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

Development of a V-shaped Semi-submersible Floating Structurefor 7MW Offshore Wind Turbine.
Makoto Ohta
Nagasaki Research and Development Center
Mitsubishi Heavy Industries, Ltd., Japan
Masao Komatsu, Hiroto Ito, Hitoshi Kumamoto
Ship & Ocean Engineering Division
Shipbuilding & Ocean Development
Mitsubishi Heavy Industries, Ltd., Japan

response in waves. In addition, overview of the mooring
design and dynamic stability performance, which has been
pointed out as a specific problem of FOWT, are described.

Abstract - V-shaped Semi-submersible floating
structure has been selected as a floater for the 7MW
offshore wind turbine of Fukushima Offshore
Demonstration Project. Design concepts were discussed
during the development of floaters and they aresummarized in this paper. A design method based on
various simulation and tank test of using the 1/64 scalemodel is shown. In addition, dynamic stability
performance including wind turbine control system,
which has been raised as essential system for floating
offshore wind turbine, is confirmed by the analytical
approach and judged suitable for the floater.

II. CONCEPTUAL DESIGN
It is considered that there are advantages anddisadvantages for the variously proposed FOWT. First, thefloating type is selected considering off Fukushima is sitelocation. Spar type is removed because the draft of thefloating structure for mounting the 7MW wind turbine
exceeds the installation water depth, which can be easily
assumed. TLP type is excluded also because increase of
the installation costs was assumed by using the domestic
work vessel and equipment. As a result, semi-submersible
type floating structure is selected.

I. INTRODUCTION"Fukushima floating offshore wind farm demonstration
project (Fukushima FORWARD)", which is world's firstfull-scaled empirical study of floating offshore wind farm,
has started in 2012. It plans to construct several type offloating offshore wind turbine (FOWT) at the sea of 20km
off Fukushima in Japan[1]. As part of this empiricalresearch project, the world's largest 7MW hydraulic windturbine and its floating structure are under construction in
our company.
Development of large-scale FOWT have been started
already in the demonstration test off the coast of Spain andNorway, various concepts of FOWT have been proposed
all over the world[2],[3]. Many of them have been planned
for the reference Oil & Gas platform, the type of floating
structure is divided into semi-submersible type, spar type,tension leg platform (TLP) type and barge type. The
authors has conducted the conceptual design of FOWT for
more than a decade, then technical knowledge for floating
performance evaluation and business assessment has been
accumulated[4],[5]. Based on this knowledge, new floating
structure is planned and designed for this project.
In this paper, the floating structure design concept formounting extra-large 7MW wind turbine is discussed first.
In the conceptual design, to avoid sub-optimal design that
focuses on specific requirements, floating structure shapeis selected by taking into account the circumstances of thesite and the life cycle cost. Next, tank test / wind tunnel
test and validation calculation are shown to grasp thefloating basic performance such as static stability andTable 1. Principal dimensions

Rotor diameter
Hub height from sea surface
Length floating structureWidth floating structure
Column width
Draft
Displacement

167.00105.0084.85
149.91
14.00
17.00
26,000mmmmmmton

Fig.1. V-shaped Semi-submersible FOWT
Then, in the determination of the in shape of1

construction cost and an increase the degree of freedom ofchoice the construction site in mass production stage byemploying a combination of simple shapes. Usually,
although many large steel structures are built in dock of theshipyard, construction or repair works near the site is also
desirable taking into account the maintenance cost andtowing cost from the construction site to the operation site.
This V-shaped Semi-submersible FOWT have a shape thatis integrated blocks of box-shaped as bridges and caissons,in consideration of the final assembly near the site too.

semi-submersible type floating structure, it was performedby considering that it satisfies the following requirements.
(1) Floating structure shall be self-stable(2) Construction is possible in port area of Japan(3) Building location is not limited(4) Simple shape and structure which is suitable for massproduction(5) Efficiency for wind turbine installation work
(6) Consideration of critical failures and maintenance(7) Response performance in waves

C. MOUNTING WIND TURBINE
7MW wind turbine is installed on the center column
placed on the floating structure. During construction for
mounting the wind turbine, it is possible to have craneapproaches to the floating structure. Fig.2 shows therelationship between the outreach and capacity of the cranehandling.
It is also possible to install a floating structure on theseabed mound when mounting the wind turbine in order tostabilize against wind and waves. Furthermore, lower hull
structure can be used as a docking base of floating crane
for maintenance in the future.

Principal dimension of the V-shaped Semi-submersible
FOWT newly developed is shown in Table 1, and thatconceptual diagram is shown in Fig.1.
The floating structure is floated by buoyancy of lower
hull part placed in a V-shape during construction / towing,
and it is semi-submersible condition by the water injection
to this part. The ends and intersection of lower hull part areconnected with three columns. 7MW wind turbine isinstalled on the center column. Columns and lower hullpart are adopted a box type structure with no curved
surfaces, therefore this floating structure is a very simplestructure without any complex structure nor the obliquesupport structure which is easy to lead to weld defect. This
structure design was selected based on the past experienceof Oil & Gas platform[6],[7].

III. FLOATING PERFORMANCE
In floating design, providing a floating structure as thebasement, on which super-large wind turbine can beoperated safely, is the most important. And there shall be
the serious accident like subversion or drifting. On theother hand, ease to build, convenience of construction andmaintenance, and various performances come involved inproduct value. For example, if the floater is designed with
only suboptimal view of the construction costs and/ormotion performance, business potential might be reduced
significantly if it is evaluated from the view point of theentire value chain

A. FLOATING DRAFT
The floating structure is capable of draft adjustment by
the drainage of the ballast tank inside lower hull fromshallow draft (3 meter) to design draft (17 meter)
depending on the situation (construction / towing /operation) and sea condition. Wide range of draft change isimportant for FOWT, because it is necessary to enter theport near the site for support of critical failures and theremoval in the future, not only for the construction period.

Fig.2.

A. PERFORMANCE REQUIREMENTS
In general, the importance of static stability and motion
in waves at floating design is well known. In addition,
another characteristic performance requirement for FOWT
is dynamic stability in conjunction with wind turbine
control system[8]. This performance requirement is theevaluation of whether or not to amplify the rotationalmotion of the floating structure, because the force, which is
caused by the wind turbine blade control for smoothing of
power and thrust, acts on the floating structure. It is
confirmed that this V-shaped Semi-submersible floating
structure has sufficient performance to satisfy all thesemajor performance by carrying out a detailed study in theinitial design stage.
In the design of FOWT, which shape is always top
heavy, reducing the motion leads to reduce the burden forwind turbine structure and onboard equipment. For thispurpose, motion reduction devices called MS-Board,which was originally developed, are installed near thelower end of each column [9]. This device is passive typethat does not use any electrical power generated by windturbine, and there are also adoption records to the floating
pier, etc.

Relations capacity of crane and outreachB. MODEL TEST
For the purpose of verification of performance

B. MASS PRODUCTION
This floating structure has achieved a reduction of2

And motor/gear is housed in the nacelle, rotor is rotated ata predetermined rotational speed, it is possible to change
the blade pitch in remote control. According to themooring design as mentioned below, floating structure
model was moored with chain loosely in tank. The model
overview, the status of the tank test and mooringarrangement are shown in Fig.3, Fig.4 and Fig.5.
Measurement items are floating structure motion, lower
hull surface strain, mooring tension, tower base load,
nacelle acceleration, etc.
As a tank test results, typical response amplitudeoperator (RAO) of wind turbine model-I in regular waves
is shown in Fig.6. In Fig.6, upper RAO is in E-waves,lower RAO is in S-waves, RAO of left side is heave
motion and RAO of right side is pitch motion. And triangle
mark are experimental results without MS-Board, square
mark are experimental results with MS-Board. Solid line
are the calculation results without MS-Board by panel
method using computational mesh showed in Fig.7. Thisin-house code computes diffraction / radiationhydrodynamic forces on the structure based upon thepotential theory, and floating structure response wasestimated by solving in the frequency domain equation ofmotion taking into account non-linear viscous damping
force as results of motion decay tests[10]. This mesh data
is as it was taken out the wetted parts from the FEM data
for the analysis of the floating structure response, and adetailed strength analysis has been conducted.

requirements and acquisition of verification data for designcode, many model tests were carried out using large
experimental tank and wind tunnel facilities. In Seakeeping& Maneuvering Basin (Length 160m, Breadth 30m, Depth3.1m), motion decay tests, regular/irregular waves tests,
towing test were carried out by installing the simple windgenerator.

Fig.3.

Model Overview

Fig.4. Tank Test Situation

Fig.6. Response Amplitude Operator

Fig.5.

Mooring Arrangement

Two types of wind turbine were used on scale ratio
1/64 floating structure model. Floating structure model was
made of an acrylic material in shape similarity but lower
hull part is using by sandwich structure of aluminum plate
which scaled down the actual bending stiffness. Windturbine model-I was produced using PVC pipe in weightcenter of gravity similarity. For wind turbine model-II,
blade/nacelle model were laminated C-FRP in shapesimilarity and tower model was made of aluminum plateby machining in shape/weight/bending stiffness similarity.

Fig.7. Computational Mesh (13,092mesh)
The peak of pitch motion of with MS-Board is shiftedto long-period side compared to without MS-Board. It isconsidered that non-linear damping force is increased by3the effect of MS-Board. Pitch motion has also occurred inS-waves from floating laterally, because of asymmetricshape forward and aft side of floating structure.
Assuming the rated power generation and the 50 year
storm, pitch motion of wind turbine model-II is shown in
Fig.8 in irregular waves and uniform wind. Reynolds
number of the model is 1/512 times the actual wind turbine,
aerodynamics cannot be simulated. Therefore, in view ofthe load to transmit to the floating structure from the windturbine in this study, blade pitch angle was selected to be
the equivalent thrust on rotor and bending moment acting
on the tower base, irregular waves and uniform windwhich was scaled down by Froude law were generated.
Test conditions are shown in Table 2, and statistical resultsof pitch motion in the case of rated power generation andthe 50 year storm are shown in Fig.8 and Fig.9.
Average value of pitch motion is about 1.3 degree inCase-RPG that thrust acting on the wind turbine ismaximized and is 2 degree or less in Case-50YS thatbending moment of the tower base is maximized.
Maximum / minimum value of pitch motion is 3 degree orless in Case-RPG, and is 6 degree or less in Case-50YS, soit is understood that this floating structure has both
excellent stable performance and anti-wave performance.
An example of the result of the time history analysis using
above hydrodynamic force on Case-50YS is shown inFig.10. This estimation result is safe side, because ofsimulation result of 180 minutes against test results ofabout 30 minutes.
Table 2.
Rated Power Generation(Case-RPG)
50 Years Storm(Case-50YS)

Fig.10.

Simulation Result in Case-50YS

Table 3. Wind Tunnel Test Case Summary
Waves Test

Floating Fixed Test

Fluctuating Wind Test

Fluc. Wind / Waves Test

Test Condition
Full scale
U=12.0 m/s
Hs=3.2 m
Ts=7.5 s
U=50.0 m/s
Hs=11.7 m
Ts=13.0 s

Model scale
U=1.50 m/s
Hs=50 mm
Ts=0.94 s
U=6.25 m/s
Hs=183 mm
Ts=1.63 s

Fig.11. Wind Tunnel Test Situation

Fig.8. Statistical Results of Pitch in Case-RPG(tank)Fig.12. Statistical Results on Pitch of Case-RPG(tunnel)

Fig.9. Statistical Results of Pitch in Case-50YS(tank)4

In this demonstration project, empirical observation isscheduled for two years after installation, it is planned tobe apparent safety, reliability, and economy on thisfloating structure along with the establishment oftechnology.

Since the effects of fluctuating wind cannot be grasped
by using simple wind generator in tank, wind tunnel tests
were carried out by placing simple tank and wave maker inBoundary Layer Wind Tunnel (Length 35m, Breadth 6m,
Height 5m). For the issue of Reynolds number, widening
blade models were produced which its aerodynamic
characteristics are similar to the actual wind turbine. Windtunnel test case summary is shown in Table 3, and windtunnel test situation is shown in Fig.11. As an example ofthe wind tunnel test results, the statistical results on pitch
motion of Case-RPG are shown in Fig.12. The steady
gradient is dominant in fluctuating wind.

ACKNOWLEDGEMENTS“Fukushima floating offshore wind farm demonstration
project (Fukushima FORWARD)” is promoted by consortium
which consists of Marubeni Corporation (Project integrator),
the University of Tokyo (Technical advisor) and several
companies. The authors are grateful to people of consortium
members striving for realization of this verification test.

C. MOORING DESIGN
The environmental conditions in the mooring design ofthis project are adopted the extreme value statistical resultsof 50 years return period based on various types of datasuch as waves and wind speed of installation area . The
design wave condition is shown in Fig.13 compared withobserved maximum high waves which were measured at
58 locations in Japan coastal waters for 30 years. As results,
high wave height to exceed the observed maximum wave
in western Japan where large typhoon passes frequently
has been set.

REFERENCES[1] http://www.fukushima-forward.jp/index.html[2] http://www.statoil.com/en/technologyinnovation/
newenergy/pages/default.aspx[3] http://www.principlepowerinc.com/[4] Izumi Ushiyama, et.al. , 2004, A Feasibility Study on
Floating Offshore Wind Farms in Japanese Waters,
WIND ENGINEERING VOLUME 28, NO. 4[5] Makoto Ohta, et.al., 2003, A Study on Antiwave
Performance of Box Girder Type Floating Wind Farm,
17th Ocean Engineering Symposium (in Japanese)[6] T. Moan, 1981, The Alexander L. Kielland Accident,
Proceeding from The First Robert Bruce Wallance
Lecture, Department of Ocean Engineering, M.I.T[7] Marine Casualty Report “Mobile Offshore DrillingUnit (MODU) OCEAN RANGER” O.N.615641,
United States Coast Guard , 1983
[8] Larsen T., Hanson T., 2007, A method to avoid
negative damped low frequent tower vibrations for afloating, pitch controlled wind turbine, The science ofmaking torque from wind. Journal of Physics:
Conference Series 75[9] Masami Matsuura, et.al., 2001, A New Concept ofMotion Suppression Board Attached to Bottom End of
Floating Structures, Offshore Mechanics and ArcticEngineering[10] Masami Matsuura, et.al., 1994, Dynamic Response
Analysis of Composite Floating Bodies in wave,Mitsubishi Technical Review VOLUME 31 (inJapanese)[11] Toshihiko Nagai, 2002, Long Term Statistics Reporton Nationwaide Ocean Wave information network forPorts and HArbourS (NOWPHAS 1970-1999),
Technical Note of The Port and Airport ResearchInstitute No.1035 (in Japanese)

Fig.13. Design Wave and Observed High Waves
Since there is potential flow strength under theinfluence of ocean current Kuroshio, it is expected thatdrag force acting on the floating structure in submerged
section also becomes very large load. Mooring design toensure adequate security has been carried out, to be able tosafely position FOWT acted maximum load due to wind,
wave and current even if all of them comes from the same
direction based many tank tests and mooring simulation.
IV. CONCLUSION
The concept design on a V-shaped semi-submersible
floating structure is discussed. Tank tests and wind tunnel
tests for the purpose of acquisition of the design data areshown. It is shown that this floating structure has both
excellent stable performance and anti-wave performance,
and also satisfies the rules for the safety of mooring
equipment. About the serious issue on dynamic stability
performance including negative damping due to windturbine control system, many simulations were conducted
and it is prospected that it might be avoided.

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