Floating offshore wind measurement system
by using LIDAR and its verification

PO.154

Atsushi YAMAGUCHI, Takeshi ISHIHARASchool of Engineering, The University of Tokyo

Verification

Motion compensation of LIDAR■ Correction of tilting

The use of floating LIDAR will be an important technology toreduce the cost of offshore wind measurement. Floater motion
due to wave force may result in the error of measured windspeed and floater motion compensation is necessary.

Proposed correction method was verified by numericalsimulation.

verticalp1

p4

Several studies have been carried out focusing on floatermotion compensation1)2). However, the detailed algorithm for
motion compensation is not described. The most simple motion
compensation algorithm is simply to perform coordinatetransformation. Fujitani3) applied coordinate transformation forsonic anemometer on ship. But due to the difference in themechanism of wind speed measurement between sonicanemometer and lidar, the coordinate transformation may not
be used.

p2

p5
p3

tilted lidar andits LoSs

Another problem is that there is no validated method for themeasurement of the motion of the floater. Typically gyro andaccelerometer are used to measure the floater motion. But
numerical integral may introduce additional error. On the otherhand, GPS can give the position of the floater directly, but theaccuracy easily changes depending on satellite conditions.

W2,4 axis

V4 w

w2, 42, 4• Three different synthetic floater motion data weregenerated.
• The maximum pitch and roll angle of the motion data are 5,
15 and 30 degree.
• 10 minutes average wind speed in three component areverified.
Horizontal wind speed component
at maximum inclination of 6 deg., 18deg and 36 deg.
20.0

ϕ2

8.0

8.0

4.0

4.0

4.0

0.0

0.0

0.0

Ref.

V2p2

p5

1.E-050.02

Time (s)

0.04

0.06

0.08

wind speed
component inhorizontal plane at thetarget height

1.E+0113

1.E+0012

1.E-01
1.E-0211i

10u

1.E-03

40

060

0.1

0.2

0.3

Frequency (Hz)0.4

• Integration of acceleration shows good agreement with thereference data for high frequency.
• GPS data with missing value shows good agreement withthe reference data for low frequency.
14u1,3

ψwhereψuVertical wind speed

0.015u 2,4horizontal wind speed
and direction at thetarget height

relative error (%)

1312
reference
n=0
n=1
n=2
n=3
n=4
n=5

11
10
9

Vertical wind speed can be calculated as the averageof vertical wind speed calculated from each LoS
speeds by the equation as follows.

0.01

0.005

2030

2.0%

No correction
Conventional
Proposed

1.5%
1.0%
0.5%
0.0%
010
2030
Maximum inclination angle (deg)

40

2.5

2.0

2.0

2.0

1.5

1.5

1.5

1.0

1.0

2.5

0.30

1.01
1.0

0.5

0.5

-0.11-0.24

0.0-0.06

-0.06

Ref.

Prop. Conv. No cor.

-0.5

0.0-0.06

-0.07
-0.5

Ref.

Prop. Conv. No cor.
2.5-0.06

Ref.

-0.08

Prop. Conv. No cor.

No correction
Conventional
Proposed

21.5
10.5
00

10
2030
Maximum inclination angle (deg)

40

40

50

60

time (s)Displacement calculated byproposed method

GPS data with missing values
and the displacement obtainedby integrating acceleration arecombined in frequency domain.
GPS data with missing value isupdated based on the error of
the missing intervals and sameprocedure is performed.

Conclusion
In this study, a method to measure the floater motion
and a motion compensation method for Lidar areproposed and verified. Following results were obtained.

0123
number of iteration

42) Proposed motion compensation method for lidar canpredict the wind speed regardless of the maximum
pitch angle. When maximum pitch is 36degree, theerror in horizontal wind speed becomes 1.5% without
motion compensation., but can be reduced to 0.3%
by using motion compensation.

Acknowledgement
This study was carried out as a part of FukushimaFORWARD project funded by Ministry of Economy,
Trade and Industry (METI), Japan.

010Prop. Conv. No cor.

1) Proposed method can measure the six degree offreedom floater motion even when some of the RTKGPS data are missing.

0.5

Comparison of reference data and GPS data with missing value in time
domain (left) and frequency domain (right)

017.42

1.E-04
1.E-05

9surge displacement (m)p3

1.E+02Time (s)17.62

Frequency (Hz)

1420

17.64

0.1

Comparison of reference data and displacement obtained from numericalintegral of acceleration in time domain (left) and frequency domain (right)
Displacement (m)

u2,4

p4

1.E-04

0

17.69

Ref.

2.5

• The estimation error by
conventional method
increase when inclination
becomes large while
proposed method canmeasure wind speed
regardless of theinclination angle

u
u1,30

Prop. Conv. No cor.

error (m/s)

Displacement (m)Horizontal wind speed and direction
p1600

Ref.

• The estimation error by
conventional method
increase when
inclination becomeslarge while proposed
method can measurewind speed regardlessof the inclination angle-0.5

1.E-03400

Prop. Conv. No cor.

0.0

1.E-02

200

20.0

8.0

u2,4 axis

1.E-010

17.60

12.0-0.07

1.E+00

-100

17.65

12.0

0.5

1.E+01

-50

17.65

2.05

1.E+02

0

17.69

Vertical wind speed component
at maximum inclination of 6 deg., 18deg and 36 deg.

The substation of Fukushima
FORWARD project4) is equipped with
RTK-GPS sensor, accelerometer andgyro in order to measure the floater
motion in addition to the Lidar.
Theoretically, one GPS sensor and gyro
can be used to identify the six degree offreedom motion of the floater. However,sometimes the GPS measurement in
RTK mode is not available andinterpolation of GPS measurement isrequired.
50

20.0

12.0

Floater motion measurement

100

17.65

16.0

βϕ4

17.65

16.0

u2, 4

θ0 θ0

17.65

16.0

u2, 4

In this study, first, a method to measure the motion of the floater
is established, and then, different motion compensation
algorithms are applied and their applicability is investigated.

17.69

Relative error (%)

Introduction

5

Relative RMSE of the estimate
for different number of iterations

References■ Correction of translational motion

EWEA 2015 – Paris – 17-20 November 20151) Gerrit Wolken-Möhlmann : Simulation of motion induced measurement errors for windmeasurements using LIDAR on floating platforms, Fraunhofer IWES, Am Seedeich 45,27572 Bremerhaven, Germany,20112) Julia Gottschall, Hristo Lilov, Gerrit Wolken-Möhlmann, Bernhard Lange : Lidars on
floating offshore platforms /About the correction of motion-induced lidar measurement
errors ( simulations and first experiments ) , EWEA, 2012.4
3) Fujitani,T.: Direct measurement of turbulent fluxes over thesea during AMTEX. Pap.
Meteor. Geophys., 32, 119–134, 1981.
4) Fukushima FORWARD Project, http://www.fukushima-forward.jp/QR
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