論文番号 2015S‑OS1‑1

日本船舶海洋工学会講演会論文集 第 20 号

Verification of Precision Concerning the Design ofAdvanced Spar Type Structure
byRyo Matsuoka*

Haruki Yoshimoto*, Member

Key Words: Floating Substation, Floating Wind Turbine, Advanced Spar, Observed Data

1. INTRODUCTION
In Japan, followed by the Great East Japan Earthquake andnuclear accident, the energy composition has been
fundamentally reviewed. As a result, the demand forrenewable energy is growing. Among them, the potential ofoffshore wind energy is very high, which is estimated at 1.6
million MW. Many kind of research has been conducted tointroduce it. The government of Japan has started theexperimental research project of the world’s first floating
offshore wind farm, which is conducted by the consortium
made up of industry-academic-government organization1).
This project is named as “Fukushima FORWARD (Fukushima
Floating Offshore Wind Farm Demonstration Project)”.
Concerning offshore wind energy, there are two types ofplatform, bottom mounted type and floating type. Since
Japanese coast is topographically steep and has less area ofshoal, floating type which isn’t affected by water depth is moresuitable. Until now, a number of concepts represented bysemi-submersible type, spar type and TLP type were proposedto accomplish sufficient stability and low motion performance
which is required with the floater part of floating offshore
wind turbine (FOWT).
Advanced spar type is developed by JMU based on spartype2). This type has advantage in terms of motion
performance, installation cost and ease of maintenance.
Advanced spar type Floating substation was installed in 2013,
and began to observe meteorological data, hydrographic data,and motion data.
In this paper, we discuss about verification of precision
concerning the design of advanced type structure using
observed data. We also introduce how we make use ofverification data in next floating structure.

wave force is not so much big since the shape of Floating
substation is almost axisymmetric shape. So, we don’t include
the analysis of Yaw motion.
Concerning mooring characteristic, we compare the result of
the position where tension of mooring balances to the externalforce.
We show the definition of Fore/Aft and target positions ofevaluation for structural stress in Fig.2. C1, D1, and F1 are set
in the direction of aft. C2, D2, and F2 are set in the direction ofstarboard side.

Fig.1 Flow of Verification.
Table.1 Target of Comparison.
Verification
Item

Target of Comparison

ExternalForce

MotionPerformance

Surge, Sway, Heave,Roll, Pitch [m,deg]

Wave

Mooring
Characteristic

Horizontal
Displacement [m]Wave, Wind,
Current

Structural Stress

Axial Stress

[MPa]

Wave

2. METHOD OF VERIFICATION
2. 1 Outline of Verification Method
We verify the design method by comparing observed value
and design value concerning motion performance, mooring
characteristic and structural stress. Flow of verification isshown in Fig.1.
Period of verification is set as 3 hours which is generally
considered to be duration time of meteorological condition.
When we calculate design value, we use average orsignificant value of meteorological data. Targets forcomparison are summarized in Table.1.
Concerning motion performance, Yaw motion induced byC1
Aft

D1

F1

* Japan Marine United Corporation

C2ForeD2
F2Fig.2 Model of Floating Substation.

Received 10th April 2015
Read at the spring meeting 25th and 26th MAY 2015
©The Japan Society of Naval Architects and Ocean Engineers─1─

2. 2 Calculating Method of Design Value(1)Acquisition of Meteorological data
Meteorological data which is used in the verification issummarized in Table.2. Each data is acquired by the devices
which have been installed with Floating substation. In thisverification, we don’t consider the effect of floater’s motion as
effect of floater’s motion is not considered to be large.
Concerning meteorological data, we calculate average orsignificant value of 3 hours for each observed data by applying
statistical processing to time-series data.
Table.2 Meteorological Data.
ExternalForceWind
Wave
Current

Sensing
Data
Velocity,Direction
Height,Period,Direction
Velocity,
Direction

Measuring
DeviceSupersonic Anemometer

3. RESULT OF VERIFICATION
3. 1 Meteorological Condition

Directional Wave Meter

We selected below period as the period of verification.
Period : 0:00～3:00 1 May 2014

Directional Wave Meter(2)Calculating the Response to Meteorological Condition
Methods of analyzing each response to meteorologicalcondition are indicated below.
①Motion Performance
By inputting significant values calculated in (1) to theresponse function which was calculated by the threedimensional sink-source method program, we conduct
short-term prediction. We assume Bretschneider-Mitsuyasu
spectrum and cos2χ distribution as a spectrum of wave anddirection distribution function respectively.
②Mooring Characteristic
We calculate external force using each average orsignificant value of wave, wind, and current calculated in (1).
Then, we calculate the position where restoring force ofmooring balances to the external force.
③Structural StressWe calculate stress function of each position by FEM using
motion function calculated in ① . By inputting eachsignificant value calculated in (1), we conduct a short-term
prediction. We assume Bretschneider-Mitsuyasu
spectrumand cos2χ distribution as a spectrum of wave and direction
distribution function respectively as same as ① .

2. 3 Calculating Method of Observed Value
Observed data of motion and response used in verification issummarized in Table.3.
Table.3 Motion and Response Data.
Target ofSensing
Measuring
Observation
Data
DeviceSurge, Sway,
Heave

Position X, Y, Z

RTK-GPSRoll, Pitch

Rotation θ, φFiber Optic Gyro(FOG)

Position X,Y

RTK-GPSStrain ε

Strain Gauge

Horizontal
Displacement
Axial Stress

Concerning observed data of motion and response, wecalculate average or significant value of 3 hour for eachobserved data by applying statistical processing to time-series
data. Methods of analyzing each data are indicated below.
①Motion Performance
By processing time-series data of position X, Y, Z, androtation θ, φ, we calculate the standard deviation of eachmotion.
②Mooring Characteristic
By calculating average value of position X, Y, we calculate
horizontal displacement during each period of verification.
③Structural Stress
By processing time-series data of stress which is obtainedby multiplying young’s modulus to strain, we calculate thestandard deviation of stress for each position.

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The analytical values of meteorological data during theperiod of verification are summarized in Table.4. The directionof wave, current and wind is defined according to thecoordinate system in which 0 degrees conform to North of theEarth in a clockwise manner.

WindWave

Current

Table.4 Meteorological Codndition.
Average Velocity
13.2 m/s
Average Direction

178.0 degSignificant Wave-Height3.9 m

Significant Wave-Period

8.3 s

Significant Wave-Direction

166.3 deg

Average Velocity

0.2 m/s

Average Direction

220.1 deg

When we select the period of verification, we consideredbelow matters.
・All datum don’t have a deficit.
・Meteorological conditions are relatively severe.
→It is considered that the precision of the observed data
will be relatively fine, as noise of datum is suppressed if
absolute values of observed data are large.
・Directions of wave, wind and current are almost constantduring each period.
→It is expected to be complicated to analyze if directions
change frequently during the period.

3. 2 Result of Comparison①Motion Performance
Result of comparison between design values and observed
values is shown in Fig.3. Regarding the result, design values
are higher than observed values. Possible factor of this result isthat wave-spectrum or direction distribute function doesn’taccord.
Plausibility of wave spectrum and direction distributefunction is to be verified based on detail analysis of observed
data.

Fig.5 Result of Structural Stress.

4. MAKE USE OF VERIFICATION DATA IN
NEXT FLOATING STRUCTURE
As second phase of the project, construction of two large
FOWT is planned. JMU is in charge of constructing floater
part of one (We call it as 2nd floater). As 2nd floater is to beinstalled large wind turbine, sufficient stability and low motion
performance are required as same as Floating substation. And
the dock where we can construct is limited. So, we decided
adopting advanced spar type to 2nd floater.
We used the knowledge about motion performance which
was acquired through the previous described verification todecide principle particulars of the floater. The shape andprinciple particulars of 2nd floater are shown in Fig.6 andTable.5 respectively.

Fig.3 Result of Motion Performance.
②Mooring Characteristic
The result of comparison between design value andobserved value is shown in Fig.4. This result shows average
position during the period. Origin of the coordinate is definedas neutral position of the floater. Right figure shows average
direction of wind, wave, and current.
The results are well accorded.

Fig.6 Shape of 2nd floater.
Table.5 Principle Particulars of 2nd floater.
ItemFig.4 Result of Tensile Force of Mooring.
③Structural StressResult of comparison between design values and observed
values is shown in Fig.5. Order of the result is almost same as
we assumed. One of the possible factors of error is thatwave-spectrum or direction distribute function doesn’t accord.
Another factor is that observed value is the result of surface,despite design value is the result of neutral axis. We have toconsider local bending stress to conduct more precise
evaluation.

─3─Length
BreadthHeight(Bottom to Upper Dk.)
Draft (Operating)
Displacement

Value
58.9 m51.0 m
48.0 m33.0 mAbt. 30,000 ton

One of the different things from Floating substation is that
2nd floater has a big upper hull. The role of it is increasingstability of the floater by enlarging area of water plane. It isalso different that 2nd floater has two hulls which is under thewater surface while Floating substation has 3 hulls. The
purpose is improving ease of construction considering
installation of wind turbine and improving motion
performance. We used some kind of parameters which wereobtained in the previous described verification to estimate

motion performance and structural stress of 2nd floater.
Concerning mooring points, we ensure ease of operation andcorrosion resistance by installing them at submerged positionof the floater as same as Floating substation.
2nd floater is to be installed in current year.

5. SUMMARY
We compared results of design value and observed value formotion performance, mooring characteristic, and structuralstress respectively. We confirmed that results were almost
coincident.
We will research about wave conditions based on theanalysis of wave spectrum and direction distribution function.

ACKNOWLEDGMENT
We are grateful to Ministry of Economy, Trade and Industry,and members of consortium which promote Fukushima
FORWARD project together.

REFERENCES
1) T. Ishihara: Current status and future view of Fukushima
floating offshore wind farm demonstration project, Journal ofWind Energy, Vol. 36, 2013, pp.553-556.
2) H. Yoshimoto et.al. : Development of floating offshore
substation and wind turbine for Fukushima FORWARD,
Proceedings of the International Symposium on Marine andOffshore Renewable Energy, Oct 28-30, 2013.

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