Special Issue

Smart Grid

The Development of the Power Transmission
System for Fukushima FORWARD Project
Shigeru Fujii*5, Yuji Tateno*, Koji Tomikuda*2, Yasuhiro Tominaga*3, Noriaki Horiguchi*3,
Hiroshi Nakano*3, Hiroyuki Kon*, Tooru Kagoura*4, Takumi Yamaguchi*5, Hiroyuki Sakakibara*5

Furukawa Electric Company (FEC) has proceeded with a Fukushima
FORWARD Project (FF project) as one of the commissioning manufacturers of
the Ministry of Economy, Trade and Industry. The Ministry has a plan of a floating wind turbine
development for a powerful choice of renewable energies in Japan, which has the 6th largest
exclusive economic zone (EEZ) in the world. Companies representing Japan entered into a consortium in which FEC is responsible of a transmission system. This paper reports the development condition of the power transmission system, mainly the development of a high voltage
dynamic cable, which is considered as an essential technology. A behavior of dynamic cables
subjected to under-water rolling caused by the steadily wave movement is different from that of
the static submarine cables. Therefore a development point is the improvement of a fatigue withstands property. For a practical application of riser cables, a target is to achieve the similar
endurance life of a floating body or of a windmill.

ABSTRACT1.

INTRODUCTION

An offshore floating wind turbine is a promising power
generation method in Japan, because Japan has the 6thlargest exclusive economic sea zone in the world and has
few suitable shallow shores for the implantation of the offshore type wind power generations.
Offshore floating wind power generations are very few
in the world.
The construction of a wind farm possessing complex
floating windmills with an offshore floating substation will
be a first in the world.
FEC is responsible for a transmission system (including
a power and an optical transmission lines) for the wholeproject and plays a role in the system interconnection of awind turbine power to a power utility by means of submarine cables and onshore associated cables.
This paper reports the development condition of the
project dynamic cable system, which is an essential technology for the offshore floating system.
* Research & Development Department, Technological
Administration Division, Viscas Corporation*2 Distribution Cable Engineering Department, Distribution
Cable Division, Viscas Corporation*3 Power Cable Engineering Department, Power Cable
Division, Viscas Corporation*4 Power & System Laboratories, Research & Development
Division*5 Offshore Wind Project Team, Energy Division

The FF project is proceeding by establishing a prototype system for a demonstration research in Fukushima
prefecture offshore.
The development of the demonstration results is designated as a new industrial job creation task originated in a
renewable energy for the Great East Japan Earthquake
restoration.

2.

OVERVIEW OF THE FF PROJECT

2.1 Project Overview
The FF project consists of a 1st stage (2011-2013) and a
2nd stage (2014-2015).
In the 1st stage, a 2 MW down-wind type offshore floating wind turbine generator and a 25 MVA offshore floating
substation, for the first time in the world, are planned tobe constructed.
FEC installs a dynamic cable (referred to as riser cable)
to connect between a floating body, a long distant submarine cable to reach an onshore facility and an onshore
associated power line.
In the 2nd stage, it is planned to construct two new 7
MW floating wind turbine generation facilities and to
install the riser cable toward an offshore substation.
Figure 1 shows the project overview.

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The Development Of The Power Transmission System For Fukushima FORWARD Project3.

Floating
substation
Advanced
spar

Compact
semisubmersible
floating
structure

V-type semisubmersible
floating
structure

Figure 1 Project Overview.

2.2 Transmission and Substation System
The wind power generation capacities are 2 MW and 7 MW
respectively.
A 22 kV cable (inter array cable) is selected for thetransmission line towards the offshore substation.
The route distance from the propose shore position to awindmill installing potential position is approx.25 km longwith a submarine cable.
It is planned in the FF project to construct an offshore
substation and to raise the cable (export cable) voltage up
to 66 kV for the transmission to the shore port because a
22 kV cable has higher transmission loss.
The export cable on shore is connected with an alreadyinstalled nearby 66 kV overhead transmission line which
is operated by a power utility, a transformer station will be
installed on the way, and so a systematic grid link will becompleted.
Figure 2 illustrates the transmission and substation system.
FEC is responsible for the transmission system part.

Shore switching station&
Shore system gridtransmission line

DEVELOPMENT OF THE RISER CABLE

3.1 Fundamental Design
FEC has a lot of experiences in submarine cables, submarine oil pipes, deep-sea water suction pipes etc., andwealth of investigation experiences in dynamic applications.
For submarine cable experiences in 6.6 kV voltage
class, FEC has a lot of operational know-how in loading
and unloading of floating berth for crude oil since 1992,
starting from the “KAIMEI” in 1977 for wave turbine facilities on Yura offshore in Yamagata prefecture, and the“POSEIDON” in 1986.
An analytic simulation technology is also important forthe dynamic application that an entire investigation of thesubmersible floating shape requires.
Figure 4 illustrates the riser cable design flow.
A fixed wave condition and a floating body’s rolling
condition are needed at first for external conditions.
A static behavior analysis, a dynamic behavior analysis
and a fatigue analysis are in turn conducted combining of
the various parameters and the shapes for a riser cable
under above conditions.
The analysis and feedbacks are reiterated and finallythe most suitable submersible shape design and its
detailed riser cable design are defined.
The riser cable required in the FF project is different
from the past experienced 6.6 kV cables, with respect toits application to a 22 kV and to a 66 kV voltage cables,
because their cable outer diameters and weights becomelarger and their electrical stress are more severe.
The riser cable is therefore required for the power cable
construction to provide a superior fatigue withstand characteristic.

2 MW windmill &semi-submersible
floating structure

Floating sea construction
Total length 34 m
Total width 24 m
Height13.5 mCold water 5.5 m
Water depth 41 m

Floating substation(66/22 kV substation / observation tower)

66 kV
submarine
cableIntermediate
buoy

66 kV
riser
cable

Intermediate Intermediate
buoy
buoy
22 kV
riser
cable

Sea-bed splice

Bend-stiffener

Figure 2 Transmission and substation system.
Flexible riser

A riser cable developed by FEC is required for the
dynamic movement, because the cable is affected by thesea current conditions caused by wave, current movements and the rolling of the floating body itself.
The FF project is designed with a submarine riser cableconsisting of optical fibers to control a windmill, a floating
body also to transmit measured data and images at eachfloating structure and an observation tower installed parallel to an offshore substation.

Echo sounder receiver(on column at four corners)
Ultrasonic transmitter

Ultrasonictransmitter

Ultrasonictransmitter

Intermediate buoy
Anchor devices
Ultrasonictransmitter

Accelerometer
Ultrasonictransmitter

SinkerOptical fiber cable

Figure 3 POSEIDON system.

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The Development Of The Power Transmission System For Fukushima FORWARD Project

Start
Riser parameter &fundamental property

Wave condition &floating condition
Decision on Riser shape

Wire armor

Cushionlayer

Anticorrosion
layer

Metal shield layer
Water
shieldinglayer

Static behavior analysis
Yes
No

Inner
Conductor
semiconductor
layer
Insulation
Outer
semiconductor
layer

Floating structure
static displacement
absorption amount

SheathFillerDynamic behavior analysis

Floating structure
responsecharacteristics
No

Figure 5 Structure of 66 kV riser cable.
Floating structure
dynamic displacement
absorption amount

Yes
Fatigue analysis

Current wave
statistic data
No

Cable fatigue data
(SN characteristics)

Durability
satisfaction
Yes
End

Figure 4 Design flow of riser cable.

It is essential for a conceptual construction of the submarine cable to meet the specification in accordance withthe electrical equipment technical standards Clause 127“Facilities of electric power line over water and underwater” and for its electrical and mechanical characteristics to satisfy JEC-3408, CIGRE TB 490 and CIGRE
Electra No.171. The requirements of the riser cable characteristics for dynamic cables are shown in table 1.
Table 1

Target characteristics.

Item

Target characteristics

Sea condition at
installation site

・Meet allowable tension and minimum
bending radius in floating condition
・Floating part of the cable should not touch
the sea bed
・Cable should not be kinked

Floating structurerolling condition

Optical fiber cable

3.2 Detailed Design and Behavior Analysis
3.2.1 Behavior analysis
There are three types of floating bodies, differing in structure and dimension, connected with the riser cables.
Behavior analysis is carried out for each floating body,as each floating body has different rolling characteristics.
Cable lightness was the first designated target of thisdevelopment, in order to reduce the cable tension.
Static behavior analysis was conducted considering awave and a floating rolling condition in the conceptual
design of the applied riser cable.
Table 2 shows the wave conditions used for the analysis.
Table 2

Wave conditions.

Item

Adoption value

The 50-year-period-return value
associated with a storm wave
Sea current

Significant wave height(note)
11.71 mSignificant wave period 13 sec.
1.5 m/s (including drift current)(Note) The average wave height is defined as the average values
from the highest record and the consecutive values of 1/3
measurements of the total measurements recorded of the period ofrecording (for example 20 min) at a certain point.

Fatigue life design ・Similar to windmill or floating structure

Based on the previous findings, the riser cable construction is defined as shown in figure 5. The cable has
double layers of strand armor wires twisted in opposite
directions in order to prevent cable torsion.

It became clear that the riser cable was largely flown by
tide current, so that a first cable lightness design could
not be applied to the riser cable from the results of theanalysis.
Another analysis was conducted by increasing theweight up to the maximum amount attainable in the cable
construction.
However the cable design could not satisfy its property
requirements, because a kink was formed at the sea-bed
portion, irrespective of the intermediate buoy structure, as
shown in Figure 6 and Figure 7 (refer to circle part).
Finally a submersible floating shape and a riser cable
construction were confirmed to meet its characteristics
requirements as illustrated in Figure 8 by increasing its
cable weight with weight attached at its sea-bed portion.
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Figure 6 Cases where the submarine cable is kinked.
(Modular buoy)

Figure 7 Cases where the submarine cable is kinked.
(Mid arch buoy)

Figure 8 Cases where the submarine cable is not kinked.
(Modular buoy)

For the next step, a dynamic behavior analysis was
conducted based on the Response Amplitude Operator(RAO) data of each floating body and evaluated under theseverest condition of the tension and the bending curvature. As a result, it became clear that no defective cableportion was found by the tension and the bending curvature.
However, a maximum tension was generated at a fixing
part of the riser cable inside the floating body. Therefore asufficient safety margin is necessary in the wire armor
holding design of this portion. Hereafter, we are planningto evaluate the cable SN characteristics, and the fatigue

analysis is carried out at the end.
Table 3 shows the conceptual design parameters forthe riser cable.
Table 3Item

Specification of riser cable.
22 kV cable

66 kV cableOuter diameter

Approx.150 mm φApprox.180 mm φ

Weight (in the air)Approx.45 kg/m

Approx.55 kg/m

Inside optical fiber
number1

3

Intermediate buoy

Modular type

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3.3 Prototype cable production and its evaluation
A 66 kV prototype riser cable was manufactured and its
vertical stiffness and its bending stiffness were measured.
From now on, it is planned to determine the most suitable submersible riser cable shape based on the behavior analysis and the practical data obtained from the prototype cable.
Table 4 illustrates the main evaluation test items
planned to be conducted from now on.
A similar evaluation test will be conducted on a 22 kV
prototype cable, and the riser cable characteristics canbe confirmed.

Drag-in cable toward offshore substation

Table 4

Test items.

1. Tensile test and tension-bending test
2. Torsion test
3. Side pressure test
4. Repetitive bending fatigue test
5. Dissection investigation after above tests
6. Tension test of anchor portion

Since floating bodies are different on a case by casebasis, the investigation of dragging-in a riser cable into afloating body has to be conducted for each floating body.
An operation procedure of a riser cable installation is
illustrated in Figure 9.

Towards next floating structure

Modular buoy setting
Drag-in cable toward semi-submersible
floating structure

Pull-out cable

Cable installation completion

Cable touch-downFigure 9 Construction process.

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3.4 Towards Demonstration Test
Behavior measurement will be performed in the verification test, on the actual equipment by attaching an accelerometer in the riser cable installation.
Simultaneously, wave data and floating body rolling
data are collected by Tokyo University and other companies. These data are analyzed by linking with actual measurement data of the riser cable. And the fatigue test will
be carried out, comprehensively evaluating the desktop
study result, the actual plant data and the prototype actual measurement data.
Then the maintenance method will be eventually established.

4.

CONCLUSION

tion and its property evaluation.
Then, the property of riser cable system will be confirmed and simultaneously the actual cable will be manufactured for the practical installation in the next year term.
Further study will be carried on for the successful installation of the first wind farm in the world with multiple floating bodies and an offshore floating substation.

REFERENCES
1) JEC-3408 ”High voltage testing method for Special high voltage
cross-linked polyethylene cable and its splice for 11-275 kV”, 1997
2) CIGRE TB 490(Recommendations for Testing of Long AC
Submarine Cables with Extruded Insulation for System Voltage
above 30(36) to 500(550) kV)
3) CIGRE Electra No.171 (Recommendations for Mechanical Test on
Sub-marine Cable)

The FF project is proceeding on a very tight schedule.
At present, it is still at the stage of its prototype produc-

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