Grand Renewable Energy 2014, July27-August 1 O-WdOc-1-4

METOCEAN DESIGN CONDITION FOR “FUKUSHIMA FORWARD” PROJECT
Takeshi Ishihara 1, Kenji Shimada 2 and Akihiko Imakita 3
1

The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656 Japan
Shimizu Corporation, 3-4-17 Etchujima, Koto-ku, Tokyo, 135-8530 Japan3
Mitsui Engineering & Shipbuilding, 5-6-4 Tsukiji, Chuo-ku, Tokyo, 104-8439 Japan
2Metocean condition for the Fukushima FORWARD project is presented. Wind speeds and tsunami are predicted byMonte Carlo simulation and Tsunami simulation, respectively. For wave condition, extreme sea state and normal sea state
are evaluated by published data and newly proposed wind-wave and swell combination formula, respectively. Surface
current and water level are evaluated by extreme value analyses of hindcast simulation and historical data, respectively.
Keywords: floating wind turbine, design conditions, Monte Carlo simulation, extreme value, tsunami

INTRODUCTION
For the revitalization in Fukushima prefecture fromdisasters by the Tohoku region Pacific coast earthquakeand accident of Fukushima Daiichi nuclear power plant in2011, Japanese government initiated the world first
Floating OffshRe Wind fARm Demonstration project(FORWARD project). In this paper, results of assessment
on metocean conditions for 2MW floating wind turbine andthe world first floating substation are presented for windspeed, water level, wave, current and tsunami.
LOCATION OF THE FORWARD PROJECT
Site of the FORWARD project is located 20km
offshore of the east coast of Fukushima prefecture where
sea depth is approximately 120m.

the extratropical cyclones when estimating extreme wind.
Therefore, in this study, 50 year extreme wind speed wasevaluatedbysynthesizing
probabilities
ofnon-exceedance of two independent processes, i.e.
typhoon FT u  and extratropical cyclone FE u  by Eq.(1)[1]. Figure 2 shows the synthesis of the probability
distributions of annual maximum wind speeds by tropicaland extratropical cyclone, where probabilities ofnon-exceedance of the annual maximum wind speed wascalculated by Monte Carlo simulation of 10000 years fortropical cyclone and method by Gomes andVickery(1977)[2] for extratropical cyclone based on dataobtained by converting 2 year data observed at nearby
gas field to the hub height. 50 year extreme wind speed at
hub height of 60m was estimated as 48.3 m/s.

FM u   FE u  FT u (1)Fig. 1. Location of the FORWARD project.
METEOCEAN CONDITIONS

Fig. 2. 50 year extreme wind speed.

Wind speed, water level, wave, surface current andtsunami for design condition are assessed below bymeans of simulations and measurements.

Accumulation time for the assessment of fatigue wasalso evaluated by combining the two frequencies of wind,i.e. typhoon wind and non-typhoon wind. Mixed frequency
distribution was estimated by assuming that they can beapproximated by the Weibull distribution, where k=1.99
and c=15.27m/s for typhoon wind and k=1.73 andc=8.06m/s for non-typhoon wind. Figure 3 shows theAnnual accumulation times for each wind speed.

Wind speeds
In the mixed climate like Japan, it is important totaken into account both of wind climates, i.e. tropical andGrand Renewable Energy 2014, July27-August 1 O-WdOc-1-4

For meteorological tide, 50 year typhoon sea level
departure from normal was estimated by the Eq.(2),2  ap50  bU 10cos   c(2)where a, b, c and  are empirical constants provided by [3],U10 is a 50 year 10 min. wind speed at 10m above meansea level and p 50 is a 50 year value of the centralpressure depression. U10 was calculated from hub heightwind speed of 48.3m/s, which was obtained by Monte
Carlo simulation, with power law exponent =0.1.  is anangle to the wind of U10, and was set as zero for aconservative estimate of .

p50 was determined by extrapolation of extreme value
analysis of pressure depression data which is defined as
difference between ambient pressure and centralpressure for typhoons which passed through an area
within 500km of radius from the site and whose centralpressure was less than 985 hPa in 47 years from 1961 to2007. Figure 4 shows a log-normal approximation of p50.

Fig. 3. Annual accumulation times.
50

230

1.9
20

1.8

100-2log(p)

U(m/s)

40

Gumbel plot
Method of moment± limit120
1 3 6 12Return period (month)
02468

1.7
1.6
1.5

10-ln(-lnF)

1.4

1.2

Fig. 4. Extreme value analysis for 1 year extreme windspeed.
1 year recurrence wind speed was estimated by theextreme value analysis based on the gas field data.
U1=32.5m/s was obtained by the method of moment withuncertainty of 3%. Because U1/U50=0.67 < 0.8, an estimateof 38.6 m/s by IEC61400-1 Ed.3 was confirmed to beconservative. Power law exponent of vertical profile wasestimated by 3D analysis as 0.1 which is as same as 0.11by IEC61400-3. For the reference turbulence intensity,0.12 by IEC61400-3 was used. Since nearby observation
was lower than it, it will be a conservative estimate. ForI 90  1 U hub ,power generation and fatigue, 1  I ref 0.75U hub  5.6 and =0.14 was used.
Water level
Highest High Water Level was obtained by combining
the astronomical with the meteorological tide. According
to IEC61400-3, Highest Astronomical Tide (H.A.T.) is used
for astronomical tide, however, in Japan High Water Level(H.W.L.) has been commonly used [3]. Therefore, here,according to [3], H.H.W.L. was estimated by adding
meteorological tide which is a 50 year return period oftyphoon sea level departure from normal to H.W.L.
For astronomical Tide, Onahama data was used as
M.S.L.=C.D.L.+0.84m and H.W.L.=C.D.L.+1.44m.

Observation
Log-normal distribution

1.3-2-10123

45ln(-lnF)

Fig. 5. Probability distribution of the central pressure
depression for a use in the prediction of themeteorological tide in the highest high water level.
Non-exceedance probability of 50 year is calculated
as F  1 1 R  , where  is average occurrence peryear=N/K, N is a number of typhoon, K is an observation
period. Since N=81 for the site, =1.7234, F=0.988395 andp50=67.5hPa. In Table 1, two meteorological sites nearest
adjacent to Onahama, i.e. Chosi and Miyako, werecompared and conservative vale of C.D.L.+2.77m at
Choshi was adopted as the highest high water level of thesite.
Table 1. Water level.
Parameters
U10min (m/s)
p (hPa)
(m)H.W.L.(m)H.H.W.L.(m)Choshia=0.622b=0.056c=0Miyako
a=1.193b=0.012c=040.4
67.5

1.33

1.00
C.D.L.+1.44
C.D.L.+2.77
C.D.L.+2.44
C.D.L.+2.77

Grand Renewable Energy 2014, July27-August 1 O-WdOc-1-4

Wave height and wave period

H 0,SMB Extreme sea state was evaluated based on thepublished data in Table 2. From the table, 11.71m wasadopted for 50 year extreme wave height and 13s arecombined to determine the extreme sea state.

51.37  2U 10  2  31
1

0.
008
235000gU10 g H 0,swell  1.31  2.46  1.31U 10 12 ,Table 2. Published extreme sea state.
Sources of published data

Wave height(m)Tomioka
Fukushima
Fukushima prefecture civilengineering designmanual(2008)
INPEX gas field design value

11.71(ALL)
11.0(ENE-ESE)

Wave
period(s)138.4(E-SE)

1320/1.86=10.8

12NILIM[4]T0,SMB 22 0.3U 102  21  1  0.004 235000 gU 10   g  

T0,swell  8  max 0.4 tan 1 0.34U 10  1.88  0.39,0(4)Figure 6 shows comparisons between Eq.(4) andfollowing defined mean and equivalent significant wave
heights and harmonic mean wave period which are
obtained based on NMRI database [5] of this area. Waveheight and period in the low wind speed region are
successfully represented by this proposed model.

20
Equivalent significant wave height(NMRI database)
Averaged significant wave height(NMRI database)
Eq(4)Wind wave(SMB)
SwellNILIM
INPEX Iwaki gas field

H0,me, H0,eqv(m)1510nH 0,eqv  m1H 0m,in i 1nm  4,H 0,me 

1H 0 ,i ,n i 1(5)n

1T0,me  nTi 1 0,i

5

200

1020

30

40U10(m/s)

U(m/s)

1.6

20
NMRI database
Eq(4)Wind wave(SMB)
Swell
NILIM
INPEX Iwaki gas field

T0,me(m)

15

1.4
1.2
10.8

10

20.6(a)50Gumel plot
Method of moment± limit

1.8010203050 100-2-101234

51.640Gumbel plot
Method of moment± limit

1.4
1.2

Fig. 6. Wave height and wave period for normal sea state
where swell is combined with wind-wave.

U(m/s)

1

In the normal sea state, wave height and wave period
were estimated by weighting average of wind-wave andswell empirically as follows,

T0  T0,SMB  1   T0,swell10 20-ln(-lnF)

U10(m/s)

H 0  H 0,SMB  1   H 0,swell5

Return period(year)

0.8
0.6
0.4
0.2(3)(b)

03 6 121120

Return period(month)
-2024

68

10-ln(-lnF)

where subscripts “SMB” and “swell” indicates estimates ofsignificant wave height and corresponding wave period by
SMB method and swell respectively and  is theirweighting function which are obtained as follows,Fig. 7. Extreme value analyses of surface current for 50year(a) and 1 year recurrence(b).

Grand Renewable Energy 2014, July27-August 1 O-WdOc-1-4

TsunamiTsunami was simulated by solving nonlinear shallow
water long wave equations numerically, see Fig.8. The
fault was modeled by Mansinha and Smylie model [7]which was combined with fault parameters by Imamura etal. [8]. The simulation was validated by wave
measurements by a NOWPHAS GPS buoy [9] near thesite. In Fig.9, estimated maximum peak water level andcorresponding horizontal velocity are 3.2m and 0.77m/s,
which is slightly smaller than a value estimated by thelinear long wave theory of 0.88 m/s in which convection
and friction are not considered. Annual mean surface
current of 0.1m/s [10] was added to the former value todetermine the design value.

4321
0-1-2

FORARD siteNOWPHAS GPS buoy (Onahama)

151617181920Time (GMT+9)

Velocity(m/s)

Figure 7 shows 50 year and 1 year recurrence valuesof surface current which were evaluated by extreme value
analyses of hindcast data by JCOPE [6]. Since the total
number of annual maximum data is limited to ten, its
uncertainty of 13.5% is too large to neglect. Therefore
extreme 50 year value was determined by adding half ofthe standard deviation to the value which was obtained by
the method of moment as 1.5m/s. On the other hand,there is little uncertainty in an extreme value distribution
based on daily maximum data, 1 year value is obtained
straightforward from the method of moment as 1.0m/s.

Surfaceelevation(m)Komatsu of Mitsubishi Heavy Industry and Yuji Awashima
of Japan Marine United for their cooperation to this study.

Surface current

0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

FORWARD site

151617181920Time (GMT+9)

Fig. 9. Tsunami surface elevation and absolute velocity.
References[1] T. Ishihara and A. Yamaguchi, “Prediction of theextreme wind speed in the mixed climate region by using
Monte Carlo simulation and measure-correlate-predict
method”, Wind Energy. Wiley Online Library, 2014.
[2] L.Gomez and B.J.Vickery, “On the prediction of
extreme wind speeds from the parent distribution”,
Journal of Industrial Aerodynamics, 2, 1977, 21-36[3] Ports and Harbours Bureau of Japan’s Ministry of Land,
Infrastructure, Transport and Tourism, “TechnicalStandards and Commentaries for Port and Harbour
Facilities in Japan”.
[4] E. Takata et al., “Distributions of the wave, storm surgeand tsunami design conditions on Japanese nationwide
coastal structures”, Technical Note of National Institute forLand and Infrastructure Management, No.88, 2003 (inJapanese).

Fig. 8. Tsunami simulation for the 2011 Off the Pacific
Coast of Tohoku Earthquake. Water levels in the figure
are at the moment of site maximum.

[5] NMRI, “Statistical database of winds and waves
around Japan”,
http://www.nmri.go.jp/wwjapan/namikaze_main_e.html[6] JCOPE, https://www.jamstec.go.jp/frcgc/jcope/

CONCLUSION
Metocean condition for a 2MW floating wind turbine
and a substation of the Fukushima FORWARD project
was presented. It is worthy to note that the world first
floating metocean observation was started its operation inDecember 2013 by which the prediction methodology will
be validated.

[7] L. Mansinha and D.E. Smylie, “The displacement fields
of inclined faults”, Bulletin of the Seismological Society ofAmerica, 61(5), 1971, pp.1433-1440.
[8] F. Imamura et al.,
http://www.tsunami.civil.tohoku.ac.jp/hokusai3/J/events/to
hoku_2011/model/dcrc_ver1.2.pdf (in Japanese).

ACKNOWLEDGMENT[9] NOWPHAS,
http://www.mlit.go.jp/kowan/nowphas/index_eng.html (inJapanese).

This study was accomplished in the Fukushima
floating offshore wind farm demonstration project which isfinancially supported by the Ministry of Economy, Tradeand Industry. The authors would like to thank to Masao[10] Japan Oceanographic Data Center,
http://jdoss1.jodc.go.jp/data/current/stat-o-cur_j.html (inJapanese).