Wave loads on fixed offshore wind turbines

Wave loads on fixed offshore wind
turbines
Johan Peeringa en Erik-Jan de Ridder
Foundation types in 30 MW+ offshore wind farms
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14 Monopiles
7 gravity bases
1 Tripod and
Jacket
List of offshore wind farms - Wikipedia, the free encyclopedia
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Dowec 6MW
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Pitch regulated variable
speed
Rated power 6MW
Rotor diameter 129 m
Hub height 91.4 m
Monopile 6 m diameter
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Selected frequencies of the Dowec 6MW
[Hz]
Tower for aft
0.242
Blade flat wise
0.675
Blade edge wise 1.107
[rad/s]
1.521
4.241
6.956
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Offshore wind turbine standards
Organisations
– GL
– DNV
– IEC
Design situation IEC61400-3
– Power production (+ fault)
– Start up
– Normal shut down
– Emergency shut down
– Parked (+ fault)
– Transport assembly
maintenance and repair
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Linear and nonlinear waves
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Wave models and wave loads
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Irregular linear
waves
Nonlinear
deterministic
streamfunction wave
Morison equation
D2
D
xdz  CD  x xdz
dF  CM 
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Source: www.noordzeewind.nl
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Breaking waves
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Blyth
Wienke
Source: Wienke 2001
Fwave_break  FD  FM  FI
Source: Jan v/d Tempel 2006
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Need for validation of wave load models on
offshore wind turbines
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Code to Code Comparison
Lack of (public) full scale measurements
Lack of model tests including hydroelasticity
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Introduction
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ComFlow
Linear wave theory vs stream function and
ComFlow
Effect relative fluid velocity due to tower
motions
1st model tests
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ComFlow
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Volume of fluid CFD code
Used at MARIN for:
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Green water on deck
Wave impacts
Sloshing
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ComFlow: example
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Non linear wave forces using ComFlow
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12m 8s @ 30m waterdepth
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Influence of hydroelasticity
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Simple bending model
Linear wave theory
Morison loading, including
relative velocities due to
tower velocity
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ComFlow (CFD) does not (yet) include relative
fluid velocity due to tower motions
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Existing MARIN knowledge on segmented
models
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1st step by MARIN
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Model tests with a flexible model
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No detailed modelling of the proto type (DOWEC
6MW)
The clamping flexibility of the foundation is partly
taken into account
The 1st and 2nd natural frequencies are modelled, by
tuning the weight distribution over the height
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Full scale model
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DOWEC 6 MW turbine
Tower 80 m
Support 30 m
Water depth 21 m
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FE analyses of model
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FE package ANSYS
Analyses of model (scale 1:30)
11 beam & 6 mass elements
Clamping flexibility included
M1 M2
Tower
M3
E ,m ,I
1 1 1
M4
support
M5
E ,m ,I
1 2 2
M6
EI
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Natural frequencies
Natural mode
FE analyses
Model testing
1st mode
0.26 Hz (1.4 Hz model)
0.59 Hz
2nd mode
1.77 Hz (9.7 Hz model)
2.23 Hz
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Accelerations at
5 location
Pressure
measurements
at five location
Forces and
moments at the
bottom
Wave height at
three locations
around the tower
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Model set-up in the basin
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Picture model tests of wave impact
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First results model tests
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Acceleration at the top of the tower
Wave:
Hs= 5 m
Tp= 12s
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First results model tests
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Acceleration at the top of the tower
Wave:
H= 14 m
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2nd possible (step)tests
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The results can be used to validate numerical
software:
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Which than can be used to optimise the control
system
Optimised the turbine for specifiek locations
(waves point of view)
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4th step full scale measurements
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Joint research Marin - ECN
Joint research
– Advanced Wave modelling
– Wave model validation on scaled Wind Turbine
models
– Wave load cases as module for Aero-elastic codes
(Phatas and AnySim)
Timeline 2010 - 2011
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