Proceedings of the International Symposium on Marine and Offshore Renewable Energy
Oct 28-30, 2013, Tokyo, Japan

Development of floating offshore substation and wind turbine
for Fukushima FORWARD
Haruki Yoshimoto
Japan Marine United Corporation
Yuji Awashima
Japan Marine United Corporation
Yuka Kitakoji
Japan Marine United Corporation
Hideyuki Suzuki
University of Tokyo, Graduate School of Frontier Science

Abstract - This paper describes the development of“Advanced Spar Float” and introduces two pilot
applications (floating power substation and floating
wind turbine) for wind farm off the coast of
Fukushima.
At first, this paper describes about the concept andcharacteristic features of advanced spar. Advanced
spar, this newly developed structure enables tosuppress the motion of the float nevertheless it retains
the characteristic stability that an existing spar had.
And, it needs smaller area for construction thansemi-submersible or the other type and can be built at
once in shipyard dock. In addition, the structure isself-standing by “Lower Hull” and reduced the draft toabout 50m, so the difficulties in construction andtransportation that existing spar had are cleared up.
For these reasons, we are sure that advanced spar canbe the good solution for floating wind turbines and canrealize safe and stable generation in the offshore farms
with cost competitiveness.
For the example of the application, this paper also
describes about the advanced spar floaters adopted in
Fukushima Experimental Offshore Floating Wind
Farm Project. One floater is the world’s first floating
power substation. Another is the world’s largest 7MWclass floating wind turbine. In the description about theapplication, transitions in the optimization of the
shapes and dimensions of the structure aredemonstrated.
I. INTRODUCTION
In recent years, a social demand for introduction ofrenewable energy is increasing because of risingconventional fossil energy resource and the nuclear power
plant accident. For example, in the study of potential forrenewable energy introduction published by Ministry of
the Environment [1], Japan’s potential for introduction ofrenewable energy is estimated at more than 2 million MW.
Among them, the potential for wind energy is about 1.9
million MW and it is over 10 times the potential for

introduction of the other renewable energy sources.
Furthermore, wind energy can be divided in onshore andoffshore, the potential for introduction of offshore windenergy is estimated at about 1.6 million MW. In otherwords, 80% of the potential for introduction of renewableenergy in Japan are occupied by offshore wind energy.
In offshore wind energy, there are two types of
embodiment, bottom mounted type and floating type.
Floating type has less relationship between cost and water
depth, compared to bottom mounted type. Therefore, total
project cost of floating type may be less than bottom
mounted type in the case of more than about 80m water
depth [2][3]. Because Japanese coast is topographicallysteep and has less area of shoal, floating type which can’t
be affected by water depth has been attracting attention in
Japan.
In other countries, countries having vast shoal areasuch as UK and Germany are introducing mass bottom
mounted offshore wind energy. With regard to floating
wind power generating systems, some floating offshore
wind energy projects are conducted as demonstration tests.
Hywind project [4] and WindFloat project [5] are theexample of full scale demonstration tests. Hywind by
Statoil is equipped with 2.3 MW wind turbine and installed
at 2009 in the North Sea at a distance of 10 km off thecoast of Stavanger, Norway. WindFoat by Principal Power
Inc. is equipped with 2 MW wind turbine and installed at2011 in off the coast of Portugal.
The floater part of floating offshore wind turbine(FOWT) is required sufficient stability and low motion
performance. The stability performance is required as
restoring force to the thrust force derived from windturbine and low motion performance is required for thefloater’s soundness in the harsh environment. These
performances are often compensated each other. Until now,
a number of concepts represented by semi-submersible
type, spar type and TLP (Tension Leg Platform) type areproposed for accomplishing these performances [6].
Hywind is the spar type and WindFloat issemi-submersible type. As the two demonstrative projects
suggest, it is thought that there isn’t the best concept1adoptable to all project. The best concept will be varied by
the project’s environmental condition and some constraint
condition. These things are same as oil and gas offshore
platforms have many concepts according.
Most of FOWT concepts proposed have been diverted
from oil and gas technology (i.e. semi-submersible, spar
and TLP). For the oil and gas platforms, the purpose of thefloater is to supply vast deck area and variable deck loads.
So, the floater’s concepts are optimized for these purposes.
But, the purpose of FOWT is somewhat different from the
purpose of oil and gas platforms. FOWT is equipped with
only tower and RNA (Rotor Nacelle Assembly) while oiland gas platforms are equipped with much equipment. So,
the best concept for FOWT is likely to be the newly
concept which is not proposed in oil and gas platforms.
Since 2009, JMU (Japan Marine United Corporation)
have been developed the newly concept named as“Advanced Spar” for FOWT which can minimize theproject cost. Advanced spar concept is developed throughresearch collaboration with the University of Tokyo andapproved as AIP (Approval in Principal) by ClassNK(Nippon Kaiji Kyokai) in 2011 [7].
Meanwhile, for the reconstruction of Fukushima
Prefecture severely hit by the Great East Japan Earthquake
and the Fukushima nuclear power plant accident, thegovernment of Japan started the experimental research
project of the world’s first floating offshore wind farm [8].
This project named as “Fukushima FORWARD(Fukushima Floating Offshore Wind Farm Demonstration
Project)” is conducted by the consortium made up of
Marubeni Corporation (project integrator), the Universityof Tokyo (technical advisor), Mitsubishi Corporation,
Mitsubishi Heavy Industries, Ltd, Japan Marine United
Corporation, Mitsui Engineering & Shipbuilding Co., Ltd,
Nippon Steel & Sumitomo Metal Corporation, Hitachi,Ltd., Furukawa Electric Co., Ltd., Shimizu Corporationand Mizuho Information & Research Institute. And, thisproject has been sponsored by the Ministry of Economy,
Trade and Industry since March 2012.
In Fukushima FORWARD, JMU is in charge of EPCI(Engineering, Procurement, Construction and Installation)of floater part of the floating substation and 7MW floating
wind turbine. Floating substation will be installed in fiscal
2013 and FOWT will be installed after fiscal 2014.
In this paper, we describe the technical specification ofadvanced spar concept developed since 2009 to during
Fukushima FORWARD. Then, for the example ofapplication, we describe the design of floating substation

Fig. 2. Overview of Fukushima FORWARD
and FOWT.
II. Advanced Spar Concept
A. General Requirements
The general requirement for FOWT is to produce more
electrical power by less cost. To fulfill this requirement,
the floater has to have the specification described below.
・law steel weight (displacement) and low material cost・not so big breadth (to construct at general shipyard)
・towed in upstanding position (to simplify installationand mooring works at the offshore)・can install big ,2MW to 10MW class, wind turbine (toachieve economies of scale)・less heeling and less motion (to minimize the failure
risk of RNA)・less limitation to water depth and seabed property (toinstall the site of good wind conditions)
In addition, for the problems of the safety andenvironment, the floater must accomplish below things.
・not to restrict navigation of ships as possible
・minimize the impact to oceanic ecosystem
・minimize the impact to fishery industry
・high redundancy to collision damage
・less risk of drifting
Advanced spar is developed to fulfill theserequirements to the maximum extent as far as possible.
Until now, there are no codes and rules standardized byan international association. But, nowadays, someclassification societies publish their own guidelines[9] [10][11] [12] and IEC is preparing international standards for
FOWT as IEC61400-3-2 in TC88 [13]. Advanced spar isdeveloped to follow the guideline published by ClassNK[9].
For the environmental conditions have to be specifiedin each projects, in the conceptual design stage, we don’t
assume the specific environmental conditions. The
environmental conditions for Fukushima FORWARD aredescribed below.
B. Overview of Advanced Spar
Advanced spar is composed of a tower, column andthree hulls, and rotationally symmetric floater (Fig. 3).
Three hulls are named as “COB hull”, “Middle hull”
and “Lower hull” in order from top to bottom. The floater
has ballast tanks in hulls and can move freely fromoperating draft (COB hull) to transit draft (middle hull).
Mooring lines are connected to middle hull. The numbersof mooring lines are selected adequately by projects.
On lower hull, some “Motion suppression fins” arearranged
In the three hulls, there are two types of water ballast
tanks, permanent ballast tanks and water ballast tanks.
Permanent ballast tanks are arranged in lower hull andFig. 1. HyWind(Left), WindFloat(Right)

2

RAO

A

With COBB

The case of enlarged
waterplane area

C

Without COB

B
AC

Wave Period

Fig. 4. Schematic curves of heave motion response
Furthermore, fenders and ladders for boarding from a
support vessel are equipped on COB hull side. Internal
space of COB hull is used as water ballast tanks, void
spaces or the spaces for the other utility systems.
Fig. 3. Advanced spar wind turbine
water ballast tanks are arranged in three each hulls.
Electric cables are derived from the bottom of lower
hull and went through the cable pipe constructed as outershell. Finally, the cables are conducted in tower at thebottom of it (not shown in Fig. 3).
Below are the characteristics of floater’s performanceand the detail of each part.
C. Wind Turbine
Wind turbine to be installed is assumed a horizontal
axis lift type wind turbine. And, it is required to haveappropriate control logic for FOWT and withstand theacceleration generated by floater’s wave frequency
motions. 50 years maximum horizontal acceleration will
not be more than 0.5G for almost sea state, nevertheless it
is depending on design environmental conditions.
The assumption of the wind turbine scale is more than2MW class. Both upwind type and downwind type can beinstalled.
At the bottom of tower, tower entrance to the inside isarranged (not shown in Fig. 3). The tower entrance should
be kept sufficient air gaps from water line. The working
platform may be installed around the tower entrance forelectric cable pull in work and RNA equipment handling.
Hub height should be decided according to the site
specific wind conditions and wind turbines scale andefficiency. However, appropriate clearance between blade
chips and tower entrance should be kept.
D. COB Hull
The hull located around sea water line at operating
draft is called as “COB hull”. “COB” is an abbreviation for“Column Outer Belt” and it was developed by IHI
Corporation for the purpose of motion suppression forsemi-submersible rigs.
The detailed function of COB is described in the paper[14]. The mainly hydrodynamic function of COB is tosuppress heave motion in low wave frequency (Fig. 4). In
addition, COB has the function to set the heave natural
period to preferable frequency and keep the second orderinertia moment of waterplane for stability.

E. Middle Hull
The hull located at the middle of bottom and sea waterline is called as “Middle Hull”. The main purpose of thehull is to raise the center of buoyancy for keeping adequate
restoring force.
Mooring lines are connected to middle hull. So as theconnecting points are completely submerged in operation,
mooring lines are free from harsh corrosion. In addition, it
is easy to maintain and install the mooring lines on
offshore because the draft of the floater can be middle hulllevel by ballast control and the connecting points are to be
exposed to air.
In the case of upwind turbine, the turbine carries out
yaw control for fronting the blade plane to the winddirection. During yaw control, RNA generates yaw torqueat the top of the tower. For restoring the yaw direction of
the floater, the floater has to generate sufficient yaw
restoring force and damping force. It is difficult to get theforces from other than the mooring lines. By connecting
the mooring lines to outer edge of middle hull, this floater
generates large yaw restoring force from mooring lines.
If the mooring lines are connected at shallow point,sudden current loads can lead large overturning moment
around connecting points when RNA generates thrust loads
in the opposite direction to the current. But, if the mooringlines are connected to middle hull, such a large overturning
moment are not generated. Internal space of middle hull isused as void spaces and water ballast tanks (Fig. 5).
F. Lower Hull
The hull located at the bottom is called as “Lower
Hull”. Lower hull is often filled by permanent ballast andwater ballast. The main purpose of the lower hull is tolower the center of gravity. Lowering the center of gravity
increases GM (metacenter height) and restoring force. In
addition, heavier parts in the floater, RNA and lower hull,
are set apart from each other with a distance between
bottom to top, so radii of gyration is relatively enlarged.
Enlarged radii of gyration makes natural period of roll andpitch longer, as a result, wave frequency pitching motions
are suppressed.
Furthermore, lower hull can keep the floater in
upstanding position during construction. Because the3

If the size of wind turbine installed is similar, required
stability is not so much different between any concepts.
Restoring forces of pitching motion can be described asfollowing equation;

Mooring Line

Yaw TorqueMiddle HullWind DirectionRNA
Yaw Restoring
Force

Mooring Line

Fig. 5. Balancing of yaw torque and restoring force (TopView)
bottom of the conventional spar concept has not enougharea to stand by itself, constructors have to lay the floater
during construction. And they have to launch the floater,
straighten up and install the wind turbine on offshore
nevertheless offshore operations are dangerous and costly.
In contrast, advanced spar concept allows constructor tobuild, launch and tow in upstanding position, and install
wind turbine in dry dock. When RNA is replaced, thefloater can be landed at seabed near the installation site andconducted safety working.
At the center of lower hull, a pump room for ballast
pump is arranged. For locating pump room at the bottom offloater, the suction head of water ballast tanks in lower hull
can be lower and it needs small capacity ballast pumps.
G. Motion Suppression Fin
Some motion suppression fins are equipped on lower
hull (Fig. 6). Motion suppression fins function to suppress
the motion of the float.
There are two types of damping forces expected to begenerated by fins. One is the damping force to pitchingmotion and the other is to yawing motion. Each damping
forces bring good effect not only for motion suppression
but also for enlarging yaw restoring force and preventing
low frequency motion. In particular, it is important tosuppress low frequency motion excited by negative
damping.

(2.1)

Then, GM can be described as following equation;
GM = KB + I/V – KG
(2.2)
where;
KB: height of center of buoyancy from baseline [m]I: second order inertia momentof waterplane [m4]V: displacement volume [m3]KG:
height of center of gravity [m]How to get this restoring force is an important factor
for studying the concept of FOWT. For example,
semi-submersible type intends to increase I/V, barge type
intends to increase Δ and spar type intends to increase“KB-KG”.
In the case of advanced spar, the method to acquire
restoring forces looks like both semi-submersible type andspar type.
First, permanent ballast filled in lower hull reduce KG
and COB hull and middle hull enlarges KB. As a result,
GM is increased as spar type. Also, water surface area of
COB hull contributes to gain second order inertia moment(I) and increase GM as semi-submersible type.
In this way, because advanced spar can get restoring
force by two means, advanced spar can keep required
restoring force without enlargement of displacement.
In addition to the superiority about intact stability as
described above, advanced spar has high redundancy about
residual stability. In general, advanced spar keeps KB
greater than KG in operating draft. Thus, nevertheless
COB hull is damaged by supply boats or the other ships
and inertia moments of waterplane are reduced
significantly, the floater can keep high level residualstability. This floater can have big advantage in this highredundancy about stability when large scale wind farmproject will be carried out.
I. Motion Performance
In addition to sufficient stability, FOWT is required tohave low motion performance. The wave frequency motion

Operating Draft

KB, KG

H. Stability
Any FOWT are required sufficient stability against
overturning moments came from environmental forces (i.e.
wind turbine’s thrust, current load, wave drift force, etc.).

(Restoring Force) = GM × Δ × sinθ
where;
GM:
metacenter height [m]Δ: displacement [ton]θ: pitching angle [degree]

Draft

Fig. 6. Detailed image of motion suppression fin

Fig. 7. Variance of KG and KB vs. Draft

4

of FOWT causes damage to the RNA structure and is also
the main factor for determining the scantling of the tower.
The impact it has on moving parts such as turbine shaft andyaw gear is high and is a factor directly linked to failure
rate and equipment life.
There are three main measures to reduce the motion of
FOWT.
・lengthen natural period
・increase damping force
・decrease wave exciting force
The advanced spar takes measures to increase themotion performance on all three points. Among somemeasures adapted to advanced spar, the outline of thecharacteristic measure “2 point wave free frequency” isdescribed.
“2 point wave free frequency” is acquired by a hull
form which does not get affected by the wave exciting
force at two frequencies in heave mode, and is introducedin the paper [15] as an early study. Due to the hull form,
the heave motion is significantly reduced (Fig. 8).

Heaving Force (Wave Excited Force)

J. Instability Behavior
Furthermore, it is known that in FOWT, pitch control
and floating structure motion couples, resulting ininstability behavior at the same frequency as the natural
period of the float [16] [17].
In motion equation, when the velocity term becomes
negative, instability behavior occurs. In order to prevent
instability behavior, it is suggested that adequately
designing the control logic so as to reduce negative
damping as much as possible is the most effective way.
But, giving the float a certain level of damping force isalso important.
Among the terms, radiation force and the viscosity
damping force depends on the float. At the time of natural
frequency of the float, compared to radiation force derived
from potential flow, viscosity damping force is dominant.
Therefore, the float is required to have high viscosity
damping force.
To increase the viscosity damping force, it is effective
to create a vortex. In the case of general commercial ships
bilge keel is installed to earn viscosity damping force, andthe fact that it is adopted in most of the commercial ships,
it is apparent that it is an effective measure.
In the advanced spar, in the same way as the bilge keel,
plate-like fins are placed in a radial fashion on top of thelower hall (refer to G. Motion Suppression Fin) to generate
viscosity damping force. The size and number of the fin isdetermined through tank test and time history coupling
analysis. Moreover, the motion suppression fin having
being placed in a radial fashion has the function to
generate yaw damping force as previously explained.
III. Offshore Substation of Fukushima FORWARD
From here on described is the offshore substation and
7MW FOWT of Fukushima FORWARD as an example of
the application of advanced spar.
The offshore substation is a float designed to raise thevoltage of the electricity generated by the offshore windturbine and transmit the electricity to land. It is the world's
first challenge to construct floating offshore electric power
substation. Also equipped with instrument to observe
meteorological phenomena, oceanographic phenomena,and float motion of the targeted ocean area, it alsofunctions as an observatory obtaining data necessary foroffshore wind farm.
In the electrical substation equipment part, a 25MVA
transformer which raises the pressure of the 22kV
electricity generated by the wind turbine to 66kV is
installed. Hitachi Ltd. is responsible for the electricalsubstation equipment.
Moreover, underwater transmission lines are used forelectrical transmission, connecting three 22kV lines andone 66kV line. Furukawa Electric Co., Ltd. is in charge ofthe transmission lines.
A. Environmental Conditions
The location where Fukushima FORWARD will be
conducted is a sea area 20km offshore from Fukushima
Prefecture where the depth of water is approximately
100-120m. The yearly average wind speed in the sea area
is more than 7.5m/s at 100m above water. Tab. 1 shows thesetting of meteorological and oceanographic conditions
with 50 years return period. These environmental
conditions have been estimated comprehensively from thefollowing data.
・observed value from a nearby offshore platform・wave prediction data base・typhoon simulation・observed value from a nearby land
・recommendation value from IEC61400-3
Table 1 Design conditions for Fukushima FORWARD(Return period = 50 years)

Wave Free Frequency

Wave Frequency

Fig. 8. 2 point wave free frequency

Wind Velocity (10min. @Hub Height)
Wind Velocity (10min. @10m)
Turbulent Intensity (Reference)
Weibull Shape Parameter

Abt.50m/s
41.8m/s
12%
2.0

Significant Wave Height
Significant Wave Period
Current Velocity

11.7 m
13.0 s
1.5 m/s

B. Design Conditions (Construction)
The float is constructed at Yokohama Shipyard Isogo
Works of JMU and needs to be towed to offshore of
Fukushima. Besides the connecting of the mooring lines,
as it is difficult to secure a calm sea area where we can doconstruction work at the sea, there is a need to complete5the float inside the dock of the works. The size of thedocks at Isogo Works are shown in Tab. 2 The width andlength of the float needs to be smaller than the width of thedock.
In the case of general ships and offshore structures,
displacement is given to the extent where there is enoughto surface the float at the time of leaving the dock.
Therefore, for operating in the same way as
semi-submersible, there is a need to have a large
displacement at the bottom of the float. However, inregard to the offshore substation which is installed at sea
for a long period of time and do not expect to haveperiodical inspections, there is little need for it to be able to
go in and out of the dock by its own buoyancy. So we
assumed of lifting it out using floating cranes at the time ofleaving the deck, reducing the displacement of the lower
hull and the entire float.
However, as inspection and maintenance of themooring lines are expected to take place periodically, there
is a need to be able to expose the connection point of themooring lines through ballasting.
C. Applied Rules and Regulations
In designing of the float of the substation, thefollowing regulations were applied. Note, however that
there are no crews, the classification society madesignificant allowances for the life saving and fire fighting
criteria.
Classification Siciety: Class NK
Code and Rules:
・ ClassNK Part P, Part PS
・ The International Regulations for Preventing
Collisions at Sea (COLREG)・ International Convention for the Prevention of
Pollution from Ships (MARPOL)・ Japanese Industrial Standards (JIS)D. Design Conditions (Motion)
The substation needs to satisfy the stability
performance and low motion performance required by theelectrical substation equipment and observationequipments that will be installed. In this project,
performance to the same level of the floating offshore windturbine is required.
Also, as the riser cable has a limited range that it canfollow, in the same way as rigs connected with drillingrisers and offshore structures for production connectedwith flexible risers, there is a need to design mooring lines
so as it will not be pushed outside of the certain range byenvironmental external force.
E. External Form of the Float
From the design conditions mentioned above, we
designed a float as shown in “Fig. 9”. The principal
particulars are shown in “Tab. 3”.
Table 2 Size of the dock at Isogo Works
Dock Name
Size
Building Dock
325m × 45m
Repair Dock
417m × 56m
Floating Dock “Sagami”
250m × 43m
Floating Dock “Negishi”
175m × 36m

Fig. 9. Isometric view of offshore substation
Table 3 Principal Particulars of Offshore Substation
Item
Value
Length (at middle hull)
33.4 mBreadth (at middle hull)
33.4 mHeight (bottom to tower top)
Abt. 110 m
Draft (Operating)50.0 mDraft (Maintenance)
32.0 m
Displacement
Abt. 13,000 ton
The electrical substation equipment is located insidethe upper hull which is at the top of the hull, a tower for
observing wind conditions and helicopter deck for accessin case of an emergency is provided. Also assuming an
operation in time of a power failure, it is equipped with an
auxiliary generator. The major equipments to be installed
are listed below.
Electric Substation Equipment
・ 25 MVA Transformer
・ 66kV GIS (Gas-Insulated Switchgear)・ 22kV Switchgear
・ NGR (Neutral Grounding Resistor)
Observation Equipment
・ Observation Tower
・ Doppler Lidar
・ FOG (Fiber-Optic Gyroscope)
・ RTK (Real Time Kinematic) GPS・ Monitoring Camera
・ Present Weather Sensor
・ Rain Gauge
・ Barometer・ Differential Temperature Meter・ Pyranometer・ GPS Compass
・ Cup Anemometer and Wind Vane
・ 3D Ultrasonic Anemometer
・ Ultrasonic Wave Meter・ Acoustic Doppler Current Profiler
・ Water Temperature Meter
・ Salinity Meter・ Subsea Monitoring Camera
・ Rader for Seabird
・ Temperature/Humidity Meter
Marine Equipment

6

・ Ballast Pump
・ Axially Generator
・ Helideck
Mooring will be done with four mooring lines. Thelines are composed by chains and the nominal diameter is
132mm.
F. Maneuverability
Frequency response calculations were calculated with
JMU's three dimensional sink-source method program [18].
Viscosity damping is calculated within also.
Moreover, to verify the calculation results, tank test
was conducted in an experimental tank for maneuverability
located at IHI Yokohama Works. The scale of the model
used was approximately 1/44 and the mooring lines weremoored loosely so that its tension would not impact the
maneuverability.
The RAO obtained from the tank tests and RAO
calculated from the three dimensional sink-source method
is shown in “Fig. 10” and ”Fig. 11”. In either heave or
pitch mode, you can see that the test and calculation results
were remarkably congruent.
Fig. 12. Installation on lower hull

Heave/ζa [m/m]

G. Construction
The float was constructed at JMU Yokohama Shipyard
Isogo Works from December 2012 to July 2013.
To begin with, the upper hull, the COB hull, the middle
hull and the lower hull were separately constructed, andwere constructed by composing the hull in order. The
block division and the picture of the constructed float are
shown in ”Fig. 12” and “Fig. 13”.

Part 4Cal.
Experiment

Upper HullPart 3

COB

Part 2
Middle Hull

Wave Period [sec]Fig. 10. Heave RAO (Cal. And Exp.)
Part 1

Pitch /ζa [deg/m]

Lower HullCal.
Experiment

Wave Period [sec]Fig. 11. Pitch RAO (Cal. And Exp.)

Fig. 13. Block division of offshore substation
H. Towing and Installation
Before the towing of the float, mooring lines andanchor were installed beforehand and a test to see its
holding power was conducted. The holding power test was
conducted by pulling together the mooring lines on the
opposite side on the barge.
The towing, with the condition of towing draft (32m),
was towed from Tokyo Bay going through Uraga Channel
to the installation sea are in the offshore of Fukushima7

taking two days. The towing was done by one main
tugboat and three assistant tugboats, navigating at the
speed of approximately 4 knots.
At the end of July, the float was connected to thepreviously installed mooring lines and was installed.
We are now preparing facilities on land and in October
2013 we plan to start operation at the same time as the startof generation of the 2MW FOWT.

Table 4 Principal Particulars of 7MW FOWT
Item
Value
LengthAbt. 70.0 m
BreadthAbt. 70.0 m
Height (bottom to nacelle)
Abt. 145 m
Draft (Operating)Abt. 39.0 m
Displacement
Abt. 40,000 ton

V. 7MW FOWT of Fukushima FORWARD
The following is about the 7MW advanced spar FOWT
for the Fukushima FORWARD. As many of theenvironmental and design conditions are in common withthat of the offshore substation as described previously,
only the part that differs are explained.
A. 7MW Wind Turbine
The float will employ the advanced spar design and thewind turbine to be installed is assumed to be a 7MW windturbine made by Mitsubishi Heavy Industries, Ltd.
This wind turbine employs a hydraulically operated
train system and brush-less synchronous generator and has
high reliability and durability compared to conventional
wind turbines [19].
B. Principal Particulars
The principal particulars of the designed 7MW FOWT
are shown in Tab. 4.
Unlike the previous advanced spars, in order to obtainenough stability, 6 upper hulls projecting greatly are
installed at the water plate. This form is like COB andmiddle hull jointed in the substation.
Moreover, in addition to COB, by adjusting the form of
the float, it is setup so the pitch frequency without waves
matches the wave period that dominates in the sea area.
C. Coupling Analysis
To calculate the motion and load of the float, time
history calculation using aero-hydro coupling analysis
program was conducted. The program used were,
UTWIND developed by Tokyo University [20] and FAST
developed by NREL [21] [22].
Currently, each classification society has publishedguidelines for designing but in either guidelines, there is a
description on the necessity of practicing time history
coupling analysis in order to check the validity of thestructure design [9] [10] [11] [12]. It also says to apply
DLC like it is described in the IEC61400-3. Programs tocalculate coupling analysis is in the midst of development
around the world [23] [24]. Each program somewhat
differs in the formulation and calculation conditions but
the validation of the calculation results are thoroughly
examined by IEC through OC3 [25] or OC4 [26], andsome of the codes have gained approval from theclassification society.
This time the program UTWind was used for theconcept design phase and FAST was used for calculating
ultimate load and fatigue load based on DLC. FAST was
selected not only because it is an open source program but
because it doesn't get affected by the constraints of thefloat form. To calculate the hydrodynamic force(additional mass, radiation force, wave exciting force) thatwill be input into FAST, WAMIT [27] and JMU's three
dimensional sink-source method program [18] were used.

D. Construction, Towing, Installation
Just like the offshore substation, the float for the windturbine is expected to be constructed at JMU Yokohama
Shipyard Isogo Works. The tower for the wind turbine andRNA is planned to be installed in the dock and to be towed
to the installation sea area.
VI. CONCLUSION
We have developed a completely new advanced spar
float as the optimal floating form for offshore floating
wind generation. The characteristics of the advanced spar
float are as follows.
・low motion performance・high redundancy restoring force
・fins to gain stability
Also, we adopted the advanced spar form for an actual
project of offshore substation and 7MW float, verifying its
feasibility.
ACKNOWLEDGEMENTS
Ministry of Economy, Trade and Industry (METI) is
acknowledged for sponsoring of Fukushima FORWARD
project. Hitachi, Ltd. and Furukawa Electric Co. are
acknowledged for deigning and construction of offshore
substation. Mitsubishi Heavy Industries, Ltd is
acknowledged for designing of 7MW floating offshore
wind turbine.
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