by

the available area, but by the natural flow of water through the system

and hence the supply of oxygen and nutrients to the fish.

Table I, showing yields from U.S. mariculture systems , illustrates

the remarkable productivity which is achievable; in some instances, foreign

169
TABLE I

Yields from U.S. Mariculture Systems

Species Live Weight

(Metric Tons/Hectare/Year)

Oysters 5

Mussels 55

Shrimp 6-1 6

Yellowtail ' 30

Salmonids 8-30

170
mariculture has provided yields up to an order of magnitude higher.

Assuming an average yield of edible species of 15 MT/hectare/year, and

that the entire rectenna area could be used for fish farming, an annual

harvest of 170,000 MT would result. This is about 11% of the annual

U.S. fish catch. A single rectenna island could thus have a significant

market impact, especially when compared to a regional fishery. If

relatively high-value species were raised, with an average value of

$1.00/kg (round weight, manufacturers' level) such mariculture operation

could gross $170 million per year, approaching 15% of the value of the

electric energy produced by the rectenna. A considerable investment

might thus be justified in modifying the rectenna design to suit mariculture needs.

Mariculture is based on raising a controlled fish population under

optimal conditions, with predators and undesired species excluded.

The catch does not deplete natural fish stocks, and thus should not be

subject to any FMP in force. In fact, insofar as a FMP implies an

excess of demand over supply, species covered by it would be good candidates for mariculture.

2.4.1 Mariculture Range Operations

The simplest type of mariculture which could be undertaken at the

rectenna island would involve the construction of several large pens, as

illustrated schematically in Fig. I (b). For surface-dwelling species,

the pens could have net bottoms as well as sides, but, in reasonably

shallow water, it would probably be cheaper to extend the net sides to

the bottom-, thereby accommodating bottom-dwelling species as well. The

171
principal objective in this type of mariculture would be to exclude

predators and trash fish from the area under the rectenna, while

providing a fairly natural environment for the desirable species, with

a relatively small impact on the design of the rectenna. Without

extensive stocking of the pens each season, the useable species would

be limited to those which can tolerate annual variations in water

temperature at the site and do not require long-distance migration for

breeding purposes; in general, this type of operation would thus be uost

suitable at sites in lower latitudes.

Controlled fish traps would provide the most convenient means for

harvesting, perhaps using the pens as fish weirs to direct fish into

them. Some form of intrarectenna freight transportation (e.g., barges)

would be required, implying sufficient clearance between the bottom of

the rectenna and the water surface. If the pens were large enough, the

additional drag on the structure would be modest. To provide reasonably

calm conditions beneath the rectenna, some form of surface-wave damping

would be desirable, and operations would be simplified if the mean

distance from the rectenna to the water surface were controlled in the

presence of tides, etc., which suggests a floating rather than a bottommounted rectenna (the design possibilities in this regard are discussed

in Sec. III.4, below). If the nets forming pens extended to the

bottom from a floating structure, they would of course need sufficient

slack to accommodate the maximum water depth expected.

It would be appropriate to locate at the rectenna a facility for

processing fish (cleaning, freezing, canning, etc.), especially as much

of the waste produced in processing could be utilized as high-protein

172
feed in the mariculture system. The processing plant would also reduce

waste and improve the quality of fish delivered to the consumer by

eliminating the current practice of storing in the hold of a fishing

vessel for up to several days before reaching shore.

To maximize the fish population which could be maintained in this

type of fenced range, without artificial aeration and with minimal

feeding requirements, the rectenna should preferably be located in an

area of reliable water flow (either a steady current or tides). In

choosing a site, a trade-off study is however required because high flow

through the structure implies high drag and hence increased mooring or

pylon costs.

2.A.2 Mariculture Feedlot Operations

A much more intensive type of mariculture may be feasible at the

rectenna island, in which carefully selected species are raised from eggs

or fingerlings to commercial size in relatively small, highly productive

pens, as illustrated in Fig. I (c). Because the requirements depend

rather strongly on the species involved, hypothetical scenarios were

constructed for several such grow-out facilities.

2.4.2.1 Pollock

Pollock are indigenous to continental shelf waters off New England

and adapted to the cool temperatures found there. While larger adults

are typically found in 40 to 200 meters of water, younger fish (in the

age range of interest here) inhabit depths at the shallower end of this

range. It is assumed that pollock would be grown in the lower third of

173
the water column under the rectenna. Enclosures made of nylon or polypropylene netting would form cages, extending from the bottom to some

middle depth (with a net cover to prevent escapes) or preferably from

the bottom to the surface. A reasonable surface area for each enclosure

would be 4 million . square meters.

Assuming that young pollock could be raised in salt-water hatcheries

(either near shore or at the rectenna island) to one year of age, the

grow-but pens would be stocked with fish 13 to 18 cm in length. Pollock

reportedly grow to about 30 cm in the second year, representing a 0.4

kg fish which would be marginally marketable. Each fish would require about

a square meter of bottom area, so each pen would produce some 4 million fish

each year, or about 1500 tons (round weight). At a retail value of $3.30/kg

(reduced because of size), the gross value of the harvest from each pen could

thus reach $5 million per year.

A second possibility would be to grow the fish for two years, when

they could be expected to average 44 cm in length and approach 1 kg in

weight, so that each pen would produce 4000 MT each two years, or 2000

MT annually. At a retail value of $4.40/kg for these larger fish, the

gross annual revenues from each pen would thus approach $9 million, so

that this seems a preferable mode of operation.

The number of pollock pens which could be installed at'a rectenna is

likely to be limited primarily by market demind. In 1976 (the latest

year for which data are available), the harvest of pollock in New England

waters amounted to 11,000 MT. Thus six pens (in the two-year-growth

scenario), covering 20% of the rectenna area, might be sufficient to
double the harvest of this fish. The demand for this desirable species

could be expected to increase if larger supplies were available, but

too large an increment would be expected to depress prices, at least

initially, so that pollock mariculture could arouse opposition from

existing fishermen.

The pollock pens assumed here are sufficiently large so that their

impact on rectenna design would be relatively modest, and the discussion

of range operations, above, applies to this case also. Because of the

homogenous population, wastage in fish traps or other harvesting

techniques would be less in this application.

The overall conclusion is that pollock pens at a single rectenna

island could singificantly increase the harvest of this fish. The gross

revenues from the operation could conceivably approach $50 million annually.

2.4.2.2. Salmonids

Anadromous salmonids (Atlantic or Pacific) are another candidate for

intensive mariculture at an offshore rectenna site in the New England area.

The pens for this surface-dwelling species would need to extend to a

depth of only 5 meters, so that they would be equipped with net bottoms

as well as sides. Young salmonids (smolts) between 1 and 2.5 years of

age would be obtained from onshore (freshwater) hatcheries (or from

on-site hatcheries using rainwater) and stocked in grow-out facilities

at the rectenna island. A three-stage grow-out, using pens of increasingly

larger surface area, would increase the efficiency of space utilization

because the smaller fish require proportionately less space. Thus,.a

17S
series of three suspended pens might be used for each 20,000 fish

2 2 2 produced, with surface areas of 400 m , 1200 m , and 4200 m , as sketched

in Fig. II. The last pen is designed to accommodate 20,000 adults, each

weighing about 0.5 kg, at a density of one fish per cubic meter.

Anadromous salmon reportedly gain 1.5 to 3 kg during their first

year at sea. Assuming that 0.5 kg salmon are marketable and that the

crowding in the pens assumed here slows growth somewhat, it is reasonable

to estimate that the three grow-out areas would be sequentially occupied

for 2, 2 and 4 months, respectively, to produce fish ready for harvest.

Thus, at a minimum, each series of three pens could be harvested

every 8 months (1.5 harvests per year), yielding 10 MT of fish. At an

average retail price of $8.50/kg, the gross value of this harvest could

approach $120,000 per year.

The smaller pens would be designed to allow hauling up the net

bottom from one end, in order to herd the fish into the next larger pen,

when this is required. Harvesting would be effected by hauling up the

net of the largest pen.

The design impact on the rectenna (e.g., on the drag of the structure)

of this type of mariculture clearly increases with the scale of the

operation. To provide an upper bound, the U.S. salmon catch in 1967

amounted to about 130,000 MT; to match this yield with the facilities

described here would require nearly 9000 sets of pens, with a total

surface area nearly half that of the rectenna. It is almost certainly

176
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not possible to operate at this density, even if all nutrients are

provided by the.operators of the system, because of problems with aeration

and the build-up of waste products. A more reasonable estimate is 500

sets of pens at each rectenna, providing a harvest with a gross retail

value of $60 million per year and a tonnage of about 5% of the U.S.

salmon catch.

It would be feasible to locate this number of sets of pens around

the periphery of the rectenna, minimizirig the design impact and, in

particular, providing access without the need to penetrate far beneath

it.. However, it is obviously essential that these shallow pens move up

and down with the water surface (e.g., by using floating supports for

them), and it is highly desirable that wave action be attenuated to

prevent undue stress to the growing salmon.

2.4.2.3 Lobsters

Lobster culture is a particularly interesting use of a rectenna

island in New England waters. Since lobsters are bottom-dwelling territorial animals, it may be possible to keep them in the vicinity of the

rectenna without artificial restraints; but, if necessary, a net fence

about 1.5 meters high and attached to the bottom, surrounding the benthic

area set aside for lobster grow-out, would be sufficient to avoid losses.

Infant lobsters can be supplied by existing shoreside hatcheries and

the stock could also be supplemented by "berried" females, which have

eggs clinging to the swimmerets on their abdomens. It is illegal to

178
market these females, so that they could be supplied at a minimal fee

by lobstermen who catch them in existing traps. If a fish processing

plant is located at the rectenna island, waste products from it could

supply most or all of the food required by the lobsters during grow-out.

In this scenario, clusters of lobster habitats would be lowered

on cables from the rectenna to the bottom, with a density of perhaps

100 clusters per square kilometer. A small fraction of the lobster

population on the bottom would occupy these habitats, and could be

harvested simply by raising the cluster. Based on experience with lobster

traps, it is reasonable to expect a harvest of about 50 kg of lobster

per cluster, and the harvest could be taken twice a month throughout

2

the year, giving an annual yield of 120 MT/km .

The design impact of such a lobster ranch would be quite modest: the

primary requirement is passage for relatively small vessels under the

rectenna for harvesting and transportation of the lobsters to a central

packing facility. It is not clear what fraction of the benthic area under

the rectenna could be utilized for this purpose without problems due to

aeration, waste products, etc.; but, if it were possible to use the

entire rectenna area, the calculated annual harvest would be about

13,000 MT, or more than 80% of U.S. lobster landings in 1978. The value

at manufacturer's level of this harvest would approximate $50 million,

and about $115 million at retail. While it may be feasible to achieve

only a fraction of this harvest, it is clear that lobster ranching could

be a very cost-effective use of the rectenna island.

179
2.4.3 Summary and Conclusions

Much further work will be required to demonstrate the technical and

economic feasibility of mariculture uses of the rectenna island — for

example, there is very little experience as yet with culturing of pollock

(or of other useful species such as cod and haddock). Range operations,

a pollock grow-out facility, and lobster ranching appear to require

relatively modest impact to the design of the island, while a salmonid

grow-out facility is expected to have a larger impact, but only over a

relatively small fraction of the rectenna area, around the periphery.

In any case, it appears quite possible that mariculture at the rectenna

could yield gross revenues (at retail) well in excess of $100 million.

Whether or not this is considered a significant contribution to the

overall revenues (including electricity) from the rectenna, it is clear

that mariculture may represent a promising industry in its own right,

if it can be accommodated without major increases in rectenna costs.

The effects on the design of the rectenna are discussed in more detail

in Chapter III.

Designing a mariculture system on the scale envisaged here amounts

essentially to developing a controlled but not entirely closed ecological

system. Table II lists some of the criteria which might be used in

developing the system, starting with the species which is to be raised,

and some of the problem areas which must be expected. In addition to the

pollock, salmonids and lobsters discussed specifically here, other

species which might be considered for culture at an off-shore New England

site include cod, haddock, flounder, sole and halibut; this particular

selection is based primarily on available markets. For most of these

180
TABLE II

MARICULTURE CRITERIA

SPECIES SELECTION

Season/Temperature Requirements

Predator/Prey/Competitor Relations

Population Density

Habitat Requirements

• HATCHERY FACILITIES

In Situ or Onshore

• FOOD SOURCES

Open Water Sources

Species Specific

Life Stage Requirements

Food Recycling

- Artificial Reefs

• DISEASE AND PREDATION

Baterial/Viral Diseases

Predator Exclusion

• WATER QUALITY

- Pollution (Oil Spills)

- Contamination (Antifouling Agents)

Waste Product Removal

181
species, very little is as yet known about such important factors as

their probable response to a somewhat artificial environment, the

population density which can be achieved, or the risks of disease under

these conditions. It would therefore be premature to attempt a more

detailed design of the system at this time. For present purposes, it

is sufficient to note that the design requirements on the rectenna

imposed by maricultur-j, although quite species-specific, may be tolerable,

and the potential revenues are in a significant range.

Beyond the revenue potential of mariculture operations, the possible

provision of a new source of fish protein for a hungry world may become

of increasing importance as the number of off-shore rectennas grows and

the productivity of open-ocean fishing declines.

2.5 Other Fishery-Related Uses of the Rectenna Island

Fish Processing Facility. A fish processing plant at the rectenna island

would be desirable to support mariculture operations, but such a plant

could also serve conventional fishing fleets, especially if the rectenna

were located closer to fishing grounds than the home ports of the vessels.

This would improve the productivity of the fishing fleets, allowing them

to spend more time fishing and less in travel; as an example, fishing

vessels operating in the Georges Bank area now often spend two days out

of each week in travel to and from port. The vessels would not need

freezing plants to avoid spoilage and deterioration in the quality of the

product during travel back to port. As a result, smaller boats would be

able to stay on station for longer periods, reducing capital costs and

allowing them to compete more effectively with the large, blue-water

182
vessels used by foreign fishermen, which are often equipped with

processing plants. In this connection, it should be pointed out that

floating processing facilities are under consideration or development

in several areas around the world, notably to serve the Alaskan FCZ.

Because the rectenna island would provide the foundation for such a plant,

as well as the electricity needed to run it, combining it with the

rectenna would significantly reduce the cost involved.

Overnight Docking Facility/Fuel Supply Depot. An artificial harbor built

into the rectenna island could provide fishermen with a comfortable and

safe overnight docking facility, especially if the fishing grounds were

reasonably close. Restaurants and other recreational facilities (perhaps

even family accommodations) could be provided on the island, leading

eventually to a small town (effectively shielded from microwaves) which

would improve the attractiveness of the island for other rectenna workers.

If the harbor were protected by wave-damping devices (or if the entire,

rectenna island were so protected), the docking facility could also provide

fishing vessels and other small craft with a refuge in the event of a

storm, without having to return to shore harbors. Fuel and other supplies

for fishing and other vessels at the rectenna island would be another

valuable service, improving efficiency and reducing energy expenditures.

International Fish Marketing Facility. It is conceivable that the rectenna

island could provide facilities where foreign and domestic fishermen

could market their catch, not only to the fish processing plant. This

would allow U.S. fishermen to sell to foreign entrepreneurs that portion

of their catch for which there is little market in this country, and it

183
would also enable U.S. wholesale fish merchants to buy seafood which

can be caught more efficiently by the larger bluewater fleets of

foreign nations, without incurring the transportation costs to and from .

foreign markets which are now involved in such transactions.- Since 42%

by weight and 58% by value of edible fish products consumed in the

United States are imported, this could lead to a significant reduction

in the average retail price of fish.

3. Ancillary Energy-Related Uses of the Rectenna Island.

3.1 Wave Power

One of the principal problems with extracting energy from ocean

waves is that of converting wave motion into a form suitable for the

generation of electric power. A variety of systems have been devised for

this purpose, using bottom mounted or submerged structures to provide a

stable reference, resonant hydrodynamic devices to produce a unidirectional

jet of water, and gyrostabilization .of reference members in floating

systems designed to undergo rotary oscillations due to wave action. Since

the rectenna, whether floating or bottom-mounted, may provide a stable

base at the wave frequencies of interest, it is possible that wave energy

systems used in conjunction with it could be simplified. Moreover, the

large size of the rectenna island presents an opportunity to extract

power at a significant level despite the diffuse character of the wave

energy resource.

The velocity of propagation of a wave (whose amplitude is much

2

smaller than the depth) is given by elementary hydrodynamics as

2 .2 2 gX , 2nd M ,

c = A v = - tanh -r— [1]

184
where X and v are the wavelength and frequency of the wave, d is

the water depth and g is the acceleration due to gravity. The power

crossing a line of length L, due to waves of amplitude a, is then

•D * T 2 n -i- -L. P = - Lgpa c [1 + -- cosech

Given the frequency, [1] can be solved for the wavelength and

propagation velocity: for example, for typical surface waves (with the

wavelength much shorter than the depth), with periods of 2 to 3 seconds,

the wavelength is 3 to 4.5 meters. If it is assumed that these waves have

a height of 1.5 m, and that the minimum diameter of the rectenna is

10 km, the power impinging on it is calculated to be 85 to 130 MW.

Quite simple devices, for example using the relative motion of floating

collars around the rectenna support masts along the periphery, could be

used to convert this energy to electricity.

After taking into account conversion efficiency, it appears probable

that surface wave energy could contribute an average of at least 50 MW to

the power output of the rectenna. Although this is a fairly small contrib

ution to the overall power from the system, and would vary considerably

with the amplitude and frequency of the waves, it would contribute about

$13 million per year (at $0.03/kWh) to the gross revenues, and would

provide an ancillary source of power for rectenna housekeeping functions,

navigation beacons, etc., during SPS outages (for maintenance or during

occultations of the satellite, etc.). Moreover, energy taken from the

waves would of course reduce their amplitude and the system could thus

be integrated with wave-damping to give smooth conditions under the

185
rectenna, which is highly desirable for most of the fishery applications

discussed previously.

The energy contained in ocean swells is much greater than in surface

waves. For example, if the water depth is 30 m and swells of height 5 m

and period 18 seconds are incident on the rectenna, the calculated power

passing beneath it exceeds 8 GW. Because of the lo./er frequency, it is

much more difficult to extract useful power from swells, but it may not

be impossible, given the size of the rectenna structure. If a suitable

means could be found, at selected sites the average swell energy output

might be comparable to that from the rectenna, so that this is clearly

an interesting area for research.

It is also more difficult to damp swells than surface waves, and

perhaps less important to do so. Chapter III gives a more detailed

discussion of possible sea-defense systems, intended to protect the

rectenna from damage, simplify its design, and ensure sea-state conditions

compatible with ancillary uses of the structure.

3.2 Other Energy-Related Uses.

At suitable sites, adjacent to cold, deepwater areas but with warm

surface water temperatures, an ocean thermal energy converstion (OTEC)

system could be colocated with the rectenna. Apart from the provision

of common support services and the capability of using a common power

transmission system to the shore, there does not seem great motivation

for this use of the rectenna island.

186
A more interesting possibility is to build a deepwater port in

conjunction with the rectenna, adapted especially to very large supertankers, in areas where no natural deepwater harbors exist on the

adjacent coastline or where there are environmental objections to

offloading tankers at shore ports. In such a case, an oil refinery

could also be built on the rectenna island; this would be particularly

desirable in regions juch as New England, where no refinery now exists

and acceptable on-shore sites have not been found. Crude or refined

petroleum would be transported ashore by pipeline. Neither a deepwater

port nor a refinery would be expected to impose significant design

constraints on the rectenna, because of their limited extent, relative

to the area of the island.

4. Industrial Uses of the Rectenna Island.

An off-shore rectenna island offers significant advantages for a

variety of industrial activities, compared to an on-shore site. They

include:

• The availability of port facilities for very large vessels.

• The lack of existing property rights or other land-use problems.

• Little or no property tax.

• Isolation from population centers

• The ample availability of seawater for cooling purposes in

industrial processes.

• An ample supply of electric power from the rectenna.

• Use of the ocean to disperse or neutralize solid or liquid

effluents (although the effects of effluents on marine flora

and fauna and the impact on fishery uses must be carefully considered)

• Possibly relaxed atmospheric emission standards.

• Removal of aesthetically undesirable facilities from shore areas.

• Improved safety (due to such factors as the availability of

sewater for controlling fires) and reduced impact of major accidents.

187
The probable disadvantages are essentially:

• Increased personnel costs (especially transportation).

• The need to protect workers from microwaves.

*

e Lack of fresh process water .

• Physical separation of management and production.

• If implemented on a large scale, major impact on the rectenna design.

The industrial activities for which the rectenna island might

provide an attractive site include the following:

• Deepwater port.

• Oil refinery.

• Port and storage facilities for liquefied natural gas (LNG).

. • Chemical and petrochemical plants.

**

• Aluminum plant (for refining imported bauxite)

**

• Liquid gas and air separation

Table III, adapted from a feasibility study of an artificial island

for industrial purposes in the North Sea (off the Hook of Holland) lists

the probable advantages and disadvantages of the rectenna island, compared

to an on-shore site, which respect, to cost factors in production. The

referenced study considered an island measuring 10 km by 6 km (i.e., an

area about half that of the rectenna island), and the basic construction

costs were estimated as $2.6 billion (1976 dollars). In the present case,

some of the construction cost could be amortized by sale of electricity

from the rectenna, probably improving the economics of an artificial

industrial island.

Individual plants of limited area could be accommodated at the rectenna

without major impact to the overall structure, but decisions would have

to be taken regarding the fraction of the total area which could eventually

be adapted to industrial purposes. The areal density of industrial facilities

would in most cases be much greater than thaL of the rectenna itself, and

provision would be needed for locating the plants beneath the groundplane

Even if the entire rectenna island were used as a catchment area, the average

available flow of fresh water would only amount to a fraction of a cubic meter

per second, depending on the rainfall.

&«fc

Taking advantage of electric power from the rectenna.

188
TABLE III

Cost Factors in Industrial Production

at a Rectenna Island

Product Cost Expense Relative to

Factor On-Shore Site*

Real Estate Costs + +

Real Estate Taxes

Insurance +

Maintenance +

Compliance with Environmental Standards -

Raw Materials 0

Labor +

Process Heat** 0

Cooling Water (Salt)

Electricity -

Process Water (Fresh) +

Petrochemical Feedstock**

Steam 0

Harbor Costs " +

Ship Transport . +

Pipeline Transport +

Hazard Prevention (Public)

Worker Safety'and Health ~+~

Port Infrastructure +

* More expensive at rectenna island: +

No significant difference: 0

Less expensive at rectenna island: -

**With refinery on site.

189
of the rectenna and for transportation of personnel and materials to them.

Areas reserved for industrial use would thus require a much more rugged

and complex structure than those to be used for the rectenna alone.

More detailed estimates of the design impact of these uses of the

rectenna island requires a study of potential industrial facilities on

a case-by-case basis.

5. Conclusions and Recommendations

In order of increasing design impact, the most promising ancillary

uses of the rectenna island appear to be wave-energy systems, a fish weir/

fish trap, mariculture using pens around the periphery, support facilities

for conventional fisheries, a deepwater port and/or oil refinery,

*\

mariculture using more of the rectenna area, and some other industrial

uses. In order to facilitate these uses, especially those requiring

access to the interior of the rectenna, consideration should be given to

the following design features:

• Shielding of the area under the rectenna from hazardous levels

of microwave radiation.

• Providing sufficient clearance between the underside of the

rectenna and the water surface and sufficient distance between

support masts to permit relatively unrestricted passage by

vessels of modest displacement.

• Wave-damping or other sea-defense systems to prove sea-state

conditions under the rectenna which would not impede operations.

• Adapting at least a limited area of the rectenna to heavy

construction, to provide port facilities for fishing boats and

perhaps much larger vessels, living quarters for fishermen,

recreational and other support facilities, and probably some

industrial plants.

190
The secondary uses discussed here should be regarded only as

preliminary suggestions. Demonstration of the technical feasibility

of many of them will require research (e.g., with respect to the

culturing of appropriate fish species for mariculture) and/or detailed

analysis of the specific design requirements. In terms of economics,

it appears that some uses (e.g., the fish weir/fish trap and a lobster

ranch) may be very cost-effective, without necessarily contributing

greatly to revenues from the island; if many of these uses were

implemented, the gross annual revenues generated could easily amount to

several hundred million dollars, justifying quite extensive modifications

to the rectenna design and perhaps providing a useful contribution to

amortization of construction costs.

191
CHAPTER II: Seabirds and the Rectenna Island.

1. Introduction

This report is a preliminary assessment of the interactions of

birds with an ocean-based rectenna for the solar power satellite system.

The postulated rectenna position is 41°N lat, 70°30'W long, or about

40 km south of Martha's Vineyard, Massachusetts. The location is on the

continental shelf with water depths of about 50 m.

The report will discuss (a) the avian species which can be expected

in the area together with descriptions of these birds, (b) observed

behavior patterns which might serve as predictors of reactions to the

SPS rectenna structure, and (c) the difficulties of making predictions

of the quality level traditionally utilized in assessing environmental

impacts of various technologies.

The source data are derived from the Manomet Bird Observatory's

continuing program to map the distribution of marine birds on the midand north-Atlantic outer continental shelf of the United States. This

effort has been underway for four years under various sponsors; currently

it is funded by the United States Department of Energy under DOE contract

no. EE-78-S-02-4706. The study utilizes trained observers on ships-ofopportunity, i.e., cruise tracks are determined not by the specific needs

of the seabird research program, but by the goals of the individual ships

concerned. This precludes a statistically rigorous survey plan, but does

allow for a great deal of coverage at relatively low cost. Seabirds away

from their breeding colonies or before they are sexually mature (3-10 years

in various species) are highly mobile in response to food resources which

are locally only available for short periods of time. Therefore, these

birds are heterogeneous populations that must be sampled by a stratified

scheme. A ships-of-opportunity scheme allows most strata to be sampled,

given sufficient observers.

Finally, much of the information presented herein is heavily weighted

by expert opinion. The data from over 100 cruises consisting of over

10,000 transect censuses are currently being reduced to computer codes

192
for machine analysis. It is anticipated that summary distribution maps

detailing birds in any specific area will soon be available which

incorporate all of the program data to date. In the interim, the seabird

observers have been queried in detail to generate these findings.

2. Marine Bird Species To Be Expected At The Proposed Rectenna Site

This listing includes only those species expected to occur at the

proposed rectenna site reasonably frequently. Rarely sighted species are

not included. Migrant land.and shorebirds will be in the vicinity of the

rectenna site especially during fall migration. Several passerine species

are transatlantic migrants, moving from Cape Cod to Tobago, and many

shorebirds migrate annually to South America. For these birds, the

rectenna would be utilizied as a stopping place when fog or overcast skies

obscure the star fields needed for orientation. When conditions improved,

the birds would be on their way once again. On the other hand, adverse

winds (either headwinds or offshore winds) might force these and other

migrants which do not normally migrate over the ocean to seek the

rectenna as a refuge. If the energy reserves of these birds are depleted,

the rectenna will not, of course, provide the necessary sustenance for

replenishment, and these birds will probably perish.

Tables IV and V listing expected species at the rectenna site are

derived from a pilot study of Georges Bank. The proposed rectenna site

is essentially similar with respect to species composition and temporal

distribution, with the exception that terns (Sterna hirunda in particular

during the summer) are more likely at the rectenna. Table VI .presents —-

size data for 21 probable site residents or visitors.

3. Marine Bird Interaction With Vessels

During the four years of seabird research cruises by the staff of

the Manomet Bird Observatory, with the single exception of gulls, none

of the marine birds listed in the accompanying tables have been observed

attempting to land on any parts of the survey vessel under normal weather

conditions. These vessels have included fisheries research ships hauling

trawl nets and processing catches much as do commercial fishing vessels.

193
Unfortunately, then, no clues are provided as to structural design of

the SPS rectenna which might minimize landing of marine birds.

194
TABLE IV: Status, relative abundance, and time of status of 16 species of

inshore birds recorded on Georges Bank, February 1976 - June 1977.

Species

Common Loon

Red-throated Loon

Great Cormorant

Double-crested

Cormorant

Canada Goose

Snow Goose

Oldsquaw

Common Scoter

White-winged

Scoter

Surf Scoter

Red-breasted

Merganser

Ring-billed Gull

Laughing Gull

Common Tern.

Arctic Tern

Status

Migrant

Migrant

Migrant

Migrant

Migrant

Migrant

Migrant

Migrant

Migrant

Migrant

Migrant

Visitor

Migrant

Migrant

Migrant

Migrant

Migrant

Relative

Abundance

Uncommon

Uncommon

Uncommon

Uncommon

Uncommon

Uncommon

Uncommon

Uncommon

Uncommon

Uncommon

Uncommon

Uncommon .

UncoiTimon

Uncommon

Rare (?)

Uncommon

Rare ( )

Seasonality (Season)

Spring - fall

Spring - fall (?)

Spring

Spring

Fall

Spring

Fall

Fall

Fall

Fall

Fall

Winter-spring

Summer

Late spring - early summer

Fall

Late spring - early summer

Fall

Sooty Tern Visitor Accidental Summer - Fall

195
TABLE V: Status, relative abundance, and time of status of 26 species of offshore birds recorded on Georges Bank, February 1976 - June 1977.

Species Status

Yellow-nosed Albatross Visitor

Northern Fulmar Visitor

Visitor

Visitor

Cory's Shearwater

Greater Shearwater

Sooty Shearwater

Manx Shearwater

Audubon's Shearwater

Leach's Storm-Petrel

Wilson's Storm-Petrel

Gannet

Red Phalarope

Northern Phalarope

Pomnrine Jaeger

Parasitic Jaeger

Visitor

Visitor

Visitor

Visitor

Visitor

Visitor

Visitor

Resident (?)

Visitor

Resident

Resident

Visitor

Visitor

Migrant

Visitor

Migrant

Migrant

Migrant

Migrant

Migrant

Relative Abundance

Accidental

Common-abundant

Rare

Uncommon

Common-abundant

Uncommon

Abundant

Rare

Uncommon

Common-abundant

Rare

Uncommon

Uncommon

Uncommon-Common

Absent - Rare

Uncommon-Common

Abundant

Common- Abundant

Uncommon

Common-Abundant

Uncommon (?)

Uncommon

Uncommon (?)

Uncommon

Uncommon

Seasonality (Month)

Jane

October - June

July - August

September

June - October

November

May - November

December (?) - April

April, August - October

May - July

November - March

April - October

May - September

April - November

December - March

April, September - Octob<

May - August •

February - May, September

November

December-January, June -

August

April - June

October - November

April - June

October - November

April - November (?;

April - November (?)

196
TABLE V: (continued)

Species Status

jong-tailed Jaeger

Skua spp.

Jlaucous Gull

[celand Gull

Jreat Black-backed

, Gull

lerring Gull

aabine's Gull

31ack-legged Kittiwake

lazorbill

Common Murre

Chick-billed Murre

Dovekie

Common Puffin

Migrant

Visitor-Migrant Rare-Uncommon

Relative Abundance Seasonality (Month)

Rare June, September

January - December

Visitor

Visitor

Visitor

Resident

Resident

Resident

Resident

Migrant

Rare

Uncommon-Common

Rare

Abundant

Common

Abundant

Common

Accidental (?)

Visitor

Visitor

Visitor

Visitor

Visitor

Visitor

Visitor

Visitor

Abundant

Uncommon

Rare

Uncommon

Uncommon

Uncommon

Uncommon

Uncommon

November - May

November - March

April - May

September - April

May - August

October - May

June - September

March, October

October - February

March - June, September (?)

July - August

November (?) - April

November (?) - April

November (?) - May

December (?) - April

December (?) - May

197
TABLE VI: Size Data for Marine Birds Expected to Occur at Proposed Rectenna

Site.

Size (cm)

Species

Northern Fulmar (Fulrr.arus glacialis)

Cory's Shearwater (Puffinus diotnedsa)

Greater Shearwater (P. gravis)

Audubon's Shearwater (P. Thenrtin-ieri)

Manx Shearwater (P. puffinus)

Sooty Shearwater (P. griseus)

Wilson's Petrel (Oceeznities oaeani-aus)

Leach's Petrel (Oceanodpoma leucorhcc)

Gannet ('-'oms bassanus)

Double-crested Cormorant (Phalacrocorcz penieillatus )

Red Phalarope (Pkalaropus fulicarius)

Northern Phalarope (Lob'ipes lobatus)

Parasitic Jaeger ( ' Stercorarius parasitiaus)

Pomarine Jaeger CS. pomarinus)

Skua (Cafharacta skua)

Great Black-backed C,ull (Larus marinus)

Herring Gull (L. argentatus)

Laughing Gull (L. atricilla)

Common Tern (Sterr.a hi.runda)

.Royal Tern (Tkaitzsssiis r'.cxixius)

Cotnmon Puffin (Fratercula arotica).

Wing Span

110

110

115

66

80

110

40

50

180

130

115

120

40

165

140

105

80

110

Body Length

45,

50

45

30

35

40

15

20

80

70

17

15

40

45

45

60

50

35

35

45

28

198
4. General Considerations

Although it appears that there is nothing unique in the seabird use

of the area planned for the sea-based rectenna, it must be recognized

that the presence of the rectenna per se will create uniqueness. The

itinerant seabird is in nearly constant flight in pursuit of its transient

food resource. Areas of oceanic upwellings and fishing fleet activity

serve to establish concentrations which are exploited by the birds as

long as the resource remains abundant.

Even without an attendant mariculture operation, the underwater

support and anchoring structures of the rectenna will inevitably support

marine fouling organisms and the attendant food chain will appear.

However, with the immense size of the rectenna, this fouling-based food

chain will cause the stabilization of a major animal population normally

given to nearly constant movement throughout a wide area of ocean. This

fixed food resource will surely attract significant numbers of seabirds

to the area to utilize the food resource. On the assumption that a mariculture activity cannot be operated without some loss of nutrients to

the areas outside the boundaries of the fish farm, the increase in prey

species will become more pronounced as the intensity of the mariculture

efforts grow, and thus the bird population will grow as well.

To a great extent, this marks the end of the predictions one can

make with some confidence. The appearance of fishing fleets and/or

relatively small localized structures such as oil well platforms

represent a rather small intrusion into a very large space. In addition,

both the fishing boats and the oil-associated structures generally

experience constant and significant levels of human activity which act

as a deterrent to birds landing on them. The SPS rectenna, on the

other hand, will not only be many orders of magnitude larger than any

other man-made oceanic structure, but may also exhibit little continuous

human activity.

Thus two very significant changes will be made in the environment

of the seabird. First, food resources which have been mobile and somewhat dispersed in schools of various sizes suddenly (over a few years)

199
become concentrated and fixed in location.. Second, birds which spend

their entire lives, with the exception of a brief annual breeding period,

on or above the surface of the open ocean may suddenly be presented with

the opportunity to come to rest on a structure immediately above or

adjacent to a major food source. It is difficult to predict responses

to these conditions, representing, as they do, significant adaptive

opportunities to the birds affected. The Larus gull species will rapidly

exploit these opportunities; they spend a great deal of time resting on

appropriate structures and are known to be clever and adaptive. Terns

also may Attempt to perch on the rectenna. Breeding sites for the Common

Tern are not far away, and non-breeders forage at sea. Terns are known to

perch on wharves, piers and pilings.

Other species not known to perch or rest may adapt quite rapidly.

The Blue-faced Booby (Sula dactylatra) is not known as a perching bird,

but the proliferation of oil platforms in the Gulf of Mexico has seen an.

accompanying change in the booby's habits in that it frequently rests on

the platforms between fishing forays. Although.the boobies are not found

in the area under discussion here, the gannets, also a member of the

family Sulidae, are common during seasonal migrations. One can thus

suggest that, given appropriate structures, the perching of gulls, terns,

and gannets <a wing-span range of 75 to 175 cin) is likely on an oceanbased platform.

Considering the list of birds likely to be present at the rectenna

site as enumerated in Section 2 of this Chapter, it appears that any

preventive measures should be effective for a group of birds whose wing

spans range from 30 to 200 cm. (body lengths of 15 to 90 cm). Active

measures, e.g., brightly flashing lights or frequent non-cyclical noises,

are probably not worth considering due to (a) the complexity and maintenance required, especially in a structure the size of the SPS rectenna,

and (b) their generally demonstrated ineffectiveness over long periods

with a"stable bird population — in other words, the learning experience

of the birds negates the effect of the devices.

200
Passive measures designed to prevent landing appear to be the

best approach to the potential problem of birds landing on the rectenna

faces. Vertical rods of microwave-transparent material spaced to

prevent the smallest birds landing (probably about 20-25 cm. on center)

and long enough to inconvenience the largest birds (about 30 cm) would

seem to be worthy of further study and field testing.

Testing of such an exclusion device on land would seem to be both

valid and cost effective. Gulls, of course, can be found at most dump

sites; a suitably placed mock-rectenna section could be built and left

unprotected until its use by birds was established. Protective measures

could then be taken and the results observed, establishing the effects

on the largest birds to be expected. In the same manner, a site at

which starlings or blackbirds were active could be used to determine the

effects on birds at the smallest size to be expected.

It is difficult to conceive of performing this study at sea for a

number of reasons. First, it is the presence of the rectenna itself and

its resultant enrichment of the food resources that will provide the

attractant for the birds. We cannot predict whether some critical

minimum area of structure might be necessary below which no significant

accretion of food resources and birds might occur. This is not true on

land where the food resources and the bird populations are already present

and are subject to considerable manipulation by the experimenter.

Second, the costs of establishing the experimental structures and of

maintaining observers on...the scene.seem prohibitive. Given the unique

behavioral patterns of the seabirds, spending most of their lives soaring

through an obstacle-free environment, counter measures which prove

effective for land birds, living as they do in an environment requiring

them to have great agility in avoiding obstacles, should be clearly

\

effective on the seabirds.

201
CHAPTER III: Site Selection and Design Implications.

1. Introduction; Siting Criteria for Off-Shore Rectennas

A review of the current studies of the off-shore rectenna suggests

that the design is strongly driven by wave height considerations and

wind-loading during occasional severe storm conditions. To reduce the

impact of these factors, attention should be paid to the aerodynamic

design of the structure and to techniques for attenuating waves under it.

An appropriate sea defense system could provide relatively calm water

under most of the rectenna, allowing the height of the structure above

the mean sea surface and, hence, the lever arm through which wind-loading

forces act on it to be minimized.

If ancillary uses for the rectenna island (e.g., for fisheries) are

to be seriously considered, it is highly desirable to provide reasonably

calm conditions in the presence of external disturbances. Many of these

uses would require a specified clearance between the rectenna and the

sea surface, but it appears unlikely that this would be high enough to

conflict with the requirement for a low profile to minimize wind-loading.

Some ancillary uses also Impose design requirements on the structure below

the waterline - for example, an uncluttered design may be necessary to

avoid snagging nets, etc.

It should be noted that it is not essential that the rectenna remains

operational in all conceivable weather conditions. For example, it is

clearly mandatory that the rectenna be capable of surviving a hurricane

(particularly if it is located in hurricane-prone areas), but it may be

possible to shut the system down during such rare events without seriously

affecting the loss-of-load probability (LOLP). In any case, there may be

significant attenuation of the microwave beam due to atmospheric water

vapor during a hurricane.

A first approach to the problem of sea defense is to locate it in

relatively protected waters. To maximize the availability of such sites,

it is assumed here that visual pollution criteria can be-adequately met

if no part of the rectenna is within 15 kin of a shoreline. Preferred

202
sites are thus those which offer oval areas of water approximately 40 km

in the E-W direction and 45 km in the N-S direction, surrounded, as

far as possible, by land. Where exposure to the ocean cannot be avoided,

broad coastal bays may be preferable to straight coastlines, so as to

reduce the length of rectenna perimeter which must be protected from

ocean waves.

The primary purpose in finding sheltered waters is, of course, to

reduce the fetch over which wind can generate surface waves. The wave

height (trough to crest, in meters) produced by a wind speed V (in knots)

4

is given by the empirical relationship

h = 0.0075 V2 [3]

but the maximum wave height which can be produced over a fetch of

length X (in nautical miles) is

1/2

h = 0.46 X i/Z [4]

m

Combining these relations, winds in excess of

V = 7.8 X 1/4 [5]

m

will not be effective in increasing the height of the waves.

• For example,- if-the fetch is 15 km (8.1 n.m.), .the maximum wave,

height will be 1.3 meters, produced by any sustained winds in excess of

about 13 knots.

An additional advantage of sheltered waters is that the expected wave

spectrum (at least in limited - fetch directions) is of the JONSWAP type,

peaking at periods of a few seconds. These short-period waves are much

easier to attenuate than ocean waves, where the spectrum is of the PiersouMoskowitz type, peaking at periods approaching 20 seconds (swells).

Based on these considerations, sites along the eastern seaboard of

the United States which may merit further investigation include: Cape Cod

203
Bay and Nantuckat Sound in Massachusetts; Delaware Bay; Chesapeake Bay;

Pamlico Sound (behind Cape Hatteras); Florida Bay (protected by the

Florida Keys); Apalachee Bay, near Tallahassee; and Chandeleur Sound, near

New Orleans. Lake Okeechobee in Florida and Lake Ponchartrain in

Louisiana are marginal possibilities, although recreational uses of

these waters may preclude rectenna installations there. Finally, there

are a variety of potential sites in the Great Lakes, although winter

freezing of the lake and/or ice accretion on the structure may cause

problems.

There are relatively few sheltered sites on the West Coast, but

fortunately there is little difficulty in finding good on-shore sites

in that area.

Choosing an actual site of course would involve consideration of a

broad variety of other factors. Some of the above sites may suffer

abnormally high tides or amplification of ocean waves due to shoaling

water or coastline constrictions. The depth of water at the site will,

of course, have an important influence on the choice between floating and

bottom-mounted structures. The distance to the intended major 10a,d will

control transmission costs. The rectenna island may pose a hazard to

navigation, so that maritime traffic patterns must be taken into account.

The structure may affect (beneficially or adversely) existing uses of the

site such as fisheries or spawning grounds. The suitability of the

rectenna for ancillary uses will also depend on its location. For present

purposes, however, it is sufficient to note that the optimal rectenna

design is very likely to be site-specific. It, therefore, does not seem

reasonable to attempt a generalized conceptual design of an off-shore

rectenna; instead, a specific site should be carefully chosen and its

characteristics evaluated as an input to the design.

2. Sea Defense

In general, the rectenna must be designed to operate in the presence

of tides, ocean swells and surface waves. It must also be capable of

surviving rare events such as surge from a nearby hurricane or, in some

204
areas (e.g. Japan), even a tsunami. Tides are of little consequence

to floating structures (apart from possible mooring problems) but a

bottom-mounted rectenna clearly must have sufficient clearance above

the water for both the maximum expected tide and waves.

As noted in Chapter I, Sec. 3.1, the velocity of propagation of

a (small amplitude) wave is given by

2 .22 gX r.,

c = X v =T- tanh — :— [6]

Z7T A

where X and v are the wavelength and frequency of the wave, d is

the water depth and g is the acceleration due to gravity. For surface

waves (X«d) , the wavelength is thus given by

X = g/2TM2 [7]

and for long waves (X»d) by

X = v/id/v [8]

As an example, ocean swells in deep water, with a typical period of

18 seconds, have a wavelength of about 500 meters. As the water shoals,

the swell wavelength decreases (to about 250 meters in 20 meter depth) ,

the waves assume a steeper, trochoidal shape and the amplitude increases

slowly. On the other hand, the surface waves generated over a limited

fetch, with typical periods of 2 to 3 seconds, have wavelengths which

are only of order 10 meters.

There are three possible approaches to rectenna swell defense:

i) In sufficiently shallow water, a conventional massive breakwater,

built up from the bottom, could conceivably be used. A rough

estimate of the cost of this alternative may be obtained from

3

a feasibility study of an artificial island for industrial

purposes off the Hook of Holland. The proposed island is

comparable in its dimensions to a rectenna and the chosen site

has a water depth of about 25 meters. The cost of the sea

205
defense is quoted as about $900 million. While this figure

is rather high for the rectenna application (a capital burden

of $180/kw), it could conceivably be reduced to an acceptable

level by building in shallower water and using this type of

breakwater only along the part of the rectenna periphery which

is exposed to the open ocean.

A variant of this approach, perhaps useable in very shallow water,

is the "polder" type of construction, in which a dyke is built around

the entire area and water pumped out (as in the Zuyder Zee in the

Netherlands), leaving a dry surface for rectenna construction. As an

example of the potential cost, a study of this technique (in connection

with off-shore airports) leads to an estimate of about $200 million

for the rectenna application, if the water depth is 5 meters.

ii) If the rectenna is floating, it could be designed so that the

surface follows the contour of ocean swells. The angular

frequency of vertical bobbing of a spar buoy is given by

w

2 = gA /V. [9]

S D

where A is the cross-sectional area of the spar and V, the

s b

(steady-state) submerged volume. For example, a bobbing period of 12

seconds requires that V,/A - 36 meters, an easily attainable value.

D S

With appropriate damping (e.g., by suspending a disc on a cable below

the buoy, as in a wave staff), it should be possible to design a spar

buoy which follows swells up and down, but which is relatively unaffected

by surface waves. The buoy would exhibit reduced response to swell

motions at harmonics of the fundamental swell frequency - in other words,

the rectenna surface should have a sinusoidal profile, even if the swells

are beginning to crest. The maximum slope in this profile, for swells of

wavelength of 250 meters and height 10 meters, is only about 7°, so

cosine losses due to misalignment of the rectenna elements would be small

even under quite extreme swell conditions.

206
ill) The seaward periphery of the rectenna could be designed as a

massive floating breakwater. There are two distinct classes

of floating breakwater: those which dissipate wave energy and

those which attenuate by reflection of the incident wave, as

well as combinations of those types. As far as is known, there

has been relatively little work as yet on floating breakwaters

for swell defense, but some general conclusions may be drawn.

At first sight, dissipative floating breakwaters (DFB's) for swell

defense appear impractical, because of the low frequencies involved.

However, the peak vertical velocity in a wave is of course proportional

to the product of amplitude and frequency, so this parameter for a swell

may be comparable to that for a surface wave, the higher amplitude offsetting the lower frequency. It may thus be possible to extract energy

from the swell, at least during part of the cycle, using dissipative

devices of the same generic type as those used for attenuating surface

waves. The energy in the swell is, however, so much greater than that

in a surface wave that the relative attenuating effect of a single

device will be very small. In other words, a much larger number of

dissipative devices would be required for swell attenuation. It is

probable that the first few hundred meters of the rectenna, a distance

comparable to the swell wavelength, would need to be equipped with such

devices.

A possible dissipative device consists of a floating collar

around the shaft of. a spar buoy (with a bobbing period considerably

longer than the swell period) or bottom-mounted spar. A simple means

for extracting energy from the relative motion of the collar and shaft is

hydrodynamic (i.e., using water jets or induced turbulence). However, as

noted in Chapter I, Sec. 3.1, average power passing under the rectenna

due to ocean swells of height 5 meters amounts to perhaps 8 gigawatts

(it varies as the square of the wave height). If a means could be found

to extract this energy in useful form, it would significantly increase

the power output of the system and the increased revenues might justify

an elaborate swell-attenuation system.

207
Reflective floating breakwaters (RFB's) must have a beam comparable to the wavelength, so that the first several hundred meters

of the rectenna would need to be a massive floating raft. Although

part of the cost might be charged to the rectenna itself, as the

RFB could support rectenna elements, it appears probable that the cost

of this approach would be prohibitive. Surface-wave RFB's exhibit

annual costs of order $1000/meter (including amortization of capital

and maintenance); extended over a semi-perimeter of the rectenna

(19 km), the estimated annual cost of such a small RFB would thus be of

order $19,000,000, amounting to about one half mill for each kilowatthour produced by the rectenna. The cost of a swell-defense RFB would

surely be orders of magnitude greater.

The technical feasibility of these systems requires much further

investigation but the tentative preliminary conclusion is that the most

cost-effective approach to the problems of tides, heavy storm surges, and

ocean swells is to use a floating rectenna whose surface conforms to

the contour of the sea. However, given the scale of the rectenna and the.

fact that its defense against swells can justify part of the cost,

extraction of swell energy with a DFB is an intriguing possibility. If

combined with electric power generation, this could be one of the

promising ancillary uses of the rectenna.

The obvious defense against surface waves is a floating breakwater.

The dissipative type is preferred because it may provide broader spectral

bandwidth and may consist of nothing more complex than floating collars

around the first few rows of spars which are needed anyway for support

of the rectenna elements. The total power incident on the rectenna

diameter in the form of surface waves of period 2 seconds and height 1.5

meters is calculated as about 100 MW, which may be too small a contribution

to system power output to justify anything other than the simplest

hydro-dynamic loss mechanisms. However, if means were "provided for

extracting at least some of the energy as electric power, it could

provide an auxiliary source for powering rectenna housekeeping functions,

208
navigation beacons, etc., during SPS outages (e.g., during occultations

of the satellite).

Techniques for extracting surface wave and possibly swell energy

as an ancillary use of the rectenna island are worthy of detailed study.

It should not be difficult to provide an order of magnitude

attenuation of surface waves with a simple DFB giving quite calm water under

most of the rectenna. If swell defense is of type (ii), swells will

still be ^resent, but the rectenna structure will move up and down with

them, so that relative motion of boats or other systems working under

the rectenna may be minimized.

209
3. Aerodynamic Design

One of the principal purposes of the sea-defense systems is to allow

the rectenna island to have a low exposed profile, to minimize wind loading effects. A complementary approach to this problem is to optimize the

aerodynamic characteristics of the recetenna.

The overall drag on the rectenna is primarily important to the design

of the mooring system. It is the local forces which determine the strvctural requirements for individual rectenna modules (spar buoys, towers,

etc.).

Even if a "conventional" billboard array is used for the rectenna, it

is clearly unduly pessimistic to assume that each billboard will be subjected to the full force of the wind; depending on the wind direction,

each billboard, except those on the windward periphery, will be to some

extent in the wind shadow of others. However, the conventional array is

very likely to generate strong turbulence over the rectenna in strong winds,

leading to billboard buffeting even if the wind is steady. Because of the

billboard orientation, these problems would probably be most severe in

northerly winds.

Several techniques might be suggested for alleviating wind-loading

problems:

i) Billboards could be designed as open structures, so as to

minimize wind resistance. Resonant dipoles behind each

dipole rectifier could be used to eliminate the groundplane,

but an open mesh groundplane may provide better microwave

shielding for workers under the rectenna.

ii) A roof over the x/hole structure, transparent to microwaves

and sunlight, would eliminate local wind-loading problems.

At least in areas where ice accretion is not a problem, the

210
roof could be relatively fragile except near the periphery

where wind loading forces, spray and perhaps occasional very

large breaking waves would have the greatest effect. Cutouts

in the roof would provide drainage and equalization of aerodynamic pressure differentials. One difficulty with this

approach is that the roof must be capable of accommodating

flexing of the rectenna and local disturbances such as sway

of the supporting masts (especially in the swell-conforming

design, using spar buoys as supports). The roof might also

be used to help protect the dipole rectifiers from corrosion

and salt and guano accretion, perhaps reducing unit costs.

However, the roof constitutes an additional major element in

the rectenna design so that this solution may be relatively

costly.

iii) The billboards could be horizontal instead of perpendicular

to the microwave beam, thereby reducing the frontal area

exposed to wind. The total area of active elements would,

of course, be increased by the secant of the zenith angle of

the satellite, but this could be offset by increasing the

antenna gain of individual elements (e.g., using simple yagi

antennas), thereby reducing the density of elements required.

The beamwidth of each yagi-rectifier must be broad enough to

accommodate deflections of the system, but this is not expected

to constitute a constraint because the gain required to compensate for the cosine effect is so low.

iv) As noted earlier, it may be sufficient to design the rectenna

to survive severe storms without .being operational during them.

For a floating rectenna in deep water, a radical approach to

hurricane defense which may be worth consideration is to submerge it. For this to be feasible, major rectenna systems,

dipole rectifiers, wiring, etc., must be waterproof rather

211
than just splashproof. Sensitive special systems (e.g., the

pilot-beam transmitter, high voltage switchgear- 3tc.) might

be installed in sealable waterproof compartments or dismantled

on definite prediction of weather conditions beyond design

limits. Some systems (e.g., crew housing) could be built on

barges, ready to be towed away.

In crder to sink the rectenna, flotation must be reduced by a

volume greater than that normally above water. This can be

achieved by having some of the flotation ia the form of airfilled tanks, open at the bottom, connected by hoses to

centrally-located valves. If the water is of reasonable depth

and bottom conditions permit, the rectenna could rest on the

bottom during a hurricane; alternatively, it could be moored

above the bottom. A rough estimate of the work required to

pump air back into the flotation system, in 50 meters of water

(5 atmospheres) is 11000 kwh, so that bringing the rectenna

back to the surface would take about A hours if an air compressor of output 3000 hp were used.

4. Conceptual Designs for Off-Shore Rectennas

Based on the above considerations, a tentative conceptual design is

proposed for a floating rectenna in deep water, consisting of a checkerboard of square, horizontal billboards. A portion of the layout is shown

in Figure III. Every second billboard (white squares in the figure) is

mounted on a tabular buoy, i.e., it consists of a square constructed from

a truss framework, supported at each corner by a spar buoy. The intervening billboards (cross-hatched in the figure) are suspended on cables from

the tabular buoys, sufficiently far below them to avoid interference with

the yagi-rectifiers on the lower billboard, during flexing of the system

under wave and wind forces. All billboards have cut-off corners, to avoid

interference at these points; if economically justified, the microwave

radiation falling on the corner areas could be intercepted by yagi-rectifiers

mounted on raised "ears" attached to the northern corners of the tabular buoys.

212
Tabular

Buoy

Billboard

Suspended

Billboard

Fig. Ill; SWELL CONFORMING HORIZONTAL BILLBOARD RECTENNA

213
The billboards in this system are constructed from trusses supporting

an open raesh groundplane, the yagi-rectifiers, and power-conditioning and

distribution networks. Flexible connections (perhaps hinged, so as to

avoid fatigue problems) are needed to transfer power from one billboard to

another and eventually to the cable-head for transmission ashore.

The individual volumes of the floats supporting the tabular buoys are

determined by the mass of the system, and the cross-sectional area of Lhe

shafts supporting the tabular billboards are then chosen to give a waveresponse characteristic frequency which is high compared to swell frequencies, but low compared to surface wave frequencies. The rectenna is thus

of the swell-conforming type. Surface waves are attenuated by damping

devices -along the periphery of the rectenna, especially in directions

exposed to a significant fetch.

This design offers low wind resistance, and an uncluttered area under

the rectenna for ancillary uses. The underwater structures may also be

clean, facilitating fishing operations, etc., although the intended use

needs to be taken into account in designing mooring systems. With appropriate design, the forest of yagi antennas may provide fe\7 roosting places

for web-footed sea birds.

Much further work is needed to establish the optimal size of the

individual billboards in this system (so as to minimize mass and cost,

subject to constraints imposed by operations beneath the rectenna), to

analyze the dynamical response to wind .and waves, to determine a mooring

system and pattern of anchors to avoid unacceptable compressive stresses

across the rectenna while minimizing interference with ancillary uses, to

design the yagi-rectifiers and the power distribution system, to develop

appropriate surface-wave damping devices, and to calculate the stresses

imposed by severe weather. The purpose of this note is only to suggest

that it is conceptually possible to design a floating rectenna with a

214
relatively low clearance above the water and an open deck which may not

be much more than a meter thick.

Any rectenna floating on an array of spar buoys is dynamically similar

to a flexible sheet suspended by.springs, and will exhibit a variety of

oscillation modes. Short wavelength modes will have higher natural frequencies than the fundamental mode, in which the rectenna moves vertically

as a unit. It may therefore be possible for surface waves which penetrate'

beneath the rectenna to excite resonant oscillations of large amplitude.

It may be possible to reduce response in at least one of these modes by

choosing the separation between buoys as an integral multiple of the surface wavelength at the corresponding frequency (as given by [7]); waves at

this frequency would then be in phase at each of the buoys, tending to

excite only the fundamental mode. It should also be possible to provide

heavy hydrodynamic. damping of high-frequency modes.

Mooring lines of reasonable scope can accommodate motions of the

rectenna in response to swells, but it may be necessary to provide constanttension devices on the lines to allow for tidal variations in water depth.

Swell-conforming spar buoys could also be used to support a taut-wire

rectenna of the type recently proposed by Rice University. The principal

apparent differences include the following:

• The above-water spars may need to be. somewhat taller in order

to give adequate clearance above the water surface at the lowest point of the catenary nets hanging between supports.

• Each buoy should be designed to have as much rigidity in roll

and pitch as possible, to prevent instabilities due to the

fact that the center of buoyancy is well below the point on

the spar where the load is attached.

• The structure will exhibit more complex modes of oscillation,

and those which primarily involve motions of the above-water

cables may be difficult to damp.

215
• The rectenna would probably be considerably lighter than the

horizontal-billboard design.

• Failure of a mooring line or of one of the perimeter cables

could result in heavy unbalanced horizontal forces on the

nearest spar buoy, causing it to tip over. This could unbalance the horizontal forces on adjacent spars, leading to

collapse of an entire independent section of the rectenna.

Avoidance of this failure mode may require redundant mooring

or support cables, increasing costs.

Floating rectennas are essental in very deep water, but pile-mounted

masts could be used to support either the taut-wire or horizontal-billboard

rectenna in shallower water — for example, in most of the sheltered-water

sites suggested earlier in this chapter. Compromise designs are also possible, in which some of the compressive stress on underwater, bottommounted masts is carried by submerged floats or perhaps by using buoyant

structural members. For either design, the support masts must be taller

if they are bottom-mounted, in order to allow sufficient clearance between

the rectenna and the water surface in the presence of tides, surge from

distant hurricanes, etc., and waves (both surface waves and swells), and

this will to some extent offset the advantages of these designs in terms

of aerodynamic loading. Bottom-mounted systems thus seem most appropriate

in shallow waters exhibiting small tidal variations in depth, protected

from ocean swells (unless swell-defense/swell-energy systems prove practical) . If it is intended to permit-operations beneath the rectenna in

support of secondary uses, low-profile boats (e.g., without tall masts)

could be used, reducing the necessary clearance. It may also be possible

to suspend such operations at times of exceptionally high water (neap tides,

hurricane surges, etc.), in order to minimize the clearance under normal

conditions.

If appropriately designed for a specific site, both taut-wire and

horizontal-billboard rectennas seem readily adaptable to proposed secondary

216
uses of the structure. The taut-wire design may have an advantage for some

applications (e.g., trawling beneath the rectenna), because of the wider

separation between support masts.

5. Conclusions

Careful attention is needed to site selection to allow realistic work

on off-shore rectenna design and hence estimation of the costs and benefits

involved. The extent of the structure, the number of repetitive eleraer-ts,

and the costs for each rectenna are sufficient so that a standard design is

unnecessary: different types could be used at different sites. Shallow,

well-protected waters are probably the first choice, where simple pilemounted structures may be used, although alternatives such as polder construction may be competitive in some cases. The second choice is a deepwater site, where a floating system is needed; swell-conforming designs are

indicated unless there is economic justification for conversion of swell

energy to electric power as an ancillary use of the system. It is particularly important to avoid shoal water exposed to the open ocean, so that the

rectenna need not be designed to withstand large, steep, breaking waves

during storm conditions.

Aerodynamic and hydrodynamic considerations must be taken into account

in the conceptual phase of design, as well as the range of expected ancillary uses. Measures can be taken to minimize wind-loading and the lever

arm through which these forces act on underwater structures. The type of

sea-defense employed will have a- significant impact both on the design and

on its suitability for secondary uses; research is therefore needed at the

device level on wave damping and energy extraction systems before drawing

firm conclusions about the overall system or the potential of off-shore

rectennas.

217
REFERENCES

1. Sargent, W., Private Communication.

2. Coulson, C. A., Waves, 7th Ed., Oliver & Boyd (London, 1955), p. 60.

3. Stuiver, M. , and Steyn, B., "Design of an Artificial Island for

Industrial Purposes in the North Sea," Proceedings of the 1976 OffShore Technology Conference, Vol. II, Houston, Texas, May 1976.

4. Van Nostrand's Scientific Encyclopedia, 5th Ed., Van Nostrand

Reinhold (N.Y., 1979), p. 228.

5. Grey, J. (Ed.), Proceedings of the First International Conference on

Offshore Airport Technology, April 1973, AIAA, N.Y.

6. Adee, B. H. , "A Review of Developments and Problems in Using Floating

Breakwaters," Proceedings of the 1976 Offshore Technology Conference,

Vol. II, Houston, Texas, May 1976.

218
6. Summary of Results

Rice Universit y with subcontracts to Brown and Root

Development Inc. and Arthur D. Littl e Inc. has performed a

Preliminary study of the feasibilit y and cost of an offshore

rectenna to serve the upper metropolitan east coast. The study

proceeded by first locating a candidate site at which to buil d a

5 GW rectenna. The site was selected on the basis of proximity

to load centers, avoidance of shippin g lanes, sea floor terrain,

and soil conditions , etc. Several types of support structures

were selected for study based initiall y on the reference system

rectenna concept of a wire mesh ground screen and dipole s each

with its own rectifier and filter circuits. The study also

looked at possibl e secondary uses of an offshore rectenna.

The principa l results of this study are as follows:

1. Suitabl e candidate sites exist off the northeast coast

and probably all along the east coast and Gulf of

Mexi co.

2. Hurricane and winter storm conditions were examined for

this area and a set of environmenta l criteria were

establ i shed.

3. The winter storm criteria plu s tests done at Rice

University under icin g condition s lead to the conclusio n

that a protective radome wil l be required over the

active elements of the rectenna includin g a portion of

t he ground plane. This conclusion probably also holds

for land rectennas located everywhere except perhaps in

the desert southwest.

219
4. For the reference system rectenna (using a wire mesh

ground plan e and individua l dipoles) , a doubl e pendulum ,

t wo level rectenna panel , whic h can swing freely is

suitable.

5. Approximatel y 25,000 support towers would be required

for a 5 GW antenna using the above reference system

rectenna.

6. Four different types of support tower structures were

studied and costed. The least expensiv e of these was

the pile d guyed tower.

7. For the 49.4 m (162 ft) water depth site examined the

total cost of a 5 GW rectenna usin g the pile d guyed

tower and reference rectenna panel is estimated at $36

billion . This is considered too expensiv e for serious

consideration. The reference system is not suitabl e for

offshore use.

8. The water depth, wind loadin g and soil conditio n cost

sensitivitie s were examined. None of these factors

coul d be altered sufficiently to significantl y reduce

the cost.

9. Based on the foregoing, the only substantia l way to

reduce the cost of the offshore rectenna is to reduce

the number of support towers or go to a fully surface

floating system. Reducing the number of support towers

requires a change in the type and mass of the rectenna

panels.

220
10. The number of support towers can be reduced from 25,000

to 3,000 by eliminatin g the ground screen and adopting

an image dipol e reflector antenna, where each of the

dipol e plu s reflector elements are supported

individuall y by cables which also carry the power from

t he dipoles. This is calle d the clotheslin e concept.

Each dipol e plu s reflector is individuall y encapsulated

to protect it from the weather.

11. The cost of this clotheslin e concept for the 49.4 m

i

water depth site is estimated at $5.7 billio n (first

unit cost).

12. This demonstrates the cost reduction potential possible

with new rectenna concepts. The clotheslin e concept is

only one of several possible concepts. Time and fiscal

constraints have prevented us from examinin g other

concepts such as a surface floatin g rectenna.

13. Secondary uses, in particula r mariculture , appear

promisin g adjuncts to the offshore rectenna. The

possibilit y of wave energy extraction has also been

examined briefly. Such secondary uses do not appear to

constrain the basic rectenna design significantly .

14. A major problem identifie d with the reference rectenna

offshore version is the sea birds which wil l be

attracted to the vicinit y of the rectenna and wil l land

and roost on it. This requires further study, but it

appears that the more open structure of the clothesline

concept wil l reduce the bird problem somewhat.

221
The following areas require further study:

1. We have not yet determined the.optimum design from the

standpoint of cost and reliability . A surface floating

system has not yet been studied. Because of the

different cost per unit area for a sea antenna the

optimum size may not be 10km.

2. A great deal of research needs to be done on the

efficiency of various types of receivin g elements. We

do not know the conversion efficiency of the dipol e

without a ground plane. A Rensselaer Polytechnic

Institute study looked at highe r gai n antennas and

suggested that they might have substantial advantages

for land rectennas. They need to be examined for

offshore rectennas as well.

3. Much remains to be done in the area of survivabi1it y and

environmental protection of the rectenna, particularl y

agains t birds and corrosion.

4. Secondary use potential needs further research. A

careful study should be performed on the feasibility of

combinin g the rectenna with a hydrogen generation

plant. The electricity from the rectenna could be used

to generate hydrogen via electrolysis. The wave energy

extraction adjunct needs further study. Also, mineral

extraction from sea water should be examined.

222
7. Conclusions

We have demonstrated that an offshore rectenna near east

coast load centers is feasible and cost competitive with land

rectennas, however, the ground plane reference design is not an

appropriate design. An alternate design of the non-ground plane

type has been investigated. Other designs such as a floating

design may also be feasible and cost effective. The secondary

and fuel generation uses remain to be fully explored.

We believe that this study demonstrates that feasibility is

sufficiently great and cost sufficiently low that, with the

significan t advantages of no land requirements and removal of the

radiation from populated areas, further investigatio n of the

offshore rectenna should be vigorously pursued. Also, the

alternativ e designs suggested for the offshore rectenna should be

applie d to land rectennas to see if cost saving s can be realized.

223
Appendi x A

Detailed Environmental Data

224
ENVIRONMENTA L DATA FOR SITE III

I. LOCATION - 40° 59' N 70° 44' W

II. DISTANCE FROM NEW YORK - 175 mile s (282 km)

DISTANCE FROM BOSTON - 75 mile s (121 km)

*From NOAA Chart #12300

III. CLOSEST DISTANCE TO NEAREST LAND « 25 mile s (40km)

*From NOAA Chart #12300

IV. WATER DEPTH - 27 fathoms (49.4 m)

*From NOAA Chart #12300

225
V. WIND SPEED (ANNUAL AVERAGE)

38.0° N 71.0° N

(METERS/SEC) WINDSPEED PER CENT

(KNOTS) FREQUENCIES

<2.1 <4 4.5%

2.1 - 5.1 4 - 10 21.8%

5.7 - 10.8 11 - 21 40.1%

11.3 - 17.0 22 - 33 24.2%

17.5 - 24.2 34 - 47 8.6%

>24.2 >47 0.9%

*averaged from data obtained by ocean weather station HOTEL and

compile d in Mariner's Weather Log, years 1973-1977.

MEAN: 18.6 knots (9.6 m/s)

MAXIMUM : 78.0 knots (40.1 m/s)

226
V. HIND SPEED (ANNUAL AVERAGE )

(Continued)

2.1 - 5.1

5.7 - 10.8

11.3 - 17.0

17.5 - 24.2

>24.2

39.0° N

(METERS/SEC) MINDSPEED

(KNOTS)

<2.1 <4

4-1 0

11 - 21

22 - 33

34 - 47

>47

70.0° W

PER CENT

FREQUENCIES

3.0%

21.7%

52.4%

20.4%

2.7%

0.0%

*averaged from data obtained from NOAA ocean buoy 44004 and

compiled in Mariner's Weather Log, years 1977 and 1978.

MEAN: 16.7 knots (8.6 m/s)

MAXIMUM : 46.0 knots (23.7 m/s)

227
V. WIND SPEED (ANNUAL AVERAGE)

(Conti nued)

(METERS/SEC)

2.1 - 5.1

5.7 - 10.8

11.3 - 17.0

17.5 - 24.2

>24.2

40.1° N

WINDSPEED

(KNOTS)

<4

4 - 10

11 - 21

22 - 33

34 - 47

>47

73.0° W

PER CENT

FREQUENCIES

4/6%

36.4%

50.5%

8.4%

0.2%

0.0%

*averaged from data obtained from NOAA ocean buoy 44002 and

compile d in Mariner's Weather Log, years 1976-1978.

MEAN: 13.1 knots (6.7 m/s)

MAXIMUM : 43.0 knots (22.1 m/s)

228
V. WIND SPEED (ANNUAL AVERAGE)

(Continued)

40.8° N 68.5° W

(METERS/SEC) WINDSPEED PER CENT

(KNOTS) FREQUENCIES

<2.1 <4 10.2%

2.1 - 5.1 4 - 10 45.7%

5.7 - 10.8 11 - 21 40.4%

t

11.3 - 17.0 22 - 33 3.6%

17.5 - 24.2 34 - 47 0.2%

>24.2 >47 0.0%

*averaged from data obtained from NOAA ocean buoy 44003 and

compile d in Mariner's Weather Log, years 1977-1978.

MEAN: 10.9 knots (5.6 m/s)

MAXIMUM : 35 knots (18.0 m/s)

229
VI. WIND DIRECTION (ANNUAL AVERAGE)

38.0° N 71.0° W

DIRECTION* PER CENT FREQUENCY**

N

NE

E

SE

S

SW

w

NW

13.0%

8.8%

5.3%

4.4%

9.3%

13.9%

18.5%

25.4%

*the direction refers to where the wind is blowin g from

**averaged from data obtained by ocean weather station HOTEL and

compile d in Mariner's Weather Log years 1973-1977.

230
V I. WIND DIRECTION (ANNUAL AVERAGE)

(Continued)

39.0° N 70.0° W

DIRECTION* PER CENT FREQUENCY**

N 12.1%

NE 10.2%

E 12.2%

SE 8.5%

S 11.9%

SW 11.8%

W 18.6%

NW 15.0%

*direction refers to where the wind is blowin g from

**averaged from data obtained from NOAA ocean buoy 44004 and

compiled in Mariner's Weather Log years 1977 and 1978.

231
V I. WIND DIRECTION (ANNUAL AVERAGE)

[Continued)

40.1° N 73.0° W

DIRECTION* PER CENT FREQUENCY**

N 9.3%

ME 8.4%

E 8.0%

SE 5.3%

S 18.7%

SW 21.6%

W 17.2%

NW " 11.4%

*direction refers to where the wind is blowin g from

**averaged from data obtained from NOAA ocean buoy 44002 and

compile d in Mariner's Weather Log years 1976-1978.

232
V I. MIND DIRECTION (ANNUAL AVERAGE)

(Continued)

40.8° N 68.5° W

DIRECTION* PER CENT FREQUENCY**

N 9.8%

NE 7.9%

E . 7.9%

SE 6.4%

S 11.4%

SW 20.2%

W 22.7%

NW 13.4%

*direction refers to where the wind is blowin g from

**averaged from data obtained from NOAA ocean buoy 44003 and

compiled in Mariner's Weather Log, years 1977 and 1978.

233
VII . MONTHLY AND ANNUAL SCALAR MEAN WINDSPEED AND PREVAILIN G

DIRECTION FOR THE GEORGES BANK/NANTUCKET SHOALS AREA

MONTH WINDSPEED

(KNOTS) (m/s)

PREVAILIN G

DIRECTION

January

February

March

Apri 1

May

June

July

August

September

October

November

December

17.0

16.2

15.2

12.8

10.6

9.9

8.9

9.6

11.1

13.1

15.1

17.2

(8.7)

(8.3)

(7.8)

(6.6)

(5.5)

(5.1)

(4.6)

(4.9)

(5.7)

(6.7)

(7.8)

(8.8)

NW

NW

W

W

SW

SW

SW

SW

SW

W

W

NW

ANNUAL 13.1 knots West

*From "Wind and Wave Statistics for the North American Atlantic

and Gulf Coasts", Robert G. Quayle and Daniel C. Fulbright ,

Mariner's Weather Log, January, 1977, Vol. 21, #1.

234
VIII . WIND GUSTS

We were not able to acquire any empirica l data on the

frequency, strength or directio n of wind gusts for site III.

However, it was stated in one of the sources of

meteorological data* that gusts usually average about 1.4 times

the sustained windspeed.

*From "Extreme Wind and Wave Return Periods for the U. S.Coast",

Robert G. Quayle and DanielC. Fulbright , Mariner's Weather Log,

March, 1975, Vol. 19, #2.

235
IX. WAVE HEIGHT (ANNUAL AVERAGE)

38.0° N

WAV E HEIGHT

(METERS)

71.0° W

PER CENT

FREQUENCY

< 1

1 - 1.5

2-2.5

3-3.5

4-5.5

6-7.5

8-9.5

> 9.5

4.6%

39.4%

26.5%

16.3%

10.4%

2.4%

0.4%

0.0%

MEAN: 2.3 meters

MAXIMUM : 9.0 meters

*averaged from data obtained by ocean weather station HOTEL and

compiled in Mariner's Weather Log, years 1973-1977.

236
IX. WAVE HEIGHT (ANNUAL AVERAGE)

(Conti nued)

39.0° N

WAVE HEIGHT

(METERS)

70.0° W

PER CENT

FREQUENCY

< 1

1 - 1.5

2 - 2.5

3 - 3.5

4 - 5.5

6 - 7.5

8 - 9.5

> 9.5

1.7%

14.7%

34.8%

29.2%

15.3%

4.4%

0.1%

0.0%

MEAN: 3.0 meters

MAXIMUM: 8.0 meters

*averaged from data obtained from NOAA ocean buoy 44004 and

compiled by Mariner's Weather Log, years 1977 and 1978.

237
X. WAVE HEIGHTS (ANNUAL AVERAGE) FOR THE GENERA L GEORGES

BANK/NANTUCKET SHOALS AREA

WAVE HEIGHT

(METERS)

PER CENT

FREQUENCY

0

0.5

1

1.5

2

2.5

3

3.5

4 - 4.5

5 - 5.5

6 - 6.5

7 - 7.5

8 - 9.5

10 - 12

12.1%

23.3%

27.2%

15.7%

8.9%

4.7%

3.3%

1.6%

2.1%

0.5%

0.4%

0.1%

0.1%

< 0.05%

*from "Wind and -Wave Statistics for the North American Atlantic

and Gulf Coasts", Robert G. Quayle and Daniel C. Fulbright ,

Mariner's Weather Log, January, 1977, Vol. 21, #1.

238
XI. WAVE DIRECTION (ANNUAL AVERAGE) .

38.0° N 71.0° W

WAVE DIRECTION PER CENT FREQUENCY

N 11.8%

NE 9.2%

E 4.7%

SE 4.5%

S 11.2%

SW 13.8%

W 14.1%

NW 23.3%

*direction refers to from where the wave are approaching.

**averaged from data obtained by ocean weather station HOTEL and

compiled in Mariner's Weather Log, years 1973-1977.

239
XII. WAV E PERIODS (ANNUAL ^AVERAGE)

38.0° N 71.0° W

PERIOD PER CENT

(SECONDS) FREQUENCY

< 6 30.3%

6 - 7 42.1%

8 -9 17.6%

10 - 11 2.0%

12-1 3 0.5%

> 13 0.1%

MEAN: 5.7 seconds

*averaged from data obtained by ocean weather station HOTEL and

compiled in Mariner's Weathert Log, years 1973-1977.

240
XIII. MEAN WAVE PERIODS FOR CERTAIN WAVE HEIGHT RANGES (ANNUAL

AVERAGES)

38.0° N 71.0° W

WAVE HEIGHT MEAN PERIOD

(METERS) (SECONDS)

< 1 4.9

1 - 1.5 5.5

2 - 2.5 6.3

3 - 3.5 6.9

4 - 5.5 7.7

6 - 7.5 9.1

8-9. 5 10.4

*averaged from data obtained by ocean weather station HOTEL and

compiled in Mariner's Weather Log, years 1973-1977.

241
XIV. EXTREME WINDSPEEDS AND WAVE HEIGHTS FOR THE GENERAL GEORGES

BANK/NANTUCKET SHOALS AREA

A.

Extreme Sustained Windspeed Estimate for 100 year Return

Period

 g 100 knots (51.4 meters/second)

Significan t Wave Height Estimate for 100 year Return

Period » 18.0 meters

Extreme Wave Height Estimate for 100 year Return

Period « 32.6 meters

*From "Extreme Wind and Wave Return Periods for the

U. S. Coast", Robert G. Quayle and Daniel C. Fulbright ,

Mariner's Weather Log, March 1975, Vol. 19, #2.

B.

Extreme Wave Height Estimate Caused by Hurricane for 100

year Return Period

 a 24.7 meters

Significan t Wave Height Estimate for Hurricane Generated

Waves for 100 year Return Period » 13.1 meters

242
XIV. EXTREME WINDSPEEDS AND WAVE HEIGHTS FOR THE GENERAL GEORGES

BANK/NANTUCKET SHOALS AREA (Continued)

C.

Extreme wave height estimate for winter storm (extra

tropical cyclone) generated waves for a 100 year return

period « 25.3 meters

D.

Extreme wave height estimate for waves due to both,

hurricanes and winter storms, for a 100 year return

period » 26.5 meters

*from Extreme Wave Heights Along the Atlanti c Coast of

the United States", E. G. Ward, Shell Development Co.,

and D. J. Evans and J. A. Pompa, Evans-Hamilton , Inc.,

Offshore Technology Conference Paper 2846, 1977.

243
XV. TIDAL CURRENTS AND RANGES

A.

Tidal currents rotate clockwise at » 2.5 km/hour

*from WESTERN NORTH ATLANTIC OCEAN: Topography, Rocks,

Structure, Water, Life and Sediments, K. 0. Emery and

Elazar Uchup i , 1972.

B.

The mean range (the difference in height between mean

hig h water and mean low water) « 1 meter

During the Spring the tide range is about 10 centimeters

more.

C.

The tide is SEMIDIURNAL.

*From S. D. Hicks , A. J. Goodheart, and C. W. Iseley,

Journal of Geophysical Research, Vol. 70, No. 8, April

15, 1965.

244
XVI. AVERAGE MONTHLY FREQUENCY OF POTENTIAL "MODERATE"

SUPERSTRUCTURE ICING

MONTH LOCATION

38.7° N 73.6° W 40.1°N 73.0° W

January

February

Marqh

November

December

24.0%

9.4%

0.4%

0.6%

6.0%

22.5%

15.0%

1.5%

0.8%

12.5%

*Data was sparse and showed great variation from year to year.

**from data recorded by NOAA ocean buoys EB-07 and 44002 and

compiled in Mariner's Weather Log, years 1965-1977.

***"Potential Moderate Icing" is defined by the simultaneous

combination of an air temperature of < -2°C and windspeeds > 13

knots.

245
XVII. AVERAGE MONTHLY FREQUENCY OF PRECIPITATION FOR THE GEORGES

BANK/NANTUCKET SHOALS AREA

MONTH PER CENT FREQUENCY

January 18.9%

February 20.9%

March 13.9%

Apri l 10.2%

May 8.9%

June 8.0%

July 4.1%

August 5.3%

September 6.1%

October 6.1%

November 11.9%

December 21.3%

ANNUAL: 11.3%

*from United States Coast Pilot No. 2, Atlantic Coast: Cape Cod

to Sandy Hook, January 1979, C55. 422: 2114

246
XVIII . SEAFLOOR SEDIMENT AND SLOPE

A.

The seafloor sediment predominantly consists of coarse

sand and scattered patches of gravel.

*From "Sediments on the Continental Margin Off Eastern

United States", Elazar Uchupi , U. S. Geological Survey,

Professional Papers, 475-C: c!32 - c!37, 1963.

B.

The seafloor slope off Martha's Vineyard is about _3_

feet/mile or 0.03°.

*From "Structure of Continental Shelf Off Southern New

England" , Marine Geology, vol. 4, p. 273-289 (1966), L.

E. Garrison and R. L. McMaster.

247
XIX. SUB-SEAFLOOR COMPOSITION

A lithologi c log of a well drille d near site III, in

Nantucket Shoals, indicates the existence of alternating

layers of medium or fine and silty sand and gravel or

coarse sand up to a depth of about 40 meters.

However, another well drille d just southwest of this

first well (no more than 50 km away revealed a deep

layer of gravel and coarse sand extending 27 meters from

the seafloor followed by a layer of clay or clayey sil t

and shale at least 18 meters thick.

248
XX. CURRENT PROFILE AT 39° 20' N 70°0' W

DEPTH MEAN CURRENT

(METERS) (METERS/SEC"1)

0

100

500

1000

2000

2600

0.13

0.07

0.04

0.04

0.02

0.03

*al1 currents are directed to the WEST.

**From Fofonoff and Webster, Philosophica l Transactions of the

Royal Society (A), Vol. 270, pp. 423-436 (1971).

249
TECHNICAL. REPORT STANDARD TITLE PAGE

1. REPORT NO.

NASA CR-3348

2. GOVERNMENT ACCESSION NO.

4. TITLE AND SUBTITLE

Solar Power Satellite Offshore Rectenna Study

7. AUTHOR(S)

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Rice University

Houston, Texas 77001

12. SPONSORING AGENCY NAME AND ADDRESS

National Aeronautics and Space

Washington, DC 20546

Administration

3.

5.

6.

a.

10

11

13

M

RECIPIENT'S CATALOG NO.

REPORT DATE

November 1980

PERFORMING ORGANIZATION CODE

PERFORMING ORGANIZATION REPORT »

WORK UNIT NO.

M-310

CONTRACT OR GRANT NO.

NAS8-33023

TYPE OF REPORV & PERIOD COVERED

Contractor Report

SPONSORING AGENCY CODE

15. SUPPLEMENTARY NOTES

NASA Marshall Technical Monitor:

Final Report

Charles Guttman

16. ABSTRACT

Rice University, Brown and Root Development, Inc., and Arthur D. Little, Inc. have

jointly conducted a feasibility study of an offshore rectenna serving the Boston/New York area.

We have found that an offshore rectenna is feasible and cost-competitive with land rectennas but

that the type of rectenna which is suitable for offshore use is quite different from that specified

in the present reference system. We began by engineering the reference system rectenna to the

offshore location. When we estimated costs for the resulting system, we found that the cost was

prohibitively high. We then searched for modifications to the design which would allow significant cost reduction. The result is a non-ground plane design which minimizes the weight and

greatly reduces the number of costly support towers. This preferred design is an antenna $rray

consisting of individually encapsulated dipoles with reflectors supported on feed wires. Such a

5 GW rectenna could be built at a 50m water depth site to withstand hurricane and icing conditions for a one-time cost of 5. 7 billion dollars. Subsequent units would be about 1/3 less expensive. The east coast site chosen for this study represents an extreme case of severe environmental conditions. More benign and more shallow water sites would result in lower costs.

Secondary uses such as mariculture appear practical with only minor impact on the rectenna

design. The potential advantages of an offshore rectenna, such as no land requirements,

removal of microwave radiation from populated areas and minimal impact on the local geopolitics argue strongly that further investigation of the offshore rectenna should be vigorously

pursued.

17. KEY WORDS

Offshore rectennas

encapsulated dipoles

18. DISTRIBUTIO N STATEMENT

Unclassified - Unlimited

Subject Category 44

19. SECURITY CLASSIF. (of thl. report) 20. SECURITY CLASSIF. (of this page) 21. NO. OF PAGES

Unclassified Unclassified 257

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