Poster EWEA "Damping Estimation of an Offshore Wind Turbine on a Monopile Foundation"
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Best scientific poster EWEA 2012
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- 1. Damping Estimation of an Offshore Wind Turbine on a Monopile
Foundation Christof Devriendt, Pieter Jan Jordaens, Gert De Sitter,
Patrick Guillaume Vrije Universiteit Brussel Sirris - Offshore Wind
Infrastructure Lab Both during the overspeed test and during the
ambient Correlation driven analysis in the time domain Abstract
excitation test the tuned mass damper was switched off. By fitting
an exponential function to the relative maximaMany large scale
offshore wind farm projects use of the auto-correlation functions
of the measuredmonopile foundation to realize a cost effective
design. accelerations the damping ratio can be extracted.During the
design of these monopile structures fatigue 1.5 Data Points 1.5
Data Pointsdue to combined wind and wave loading is one of the 1
Curve 1 Curve Acceleration (g) Acceleration (g) 0.5 0.5most
important problems to face. The damping 0 0 Figure 3: Example
measured accelerations during overspeed stop (left) and ambient
-0.5 -0.5significantly influences the dynamic response of the -1 -1
0 50 100 150 200 250 300 0 50 100 150 200 250 300 excitation
(right) Time (s) Time (s) Figure 7: Exponential fitting of an
autocorrelation function of a sensor in the FA directionwind
turbine reaction and thus also the predicted (left) and SS
direction (right)lifetime. The work presented in this poster
describes a Results Overspeed Stop This approach only provides good
estimates for thecomparative study between different techniques
aimed damping when the decay consists of 1 mode. Duringat
identifying the damping values of an offshore wind Time domain
analysis the ambient tests this is not the case as there is
strongturbine on a monopile foundation. In the time domain damping
can be obtained by fitting coupling between the FA and SS mode, and
therefore an exponential function to the decaying time series and
this approach fails. Damping Effects extracting the damping ratio
from the parameters of the Correlation driven analysis in the
frequency domainThe overall damping of the first bending mode of an
fitted expression. This method assumes that the decay has only the
contribution of a single mode and therefore The Fast Fourier
Transformation of the decaying auto-offshore wind turbine consists
of a combination of proper filtering might be required.
correlations can be used as input for the operational aerodynamic
damping 0.15 1 modal analysis methods in the frequency domain.
Acceleration (g) Level 1 Data Level 2 Points Level 3 0 Curve
Stabilization Chart material damping of steel 0.1 d s s s s s s s s
-1 30 s s s s s s s s s 100 150 200 250 300 350 d s s s s s s s s
Time (s) d s s s s s s s s 0.05 s s s s s s s s s o s s s s s s s s
FA-Acceleration (g) 0.2 damping from wave creation 25 s s s s s f s
s Acceleration (g) Data s s s s s s s s 0 Points s s s s s f s s s
s s f s f s s 0 Curve s s s s f s f s s -0.2 20 s s s s f s f s s
model order s f s s s s f s s o s s s f s f s s damping due to
inner soil friction -0.4 s s s f s f s s 100 150 200 250 300 350 s
s s f s o f s -0.05 15 f s s f f s s Time (s) s s s f s s 0.2 s s f
f s s s s f f s s Acceleration (g) Data f s o f f s Points 10 s fs
s f f damping due to hydrodynamic drag -0.1 d fs o f f f 0 Curve os
o f s o s s f ss s s 5 fs f o -0.2 f f -0.1 -0.05 0 0.05 0.1 0.15
100 150 200 250 300 350 o s o damping due to constructive devices
SS-Aceleration (g) Time (s) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Figure 4: Movement seen from above (left) and exponential fitting
on FA acceleration on 3 frequency (Hz) levels after applying a
band-pass filter of 0.3-0.5Hz (right) Figure 8: Stabilization
diagram after applying operational modal analysis to the FastThe
main goal of this campaign was to identify the Fourier
Transformations of the auto-correlationsdamping ratios of the first
fore-aft (FA) and side-side The method was applied to the measured
accelerations(SS) bending mode excluding the aerodynamic damping of
the highest 3 levels in the direction of the wind. Theand damping
due to constructive devices. data was pre-filtered using 3
different band-pass filters. Offshore Measurements This approach
fits again a polynomial function with multiple modes and therefore
the results are not The measurement campaign was performed at the
affected by the fact that multiple modes are present. Belwind wind
farm, which consists of 55 Vestas V90 Periodogram driven analysis
in the frequency 3MW wind turbines. The wind farm is located in the
domain North Sea on the Bligh Bank, 46 km off the coast. Frequency
domain analysis The periodogram driven approach uses the power One
can also obtain an estimate for the damping using spectrum matrix
from the measured accelerations as an operational modal analysis
approach to the Fast input data for performing operational modal
analysis in Fourier Transformations of the free decays measured the
frequency domain. during the overspeed test. This approach fits a
Stabilization Chart 40 FA Level 1 FA Level 2 s ss s s s s 30 s ss s
s s s polynomial function with multiple modes and therefore 20 FA
Level 3 s ss s s s s SS Level 1 s ss s s s s SS Level 2 s ss s s s
s 0 s ss s s s s SS Level 3 25 s fs f f s s the results are not
affected by the fact that multiple s s f f s f f s f f s s -20 s s
f f s s f s f s s s 20 f s f f s f Amplitude (dB) modes are
present. model order f s f f s f -40 f fs f f s s s fs s f s s s ff
s f s f 15 s ff s f s s -60 f ff f s f s d f f f f s f ff o s s f
Figure 1: location Belwind wind farm (left) park layout Belwind
wind farm (right) 30 . Stabilization Chart f fs f f f FA Level 1
-80 10 s fs o f f f FA Level 2 s ff o s s s s s f f s s o f s o ff
o o f f 20 FA Level 3 30 s s f f f s f ff o o f SS Level 1 s s o f
f f o s f f -100 fs o o s The tests were performed on the
BBCO1-turbine. The s s o f f s o s 5 ff o SS Level 2 f o f s s f f
s s 10 SS Level 3 f s f s f f f f s f f o -120 25 f s o f f f s s s
s f f s f f o s f f f f 0 actual water depth at the location of
BBCO1 is 22.9m s s o o f s f ff f s s f f s s o -140 0.2 0.4 0.6
0.8 1 1.2 1.4 1.6 1.8 2 0 f s o s f f s 0 0.5 1 1.5 2 frequency
(Hz) Amplitude (dB) 20 f s f f f f Frequency (Hz) model order f s o
f f ff s s s f f s and the monopile has a penetration depth of
20.6m. -10 15 f o s s s s f f d f s s f f f f f f f f o Figure 9:
Auto and cross spectra with reference sensor 1 in FA direction
(left) Stabilization -20 f s s f o s diagram after applying
operational modal analysis to the spectrum matrix (right) The soil
is considered stiff and mainly consists of sand. f s s f s o s d f
f 10 s o f f o s f s s Note that this approach clearly identifies
both the FA -30 s f f s