GRAND RENEWABLE ENERGY 2018 Proceedings
June 17(Sun) - 22(Fri), 2018
Pacifico Yokohama, Yokohama, Japan

P-We-31METOCEAN MEASUREMENT AT FUKUSHIMA OFFSHORE SITE
1

1

1

Atsushi Yamaguchi , Shigeru Taki , Takeshi Ishihara
1

Department of Civil Engineering, The University of Tokyo, 7-3-11 Hongo Bunkyo-ku Tokyo 113-8656 Japan

Metocean measurement at Fukushima test site was carried out and metocean characteristics were investigated. The
effect of met mast tower on the measured wind speed can be mitigated by using the CFD simulation. Furthermore, thecorrected wind speed data from the cup anemometer show good agreement with the wind speed measured by Lidar. Theprevailing wind direction is NNW in winter and SSW in summer. The spectral significant wave height and period measured
by wave buoy show good agreement with the statistical significant wave height and period measured by the wave meter.
The prevailing wave direction is easterly all through the year. The current speed and direction measured by ADCP andwave meter show good agreement. The prevailing current direction is southbound cause by local current (Oya-shio)
Keywords: metocean measurement, metocean characteristics

INTRODUCTION
For the planning the design of floating offshore wind
farm, the investigation of metocean condition including thesea current is important. In this study, metocean
measurements were carried out at Fukushima Offshore
test site as a part of the Fukushima Floating Offshore
Wind Farm Demonstration (Fukushima FORWARD)
project [1, 3] by using the measurement devices installedon the floating substation and wave buoy. Measurement
data obtained from multiple devices are cross-validated
and missing data are compensated by using the datameasured by the other device to generate complete andconsistent data sets for one year. Then the metocean
characteristics of the site is investigated.

minimize the effect of the tower itself, at three different
heights (40m, 50m and 60m above sea level). An
additional wind vane is installed at the top of the tower tomeasure the unaffected wind direction, which is used todecide which anemometers can be used. A Doppler Lidaris installed on the deck, which measures wind speed anddirection from the height 60m to 290m above sea level.

OUTLINE OF THE MEASUREMENT SYSTEMOffshore met mast is located on the floating
substation, 20km offshore the Fukushima Coast, where
the water depth is 120m as shown in Fig.1. Fig. 2 shows
the met mast on the substation.
Floating substation

Approx. 20km

2M W floating offshore
w ind turbine

Fig. 2. Schematic and overview of the substation
Table 1. The list of the measurement device(a) meteorological measurement device
parameter
devicelocation
wind speed cup anemometer met mastwind direction
wind vane
met mast
wind speed
doppler lidar
deck
wind direction

Fig. 1. Fukushima offshore site.
Multiple measurement devices are employed tomeasure each metocean parameter to ensure thereliability of the system. This also allows to cross validate
the measured data to decrease the uncertainty of themeasurement. Table 1 and Fig.3 show the list of themeasurement devices.
Three sets of cup anemometers and wind vanes are
installed with the horizontal interval of 120 degree to(b) oceanography measurement device
parameter
devicelocationwave height
500m fromwave buoywave period
substation
wave heightwave period
wave meter middle hullcurrent speed
current direction
current speedADCP
Middle hullcurrent direction

GRAND RENEWABLE ENERGY 2018 Proceedings
June 17(Sun) - 22(Fri), 2018
Pacifico Yokohama, Yokohama, Japan

A wave meter and an ADCP are installed on the
middle hull of the substation and a wave buoy is installedon the sea 500m from the substation.

CFD simulation was considered to estimate theunaffected wind speed. The wind vane located at the topof the tower, the wind direction measured by which is notaffected by the tower, is used to decide the wind direction.

Wind speed ratio

2.5ObsSimulation

2.0

1.5

1.00.590180
Wind direction[deg]270360Fig. 5. Directional wind speed ratio

Wind data measured by the cup anemometer
installed on met mast is affected by met mast itself.
Computational fluid dynamics (CFD) simulations aroundthe mast is carried out to investigate the effect of the met
mast. The CFD simulation is validated by using therelative wind speeds measured at three anemometers on
the same height. Fig.4 shows the calculated wind field.
The wind speed decreases significantly at the leeside of
the tower and increases at the side of the tower. Slightdecrease of the wind speed can be seen upstream of thetower. Fig.5 shows the wind speed ratio between two
anemometers as a function of the wind direction at theheight of 50m above sea level. When one of the
anemometers is in the wake of the tower, the wind speed
ratio decreases or increases significantly. Even whennone of them is in the wake, the effect of the tower is no
negligible giving higher of smaller value of wind speed
ratio than 1.0. In any wind direction, CFD results show
good agreement with measurements.

𝑢Lidar = 𝑎𝑢Cup + 𝑏(1)𝑏 = 0.3

𝑎 = 0.99;
35

R=0.999360

R=0.99930
27025

Lidar[deg]WIND DATA

Lidar[m/s]Fig. 3. Metocean measurement device system

Corrected wind speed and direction and the results ofthe doppler lidar were compared at 60m above sea level,
where, both measurements are available (Fig. 6). Bothwind speeds and directions are consistent. In this study,
wind data measured by Lidar (𝑢Lidar ) is considered to beaccurate, and when data from Lidar is missing, the windspeed measured by the cup anemometer (𝑢Cup ) is usedconsidering the sight difference between them (Eq.(1)).

20151018090

500510152025303500

Cup anemometer[m/s]90180270360

Wind vane[deg](a) Wind speed(b) Wind direction
Fig. 6. Comparison between cup anemometer and Lidar

WAVE DATA
The data obtained by wave buoy includes theunexpected peak in low frequency part of the spectrum.
Fig.7 shows such an example. This is probably someproblem in the measurement and in this study, if the peak
period is longer than 24.9s (frequency is lower than0,04Hz), the data is treated as missing.
Fig. 4. Flow field simulated by CFD
To estimate unaffected wind speed, this CFD result
was used. The wind speed measured at the cup
anemometer located upstream of the tower is used anddepending on wind direction, speed up ratio calculated byThe evaluation time of the wave spectrum in thewave buoy is 30 minutes and the evaluation time ofsignificant wave period in the wave meter is 20 minute. On
the other hand, the international standard IEC 61400-3[2]specifies the evaluation time of 60 minutes. In this study,GRAND RENEWABLE ENERGY 2018 Proceedings
June 17(Sun) - 22(Fri), 2018
Pacifico Yokohama, Yokohama, Japanthe wave statistic measured by the wave buoy and thewave meter are converted into 60 minutes value. Fig.8
shows the comparison between spectral significant wave
height (𝐻𝑠 ) and statistical significant period (or sometimes
called energy period, 𝑇𝑚−1,0 ) obtained by the wave buoyand statistical significant wave height ( 𝐻1/3 ) andsignificant wave period (𝑇1/3 ) obtained by wave meter.
The statistical and spectrum wave statistics show fairy
good agreements. In this study these wave statics are
considered to be equal.

CURRENT DATA
Current speed and direction measured by wave
meter and ADCP are shown in Fig.9. The data measured
by wave meter is measured at 11m below sea level and20minutes average value. The data by ADCP is measured
at 11.78m below sea level and also averaged over 20minutes. Both measurements show fairy good agreement.
In this study, current data by wave meter is used becauseof better availability. If the data is missing, the datameasured by ADCP is employed.
R=0.859360

1.0

R=0.892

270

Wave meter[deg]Wave meter[m/s]0.80.60.418090

0.2

0.00.00.2

0.4

0.6

0.8

01.0090

180ADCP[m/s]Fig. 7. An example of unexpected spectrum measured by
the wave buoy (2015/06/25 5:42)
815

Wave buoy Te[s]Wave buoy Hs[m]METOCEAN CHARACTERISTICS AT
FUKUSHIMA OFFSHORE SITE

R=0.934

65432

10510360(a) Current speed(b) Current direction
Fig. 9. Comparison between wave meter and ADCP

20

R=0.9757270

ADCP[deg]

012345670805

Wave meter H1/3 [m]101520

Wave meter T1/3 [s](a) Wave height(b) Wave period
Fig. 8. Comparison between wave buoy and wave meter

The metocean characteristics at Fukushima offshoresite is investigated based on the measurement fromJanuary to December in 2015.
The statistical of the wind data is shown on Fig.10.
Monthly mean wind speed at 60m height is between
5.9m/s and 8.1m/s and annual mean wind speed is
7.2m/s. The prevailing wind direction at 60m above sea
level is NNW and SSW. The SSW wind accounts for
12.9% and typically seen in summer and NNW windaccounts for 12.6% and dominant in winter.
10
Mean wind speed[m/s]

Wave data measured by the wave buoy is usedbecause of better availability. If data is missing, the wave
data measured by the wave meter is used. Table 2 shows
frequency distribution table of wave height and wave
period. The highest frequency occurs when the significant
wave height is between 1.00m and 1.25m and significant
wave period is between 8s and 9s.

8

8.1 8.0

7.5

8.17.4

7.5

6.8
5.9

64
20

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2015(a) Monthly mean wind speed

Table 2. Frequency distribution table of wave heightWave
height

0.0～

1.0～

2.0～

3.0～

4.0～

5.0～

6.0～

7.0～

Frequency Frequency
9.0～ 10.0～ 11.0～ 12.0～ 13.0～ 14.0～ 15.0～ 16.0～
occurrence rate[%]

8.0～

0

0.0

70.1

50.1

100.1

18

0.2

1

29

0.3

62

0.7

1

186

2.1
2.7

7.00～

2

52

1

1

1

52

1

4

4

4

6.50～ 7.00
6.00～ 6.501

5.50～ 6.00

1
1

2

5.00～ 5.50

33

61

15

2

4.00～ 4.50

1421

1058

12

1

3.50～ 4.00

42246

43

25

25

182

3.00～ 3.50

7

30

66

76

30

1713

239

5

4.50～ 5.00

12355

115

154

76

16

16

460

5.3

2.00～ 2.50

794

180

317

131

81

63

23

896

10.2

1.75～ 2.00

1476

208

238

169

69

21

3

798

9.1

1.50～ 1.75

41

140

338

338

187

138

28

101220

13.9

472

210

423

360

297

89

36

6

1497

17.1

1.00～ 1.25

8

119

223

526

450

230

27

8

1591

18.2

0.75～ 1.00

17110247

419

418

153

92

1375

15.7

0.50～ 0.75

23698

149

73

71

2.50～ 3.00

1.25～ 1.50

10.25～ 0.50
～ 0.25

366

4.2

1

0.0

0

0.0

Frequency
occurrence

0000

31399

1111

2310

2366

1466

628

238

114

76

17

40

8760

100.0

Frequency
rate[%]

0.0

0.0

0.0

0.0

0.4

4.6

12.7

26.4

27.0

16.7

7.2

2.7

1.3

0.9

0.2

0.0

0.0

100.0

Frequency distribution[%]15
Wave
period

7.2 7.4

6.3 6.2

N20(%)

ObsWeibull

15Weibull(K)=1.88
Weibull(C)=8.1210NWNE

1050WE5SESWS0051015Wind speed[m/s]2025(c) Wind rose(b) Wind speed frequency distribution
Fig. 10. Statistical wind data (+60m)

GRAND RENEWABLE ENERGY 2018 Proceedings
June 17(Sun) - 22(Fri), 2018
Pacifico Yokohama, Yokohama, Japan

test site was carried out and metocean characteristics
were investigated. Following conclusions are obtained.
1) The effect of met mast tower on the measured windspeed can be mitigated by using the CFD simulation.
Furthermore, the corrected wind speed data from thecup anemometer is same as that from the Lidar. The
prevailing wind direction is NNW in winter and SSW insummer.
2) The spectral significant wave height and period
measured by wave buoy show good agreement withthe statistical significant wave height and period
measured by the wave meter. The prevailing wave
direction is easterly all through the year.
3) The current speed and direction measured by ADCPand wave meter show good agreement. The prevailingcurrent direction is southbound cause by local current(Oya-shio)
0.35
Mean current speed[m/s]

The statistics of the wave data is shown on Fig.11.
Monthly mean significant wave height is between 1.3m
and 2.0m and annual significant wave height is 1.6m.
Monthly mean significant wave period is between 7.4s
and 9.3s, Due to the influence of swell from Pacific Ocean,the significant wave period shows large value all throughthe year. The prevailing wave direction is easterly, whichmeans wave generally comes from the Pacific Ocean.
The most frequent wave period is between 8s and 9s, withthe frequency of 27%. The frequency of occurrence ofsignificant wave period shorter than 5s accounts only0.4%. The highest frequency of significant wave height
can be found between 1.00m and 1.25m with thefrequency of 18.2%. The case where the wave height issmaller than 0.5m, is very few.
The statistics of the current are shown in Fig.12.
Monthly mean current speed at 11m below sea level isbetween 0.09m/s and 0.31m/s and annual mean current
speed is 0.18m/s. The southbound current direction isdominant, which is caused by the local ocean current,
Oya-shio.
2.5
1.8 1.8 1.8
1.5

1.0

8.5
7.8
7.4

6.5

NE

5200W

E

10

0.9ー1.0SESW
S

current speed[m/s]

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2015(b) Monthly mean significant wave period(b) Frequency distribution of (c) Current direction
frequency distribution
current speed

25
2015

Fig. 12. Statistical current data

1020(%)

515

7.00～

6.50～7.00

6.00～6.50

5.50～6.00

5.00～5.50

4.50～5.00

4.00～4.50

3.50～4.00

3.00～3.50

2.50～3.00

2.00～2.50

1.75～2.00

1.50～1.75

1.25～1.50

1.00～1.25～0.25

0.50～0.75
0.75～1.00

00.25～0.50

Frequency distribution[%]

NNW

1030

07.0NNWNE

Acknowledgement

1050WEWave height[m](c) Significant wave heightfrequency distribution

SESW

30

S25(e) Wave rose

20
15

16～15～16

14～15

13～14

12～13

11～12

10～11

9～10

8～9

7～8

6～7

5～6

4～5

3～4

2～3

1～2

50This research is carried out as a part of a project ofFukushima floating offshore wind farm demonstration
project (Fukushima FORWARD) funded by the Ministry ofEconomy, Trade and Industry (METI), Japan.
Theauthors wish to express their deepest gratitude to theconcerned parties for their assistance during this study.
References

100～1

Frequency distribution[%]150.0ー0.1

7.5

20

40

0.8ー0.9

7.7

7.625(%)

50

0.2ー0.3

7.9

8.0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2015

1.0～

9.3

8.3

8.2

Frequency distribution[%]Mean significant
wave period[s]9.1 9.1

9.0

0.09(a) Monthly mean current speed(a) Monthly mean significant wave height
9.00.140.05

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2015

9.5

0.15

0.09

0.7ー0.8

0.0

0.17
0.14
0.11

0.10

0.00

0.5

8.5

0.15

0.6ー0.7

1.5

1.3 1.3

0.5ー0.6

1.9
1.5 1.5

0.2

0.20

0.4ー0.5

1.6
1.5

0.1ー0.2

Mean significant
wave height[m]

2.0

0.26

0.25

0.3ー0.4

2.0

0.31 0.31

0.30

wave period[s](d) Significant wave period
frequency distribution[1] T, Kawahigashi, A. Yamaguchi, T. Ishihara,:“Measurement of a metocean and floater motion for
Fukushima floating offshore farm project”, 39th Wind
Energy Symposium, pp.197-200, 2014 (InJapanese)

Fig. 11. Statistical wave data

[2] IEC61400-3: Wind Turbine – Part3: Design requirements for offshore wind turbines, 2009

CONCLUSIONS[3] Fukushima FORWARD:
http://www.fukushima-forward.jp/english/index.html.

In this study, metocean measurement at Fukushima