A Study on Structure Parameters of an Offshore Wind Turbine

by Excitation test Using Active Mass Damper

Shou OH, Takeshi ISHIHARA The University of Tokyo

PO.137

1. Japan Society of Civil Engineers, Design code for support structure of wind

turbine, 2010 (In Japanese)

2. AWEA/ASCE, Recommended Practice for Compliance of Large On-land Wind

Turbine Support Structures, 2011

3. Deutsches Institut für Bautechnik, Richtlinie für Windenergianlagen, 2012

4. Ishihara,T., Phuc,P.V., Fujino,Y., Takahara,K., and Mekaru,T., A field test

and full dynamic simulation on a stall regulated wind turbine, Proc. of

APCWE VI, 2005, pp. 599-612

Structure parameters are important in estimation of

loads on wind turbines. Especially the seismic load

depends strongly on the damping ratio of 1st and 2nd

mode. However, the values of damping ratio are

different among design codes. In Japanese design

code[1], 0.5~0.8% is recommended for 1st mode based

on a human-power excitation test on a 400kW wind

turbine. In the design code of United States[2], 1% is

suggested for seismic design, and 0.23% is adapted in

German code[3]. Moreover, there is little information

regarding 2nd mode damping, which is difficult to excite

by either nature wind or human power. In this study, an

excitation test using active mass damper is conducted

on a 2.4MW offshore wind turbine. Natural frequencies

and damping ratios of 1st and 2nd mode are evaluated

by sinusoidal vibration test and free vibration test. The

effect of blades to damping ratios is also evaluated by

comparing the excitation result under pitch-feathering

condition with that of pitch-fine condition.

Objectives

Free Vibration Test

Conclusions

References

EWEA OFFSHORE 2013

Outline of Excitation Test

0

10

20

30

40

50

60

70

80

0 0.2 0.4 0.6 0.8 1

FEM

Obs

He

ight

(m)

0

10

20

30

40

50

60

70

80

0 0.2 0.4 0.6 0.8 1

FEM

Obs

He

ight

(m)

-1

-0.5

0

0.5

1

0 50 100 150

Acceleration

damp=1.2%

Acceleration (m/s2)

time (s)

(c) y 1st mode

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0 2 4 6 8 10

Acceleration

damp=3.2%

Accerelation (m/s2)

time (s)

(d) y 2nd mode

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0 2 4 6 8 10

Acceleration

damp=2.4%

Acceleration (m/s2)

time (s)

(b) x 2nd mode

-1

-0.5

0

0.5

1

0 50 100 150

Acceleration

damp=0.19%

Acceleration (m/s2)

time (s)

(a) x 1st mode

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0 50 100 150

Xfree1st

damp=1.0%

Acceleration (m/s2)

time (s)

(a) x 1st mode

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0 50 100 150

Yfree1st

damp=0.3%

Acceleration (m/s2)

time (s)

(c) y 1st mode

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.3 0.32 0.34 0.36 0.38 0.4

Observation

Analytical

Normalized Amplitude

frequency (Hz)

(a)1st mode

amplitude

f=0.351Hz

=0.2%

0.0

0.1

0.1

0.2

0.2

0.3

2.6 2.8 3 3.2 3.4Normalized Amplitude

frequency(Hz)

(b)2nd mode

amplitude

f=2.98Hz

=2.4%

0.0

45.0

90.0

135.0

180.0

0.3 0.32 0.34 0.36 0.38 0.4

Phase (deg)

frequency (Hz)

(c)1st mode

phase

f=0.351Hz

=0.2%

0.0

45.0

90.0

135.0

180.0

2.6 2.8 3 3.2 3.4

Phase (deg)

frequency (Hz)

(d)2nd mode

phase

f=2.98Hz

=2.4%

X-1st X-2nd Y-1st Y-2nd

Ambient

vibration

Pitch-

Feather

0.357Hz

0.2%

3.00Hz

-

0.353Hz

0.3~1.5%

2.99Hz

-

Pitch-

Fine

0.375Hz

-

3.05Hz

-

0.363

-

-

-

Sinusoidal

vibration

Pitch-

Feather

0.351Hz

0.2%

2.94Hz

2.4%

0.357Hz

-

2.97Hz

-

Pitch-

Fine

0.350Hz

-

-

-

0.347Hz

-

3.01Hz

-

Free

vibration

Feather 0.2% 2.4% 1.2% 3.2%

Fine 1.0% - 0.2% 3.2%

Excitation test using active mass damper is performed

on an offshore wind turbine. The precise value of

resonant frequency of 1st and 2nd mode for fore-aft(X)

and side-side (Y) direction is obtained by sinusoidal test.

Structural damping ratio in the direction of blade edge-

wise is obtained as 0.2% for 1st mode and 2.4% for 2nd

mode. On the other hand, the damping ratio becomes

1.0%~1.2% for 1st mode and 2.4~3.2% for 2nd mode in

the direction of blade flap-wise. The difference is caused

by the aero-dynamic damping and the structure damping

of blade, which will be evaluated with numerical analysis

in future study. Mode shapes are obtained using

acceleration data at 5 heights and agreed with

calculation. Results obtained in this research will be

proposed in international standard for design of tower

and foundation.

X direction Y direction

Movable Mass 1700kg 1300kg

Effective Stroke 190mm 290mm

Maximum Speed 0.83m/s 0.83m/s

Frequency Range 0.1~4Hz 0.1~4Hz

Actuator power Maximum: 4400N

Continuous: 1900N

Maximum: 4400N

Continuous: 1900N

Acceleration Maximum: 2.58m/s2

Continuous: 1.11m/s2

Maximum: 3.38m/s2

Continuous: 1.46m/s2

The wind turbine targeted in this study is Mitsubishi

Heavy Industry 2.4MWturbine located 3.1km offshore

Choshi, Japan. The active mass damper(AMD) is

equipped at 55.5m height of tower for purpose of

vibration control. In this study, it is used to generate the

excitation force. During the test, the rotor and the yaw

were fixed in order to obtain pure structure parameters

without the effect of rotation. As the turbine is supported

with gravity foundation, the effect of tide and wave to

damping is neglected. Experiment date is chosen when

the wind speed is low to minimize the effect of aero-

dynamic damping.

Date 2014Feb,

21-22

2014Oct,

28-29

Mean Wind Speed 4~6m/s 5~8m/s

Mean Wind Direction 345~350

deg

20~25

deg

Yaw Angle 77.5deg 77.5deg

Sinusoidal Vibration Analysis

Mode Shapes

Sinusoidal vibration test is performed around the

approximate resonant frequency calculated from

ambient vibration monitoring to obtain more precise

value. Especially for excitation in fore-aft (X) direction

under pitch-feathering condition, the vibration can be

approximated as 1 degree of freedom, since the aero-

dynamic damping is small and the effect of nacell like

friction of gears can be neglected. Then, the analytical

solution of normalized amplitude and phase angle can

be written as

Mode shapes are

calculated from both

the acceleration

amplitude during

sinusoidal test at 5

height and from FEM

analysis [4]where

model is made from

design drawing. For

both 1st and 2nd

mode, the mode

shape shows good

agreement.

Detail of active mass damper

Left: Location of wind turbine; Right: Outline of AMD

96m

10.3m

Accelerometer

Accelerometer height Strain gage height

Strain gage

Detail of 2.4MW wind turbine and measurement devices

Active Mass Damper

Measurement Condition

Axis Definition

Excitation

X axis

Wind Direction 0°

(North)

(East)

22.5°

Excitation

Y axis Comparison of sinusoidal test with analytical solution

Left: X 1st mode; Right: X 2nd mode

Result of obtained structure parameters

Result of free vibration with pitch-feathering condition

Left: X&Y 1st mode; Right: X&Y 2nd mode

𝑎

𝐹= 𝛽2

𝜙𝑛(𝑉𝑖𝑏)𝜙𝑛(𝐴𝑐𝑐)

𝜙𝑛(𝑖)𝑖 𝑚𝑖𝜙𝑛(𝑖)

1

1 − 𝛽2 2 + 2𝜉𝑛𝛽2

𝜃 = tan−12𝜉𝛽

1 − 𝛽2

Free vibration test is conducted by stopping the

excitation force of natural frequency at the steady state.

Comparison of mode shapes of

observation and FEM

Left: 1st mode; Right: 2nd mode

Flow of excitation test

Outline of 2.4MW wind turbine and the equipment

Result of free vibration with pitch-fine condition

Left: X 1st mode; Right: Y 1st mode

Then the recorded free vibration is fitted with the

envelope equation below to obtain damping ratio.

a 𝑡 = Aexp −2𝜋𝑓𝜉𝑡

Fore-aft (X)

side-side (Y)