The Wind Observation on the Pacific Ocean for Offshore Wind Farm

Manabu TSUCHIYA *, Takeshi ISHIHARA **, Yukinari FUKUMOTO ***

* Environmental Engineering Division, Kajima Corporation 3-7-1-29F, Nishi-shinjuku Shinjuku TOKYO 163-1029, JAPAN Phone: +81-3-5321-7321, Fax: +81-3-5321-7321, E-mail: manabu@kajima.com

** Institute of Engineering Innovation, School of Engineering, The University of Tokyo 2-11-6 Yayoi Bunkyo TOKYO 113-8656, JAPAN Phone: +81-3-5841-1145, Fax: +81-3-5841-1147, E-mail: ishihara@bridge.t.u-tokyo.ac.jp

*** Engineering R&D Division, Tokyo Electric Power Company 4-1, Egasaki Tsurumi YOKOHAMA 230-8510, JAPAN Phone: +81-45-613-5505, Fax: +81-45-613-7999, E-mail: fukumoto.yukinari@tepco.co.jp

Summary

For the investigation on the possibility of wind farm in the Pacific Ocean near Japan, wind speed and direction

were observed using a platform for natural gas mining, 37 km far from a coastline. The platform is located on the

east sea of the Tohoku district, northeast part of Japan. There is no island around the platform.

The platform has a trussed tower, and a windmill-type anemometer was installed at the top of the tower. The

sensor height is 94 m above the sea level. The anemometer was non-spark type for the gas-mining site. The

observation was started in September 2004, and it will be continued for three years. We recorded wind speed and

wind direction averaged over ten minutes, maximum and turbulence of wind speed.

The characteristics of observed wind are good for wind turbines. The annual wind speed average is 7.5 m/s, the

turbulence intensity is 8 %.

1. Introduction

Offshore wind farms, which are currently being developed in Europe, are expected to be introduced in Japan.

Consequently, many attempts have been made to estimate the offshore wind energy available around Japan. Ishihara

estimated that the offshore wind energy available along the coastline of the Kanto area near Tokyo is 280 billion

kWh/year. [1]

However, all the estimates made thus far are based on numerical simulations that have not been validated by field

measurements. The full-scale development of offshore wind farms requires an investigation of the offshore wind

conditions through measurement and validation of numerical simulations. For this reason, the authors conducted

offshore wind observations at a sufficient height at an offshore natural gas platform located 37 km off the coast, in the

Pacific Ocean.

2. Outline of the wind observations

Fig. 1 shows the locations where the wind observations were carried out. The observations were carried out at

two locations — one offshore and the other onshore.

The offshore observations were made at a platform for natural gas mining, 37 km off the coast. Photo 1 shows

the platform. A windmill-type anemometer was installed on the top of the drilling tower at a height of 94 m above

sea level. (Fig. 2) In general, drilling facilities for natural gas specify that only explosion-proof electrical equipment

be used; therefore, the anemometer used was of the type that detects the number of revolutions of the propeller and

the wind direction with optical signals through an optic fiber, instead of with electrical signals. The observations

began in September 2004 and are currently in progress.

For comparison purposes, a three-cup anemometer and a wind vane were installed on the top of a 24-m-high pole

for the onshore wind observations. (Photo 2)

Offshore Observation Site(Natural Gas Platform)

N37° 17’ 49”, E141° 27’ 47”

JAPANJAPAN

Tokyo

Pacific Ocean

Sea of Japan

Onshore Observation Site(Thermal Power Plant)

N37° 13’ 53”, E141° 0’ 35”

37km

Pacific Ocean

Fig. 1 Locations of offshore and onshore observation site.

Photo 1 The natural gas platform.

528

Lightning Rod Explosion-Proof TypeAnemometer

94 m heightabove sea level

1m 2m0

Fig. 2 Anemometer installed on the top beam of the trussed tower.

AAnneemmoommeetteerr aanndd wwiinndd vvaannee

Photo 2 On shore observation site in Hirono thermal power plant.

3. Results of the wind observations

The characteristics of the annual wind, which observed from October 2004 to September 2005, are presented

below.

3.1 Average wind speed

Fig. 9 shows the changes in the time-averaged wind speed on monthly basis, respectively.

Fig. 3 shows the changes in the average monthly wind speed. The average annual offshore wind speed is 7.5 m/s,

and the average annual onshore wind speed is 3.5 m/s.

The changes in the average monthly wind speed tends to be higher in winter and lower in summer. In particular,

during the period of January to May, the average monthly offshore wind speed is high, exceeding 8 m/s. The

highest average monthly onshore wind speed — 9.1 m/s — is observed in January. In June and July, the average

monthly offshore wind speed falls below 6 m/s, the lowest being 5.7 m/s in July.

0

2

4

6

8

10

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

Mea

n W

ind

Vel

ocity

(m/s

)

Offshore (H:94m)Onshore (H:24m)

Fig. 3 Average annual and monthly wind speeds.

The average annual and monthly wind speeds shown in Fig. 4 were calibrated for a height of 80 m above sea level

(and above the ground) to allow an easy comparison between the offshore and onshore measurements. The

following formula was used for the height calibration.

1.0

9480 9480

⎟⎠⎞

⎜⎝⎛=

mmVV moffshoreHmOffshoreH

, 2.0

2480 2480

⎟⎠⎞

⎜⎝⎛=

mmVV monshoreHmOnshoreH

Equation 1

The average annual offshore wind speed at a height of 80 m is 7.4 m/s, and the average annual onshore wind speed

is 4.4 m/s. The average annual offshore wind speed is 70 % higher than the onshore wind speed.

0

2

4

6

8

10

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

Mea

n W

ind

Vel

ocity

(m/s

)

Offshore (Calibrated for H80m)Onshore (Calibrated for H80m)

Fig. 4 Average annual and monthly wind speeds calibrated for a height of 80 m above sea and ground level.

3.2 Turbulence intensity

The average annual offshore turbulence intensity is 8 %, indicating that the offshore wind has a low turbulence.

The average annual onshore turbulence intensity is 24 %. However, this value was obtained at a ground height of 24

m and is therefore considered to be significantly affected by the surrounding buildings, trees, and topography.

Fig. 5 shows the turbulence intensity for each wind direction. Fig. 11 shows the changes in the time-averaged

turbulence intensity on monthly basis, respectively.

0153045

NNNE

NE

ENE

E

ESE

SESSE

SSSW

SW

WSW

W

WNW

NWNNW

Offshore(H:94m)

Onshore(H:24m)

Fig. 5 Turbulence intensity for each wind direction. (%)

3.3 Wind speed occurrence frequency

Fig. 6 compares the wind speed occurrence frequency between the offshore and onshore observations. The two

observations reveal clear differences. The offshore observations show a smoother occurrence distribution and a

higher frequency of the occurrence of high wind speed as compared to the onshore observations. For the offshore

observations, the Weibull distribution shape factor for the wind speed occurrence frequency is 1.67 and the scale

factor is 8.5.

Assuming a rated wind speed of 12 m/s for wind power generators, the occurrence frequency of wind speeds

higher than the rated wind speed is high at 17 %.

0

5

10

15

20

25

30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Win

d sp

eed

occu

rrenc

efre

quen

cy (%

) Offshore(H:94m)Onshore(H:24m)

(m/s)

Fig. 6 Wind speed occurrence frequency.

3.4 Wind direction occurrence frequency and wind energy density

Fig. 7 shows the wind direction occurrence frequency. The most frequently occurring offshore wind direction is

northwesterly (9.4%). The occurrence frequency of northwesterly to north-northeasterly winds is 7.8% to 9.2% and

that of southerly winds is 9.3%, which indicates a relatively high occurrence frequency of north-southerly winds.

The onshore and offshore wind direction occurrence frequencies show a similar trend. Fig. 12 shows the changes in

the time-averaged wind direction on monthly basis, respectively.

The annual offshore wind energy density is 613 W/m2. Fig. 8 shows the offshore and onshore wind energy

density for each wind direction. As with the wind direction occurrence frequency, the offshore wind energy

distribution relative to the wind direction shows larger energy densities in the northwest and south directions.

05

1015

NNNE

NE

ENE

E

ESE

SESSE

SSSW

SW

WSW

W

WNW

NWNNW

Offshore(H:94m)Onshore(H:24m)

Fig. 7 Wind direction occurrence frequency.

0306090

NNNE

NE

ENE

E

ESE

SESSE

SSSW

SW

WSW

W

WNW

NWNNW

Offshore(H:94m)Onshore(H:24m)

Fig. 8 Wind energy density for each wind direction.

0

10

20

30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31January 2005

One

hou

r mea

n w

ind

spee

d (m

/s)

Offshore (H:94m)Onshore (H:24m)

Fig. 9 Example of one hour mean wind speed time history.

0

10

20

30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31January 2005

Max

imum

win

d sp

eed

(m/s

)

Offshore (H:94m)Onshore (H:24m)

Fig. 10 Example of maximum wind speed time history.

0

10

20

30

40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31January 2005

Turb

ulen

ce in

tens

ity(%

)

Offshore (H:94m)Onshore (H:24m)

Fig. 11 Example of turbulence intensity time history.

0

90

180

270

360

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31January 2005

One

hou

r mea

n w

ind

dire

ctio

n (d

eg.)

Offshore (H:94m)Onshore (H:24m)

Fig. 12 Example of one hour mean wind direction time history.

4. Discussion

Table 1 shows the results of the offshore and onshore wind observations. The average annual offshore wind speed

is 7.5 m/s. If wind power generators are installed at this location, an annual capacity factor of more than 35 % can

be expected.

The observations show that the average monthly wind speed is higher than 8 m/s in winter and about 6 m/s in

summer — a difference of more than 2 m/s.

The offshore turbulence intensity is 8 % and is thus small. The durability of wind power generators increases

with decreasing turbulence intensity. This is one of the advantages of siting wind turbines offshore, as demonstrated

by the observation.

Table 1 Results of the offshore and onshore wind observations.

Observation Site Offshore Onshore

Observation Height 94 m 24 m

Annual mean wind speed 7.5 m/s 3.5 m/s

Maximum wind speed 39.0 m/s 37.5 m/s

Weibull shape factor 1.67 1.70

Weibull scale factor 8.5 m/s 3.9 m/s

Wind energy density 613 W/m2 63 W/m2

5. Conclusions

Wind observations were made at a height of 94 m above sea level for one year at a natural gas platform in the

Pacific Ocean, located 37 km off the coast.

1） The average annual offshore wind speed (calibrated for a height of 80 m) is 7.4 m/s—70% higher than the

onshore wind speed (calibrated for a height of 80 m), which was measured at the same time.

2） The average monthly wind speed is higher in winter and lower in summer. The average monthly wind

speed drops by more than 20% over the average annual wind speed, particularly in June and July.

3） The average annual offshore turbulence intensity is 8% and is thus small.

Further observations are in progress. The measurement of time-history data with a sampling time of 1 second

began in December 2005. We will continue the analysis of these data with the aim of introducing offshore wind

farms in Japan.

Reference

[1] Takeshi Ishihara, An Assessment of Offshore Wind Energy Potential Using Mesoscale Model and GISTitle,

Poster, EWEC 2004, London.

The Wind Observation on the Pacific OceanThe Wind Observation on the Pacific Oceanfor Offshore Wind Farmfor Offshore Wind Farm

Manabu TSUCHIYA * manabu@kajima.com

Takeshi ISHIHARA ** ishihara@bridge.t.u-tokyo.ac.jp

Yukinari FUKUMOTO *** fukumoto.yukinari@tepco.co.jp

* Environmental Engineering Division, Kajima Corporation, 3-7-1-29F, Nishi-shinjuku Shinjuku TOKYO 163-1029 JAPAN

** Institute of Engineering Innovation, School of Engineering, The University of Tokyo, 2-11-6 Yayoi Bunkyo TOKYO 113-8656 JAPAN

*** Engineering R&D Division, Tokyo Electric Power Company, 4-1, Egasaki Tsurumi YOKOHAMA 230-8510 JAPAN

IntroductionIntroduction

Wind Observation ResultsWind Observation Results

Example of Time HistoryExample of Time History ConclusionConclusion

Offshore wind farms, which are currently being developed in

Europe, are expected to be introduced in Japan.

Consequently, many attempts have been made to estimate

the offshore wind energy available around Japan.

However, all the estimates made thus far are based on

numerical simulations that have not been validated by field

measurements. The full-scale development of offshore wind

farms requires an investigation of the offshore wind conditions

through measurement and validation of numerical simulations.

For this reason, the authors conducted offshore wind

observations at a sufficient height at an offshore natural gas

platform located 37 km off the coast, in the Pacific Ocean.

A windmill-type anemometer was

installed on the top of the drilling

tower at a height of 94 m above sea

level. In general, drilling facilities

specify that only explosion-proof

electrical equipment be used;

therefore, the anemometer used

was of the type that detects the

number of revolutions of the

propeller and the wind direction with

optical signals through an optic fiber,

instead of with electrical signals.

The observations began in

September 2004 and are currently

in progress.

JAPANJAPAN

Tokyo

Pacific Ocean

Sea of Japan

Offshore Observation Point

(Natural Gas Platform)

Onshore Observation Point

(Thermal Power Plant)

37km

Pacific Ocean

Offshore Wind ObservationsOffshore Wind Observations

Onshore Wind ObservationsOnshore Wind ObservationsFor comparison purposes, a three-cup anemometer and a

wind vane were installed on the top of a 24-m-high pole for

the onshore wind observations.

52

8

N37 17’ 49”, E141 27’ 47”

Lightning Rod

Explosion-Proof Type

Anemometer

1m 2m0

94 m height

above sea level

Top Beam of The Gas Platform

N37 13’ 53”, E141 0’ 35”

Hirono Thermal Power Plant

Anemometer

Wind observations were made at a height of 94 m

above sea level for one year at a natural gas

platform in the Pacific Ocean, located 37 km off the

coast.

Further observations are in progress. The

measurement of time-history data with a sampling

time of 1 s began in December 2005. We will

continue the analysis of these data with the aim of

introducing offshore wind farms in Japan.

The average annual offshore wind speed

(calibrated for a height of 80 m) is 7.4 m/s—70%

higher than the onshore wind speed (calibrated

for a height of 80 m), which was measured at the

same time.

The average monthly wind speed is higher in

winter and lower in summer. The average

monthly wind speed drops by more than 20%

over the average annual wind speed, particularly

in June and July.

The average annual offshore turbulence intensity

is 8% and is thus small.

One hour mean wind speed (m/s) Turbulence intensity (%)

Maximum wind speed (m/s) One hour mean wind direction (deg.)

0

10

20

30

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

January 2005

Offshore (H:94m)

Onshore (H:24m)

0

10

20

30

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

January 2005

Offshore (H:94m)

Onshore (H:24m)

0

10

20

30

40

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

January 2005

Offshore (H:94m)

Onshore (H:24m)

0

60

120

180

240

300

360

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

January 2005

Offshore (H:94m)Onshore (H:24m)

0

2

4

6

8

10

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

Offshore (H:94m)

Onshore (H:24m)

0

200

400

600

800

1000

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

Offshore (H:94m)

Onshore (H:24m)

Monthly mean wind speed (m/s)

Monthly mean power density (W/m2)

Wind rose (%)

Power density distribution (W/m2)

Turbulence intensity distribution (%)

Wind speed distribution (%)

0

5

10

15N

NNE

NE

ENE

E

ESE

SE

SSE

S

SSW

SW

WSW

W

WNW

NW

NNW

Offshore(H:94m)

Onshore(H:24m)

0

15

30

45N

NNENE

ENE

E

ESE

SE

SSE

S

SSWSW

WSW

W

WNW

NW

NNW

Offshore(H:94m)

Onshore(H:24m)

0

5

10

15

20

25

30

1 3 5 7 9 11 13 15 17 19 21 23 25

Offshore(H:94m)

Onshore(H:24m)

The average annual offshore wind speed is

7.5 m/s, and mean power density is 600 W/m2.

If wind power generators are installed at this

location, an annual capacity factor of more

than 35% can be expected.

The observations show that the average

monthly wind speed is higher than 8 m/s in

winter and about 6 m/s in summer — a

difference of more than 2 m/s.

The offshore turbulence intensity is 8% and is

thus small.

The durability of wind power generators

increases with decreasing turbulence intensity.

This is one of the advantages of siting wind

turbines offshore, as demonstrated by the

observation.

m/s

OctoberOctober 20042004 to September to September 20052005

0

30

60

90N

NNE

NE

ENE

E

ESE

SE

SSE

S

SSW

SW

WSW

W

WNW

NW

NNW

Offshore(H:94m)

Onshore(H:24m)