Post on 26-Dec-2015
Accuracy of calculation procedures for offshore wind turbine support structures
Pauline de Valk – 27th of August 2013
Content Introduction Approach Modeling Results Conclusions and recommendations
1
Content Introduction Approach Modeling Results Conclusions and recommendations
1
Offshore wind energy shows potential to become one of the main energy suppliers
Demand for energy continues to increase
Offshore wind energy More steady wind flow
and average wind speed is higher than onshore
Introduction Approach Modeling Results Conclusions and recommendations
2
2009 2020 20350
5000
10000
15000
20000
25000
30000
35000
40000
Re-new-ables
Nuclear
Oil
Gas
Coal
Energy demand
TW
h
Offshore wind energy shows potential to become one of the main energy suppliers
Energy demand
TW
h
Demand for energy continues to increase
Offshore wind energy More steady wind flow
and average wind speed is higher than onshore
Cost of energy (€/kWh) should be decreased Structural optimization
design
2Introduction Approach Modeling Results Conclusions and recommendations
2009 2020 20350
5000
10000
15000
20000
25000
30000
35000
40000
Re-new-ables
Nuclear
Oil
Gas
Coal
Optimize structural design of the support structure
Support structure one of the main cost items
In order to optimize one should have confidence in the outcome of calculation procedures
3Introduction Approach Modeling Results Conclusions and recommendations
Thesis objective
‘‘Investigate the validity and conservatism
4Introduction Approach Modeling Results Conclusions and recommendations
Thesis objective
‘‘Investigate the validity and conservatism
of the current calculation procedures
4Introduction Approach Modeling Results Conclusions and recommendations
Thesis objective
‘‘Investigate the validity and conservatism
of the current calculation procedures for offshore wind turbine support
structures
4Introduction Approach Modeling Results Conclusions and recommendations
Thesis objective
‘‘Investigate the validity and conservatism
of the current calculation procedures for offshore wind turbine support
structures and propose improved procedures
4Introduction Approach Modeling Results Conclusions and recommendations
Thesis objective
‘‘Investigate the validity and conservatism
of the current calculation procedures for offshore wind turbine support
structures and propose improved procedures
based on these findings.”4Introduction Approach Modeling Results Conclusions and
recommendations
Content Introduction Approach Modeling Results Conclusions and recommendations
5
Offshore wind turbine support structure is custom engineered for every wind farm
Introduction Approach Modeling Results Conclusions and recommendations
6
Foundation designer (FD) Turbine designer (TD)
Offshore wind turbine support structure is custom engineered for every wind farm
Introduction Approach Modeling Results Conclusions and recommendations
6
Foundation designer (FD) Turbine designer (TD)
(Adjust) design foundation1
Offshore wind turbine support structure is custom engineered for every wind farm
Introduction Approach Modeling Results Conclusions and recommendations
6
Foundation designer (FD) Turbine designer (TD)
Integrate foundation modelin aero-elastic model
(Adjust) design foundation1 2
Offshore wind turbine support structure is custom engineered for every wind farm
Introduction Approach Modeling Results Conclusions and recommendations
6
Foundation designer (FD) Turbine designer (TD)
Integrate foundation modelin aero-elastic model
Run aero-elastic simulation(and adjust tower design)
(Adjust) design foundation1 2
3
Offshore wind turbine support structure is custom engineered for every wind farm
Introduction Approach Modeling Results Conclusions and recommendations
6
Foundation designer (FD) Turbine designer (TD)
Integrate foundation modelin aero-elastic model
Run aero-elastic simulation(and adjust tower design)
Extract interface loads/displacements between
tower and foundation
(Adjust) design foundation1 2
4
3
Offshore wind turbine support structure is custom engineered for every wind farm
Introduction Approach Modeling Results Conclusions and recommendations
6
Foundation designer (FD) Turbine designer (TD)
Integrate foundation modelin aero-elastic model
Run aero-elastic simulation(and adjust tower design)
Extract interface loads/displacements between
tower and foundation
(Adjust) design foundation
Apply interface loads/displacements on
detailed foundation model
5
1 2
3
4
Offshore wind turbine support structure is custom engineered for every wind farm
Introduction Approach Modeling Results Conclusions and recommendations
6
Foundation designer (FD) Turbine designer (TD)
Integrate foundation modelin aero-elastic model
Run aero-elastic simulation(and adjust tower design)
Extract interface loads/displacements between
tower and foundation
(Adjust) design foundation
Run simulation
Apply interface loads/displacements on
detailed foundation model
6
5
1 2
3
4
Offshore wind turbine support structure is custom engineered for every wind farm
Introduction Approach Modeling Results Conclusions and recommendations
6
Foundation designer (FD) Turbine designer (TD)
Integrate foundation modelin aero-elastic model
Run aero-elastic simulation(and adjust tower design)
Extract interface loads/displacements between
tower and foundation
(Adjust) design foundation
Run simulation
Apply interface loads/displacements on
detailed foundation model
6
5
1 2
3
4
Calculation post-processing analyses
Introduction Approach Modeling Results Conclusions and recommendations
7
fwind
fwave
Dynamic analysis Force
controlled
g
fwind
fwav
e
fwave g
Calculation post-processing analyses
Introduction Approach Modeling Results Conclusions and recommendations
7
fwind
fwave
Dynamic analysis Force
controlled
g
fwind
fwav
e
Dynamic or Quasi-static fwave g
Calculation post-processing analyses
Introduction Approach Modeling Results Conclusions and recommendations
7
fwind
fwave
Force controlled
g
fwind
fwind
fwav
e
fwav
e
ub
Dynamic or Quasi-static
Displacement controlled
Dynamic analysis
fwave g
fwaveub
Calculation post-processing analyses
Introduction Approach Modeling Results Conclusions and recommendations
7
fwind
fwave
Force controlled
g
fwind
fwind
fwav
e
fwav
e
ub
Dynamic or Quasi-static
Displacement controlledDynamic or Quasi-
static
Dynamic analysis
fwave g
fwaveub
Calculation post-processing analyses
Introduction Approach Modeling Results Conclusions and recommendations
7
fwind
fwave
Force controlled
g
fwind
fwind
fwav
e
fwav
e
ub
Dynamic or Quasi-static
Displacement controlledDynamic or Quasi-
static
Dynamic analysis
fwave g
fwaveub
Dynamic versus quasi-static analysis
Introduction Approach Modeling Results Conclusions and recommendations
Dynamic analysis
Quasi-static analysis
Only accurate if structure is excited below first eigenfrequency
8
Dynamic versus quasi-static analysis
Introduction Approach Modeling Results Conclusions and recommendations
Dynamic analysis
Quasi-static analysis
Only accurate if structure is excited below first eigenfrequency
8
Dynamic versus quasi-static analysis
Introduction Approach Modeling Results Conclusions and recommendations
Dynamic analysis
Quasi-static analysis
Only accurate if structure is excited below first eigenfrequency
8
Frequency
Am
plit
ude
≈1K__
≈1
ω2M__
Dynamic versus quasi-static analysis
Introduction Approach Modeling Results Conclusions and recommendations
Dynamic analysis
Quasi-static analysis
Only accurate if structure is excited below first eigenfrequency
8
Frequency
Am
plit
ude
≈1K__
≈1
ω2M__
Dynamic versus quasi-static analysis
Introduction Approach Modeling Results Conclusions and recommendations
Dynamic analysis
Quasi-static analysis
Only accurate if structure is excited below first eigenfrequency
8
Frequency
Am
plit
ude
≈1K__
≈1
ω2M__
Design cycle for offshore wind turbine support structure
Introduction Approach Modeling Results Conclusions and recommendations
9
Foundation designer (FD) Turbine designer (TD)
Integrate foundation modelin aero-elastic model
Run aero-elastic simulation(and adjust tower design)
Extract interface loads/displacements between
tower and foundation
(Adjust) design foundation
Run simulation
Apply interface loads/displacements on
detailed foundation model
6
5
1 2
3
4
Design cycle for offshore wind turbine support structure
Introduction Approach Modeling Results Conclusions and recommendations
9
Foundation designer (FD) Turbine designer (TD)
Integrate foundation modelin aero-elastic model
Run aero-elastic simulation(and adjust tower design)
Extract interface loads/displacements between
tower and foundation
(Adjust) design foundation
Run simulation
Apply interface loads/displacements on
detailed foundation model
6
5
1 2
3
4
Reduction of foundation to lower computation costs
Reduce large number of DoF into smaller set of generalized DoF Size(ũ) << size(u) Lower computation costs Approximation of exact
solution Reduction basis contains
limited number of deformation shapes
Only accurate if Spectral convergence Spatial convergence
Introduction Approach Modeling Results Conclusions and recommendations
10
Reduction of foundation to lower computation costs
Reduce large number of DoF into smaller set of generalized DoF Size(ũ) << size(u) Lower computation costs Approximation of exact
solution Reduction basis contains
limited number of deformation shapes
Only accurate if Spectral convergence Spatial convergence
Introduction Approach Modeling Results Conclusions and recommendations
10
= ++
Reduction of foundation to lower computation costs
Reduce large number of DoF into smaller set of generalized DoF Size(ũ) << size(u) Lower computation costs Approximation of exact
solution Reduction basis contains
limited number of deformation shapes
Only accurate if Spectral convergence Spatial convergence
Introduction Approach Modeling Results Conclusions and recommendations
10
= ++
Reduction methods
Guyan reduction
Introduction Approach Modeling Results Conclusions and recommendations
11
+ …. +
Static constraint modes
Reduction methodsCraig-Bampton reduction
Introduction Approach Modeling Results Conclusions and recommendations
12
Static constraint modes
+ + +
Fixed interface vibration modes
Reduction methods
Augmented Craig-Bampton reduction
Introduction Approach Modeling Results Conclusions and recommendations
13
+ + +
Fixed interface vibration modes
Modal Truncation vectors
Static constraint modes
Impact on fatigue damage results
Offshore wind turbine exposed to cyclic loading
Fatigue is one of the main design drivers
Impact of error in the reponse on the accuracy of the fatigue damage results
Introduction Approach Modeling Results Conclusions and recommendations
14
Content Introduction Approach Modeling Results Conclusions and recommendations
15
Monopile versus Jacket
Introduction Approach Modeling Results Conclusions and recommendations
16
Eigenfrequency
OWT model[Hz]
Foundationωfree [Hz]
Foundation ωfixed [Hz]
1st 0.30 6.73 42.8
Eigenfrequency
OWT model[Hz]
Foundationωfree [Hz]
Foundation ωfixed [Hz]
1st 0.27 1.06 4.09
Wind, wave and operational loads
Wind loads Random load, wide
frequency spectrum Excite frequencies up to 7
Hz
Introduction Approach Modeling Results Conclusions and recommendations
17
ω
Energy
Wind, wave and operational loads
Wind loads Random load, wide
frequency spectrum Excite frequencies up to 7
Hz Wave loads
Wave frequencies are generally lower
Excite frequencies up to 0.5 Hz
Introduction Approach Modeling Results Conclusions and recommendations
17
ω
ω
Energy
Energy
Wind, wave and operational loads
Wind loads Random load, wide
frequency spectrum Excite frequencies up to 7
Hz Wave loads
Wave frequencies are generally lower
Excite frequencies up to 0.5 Hz
Operational loads Rotation frequency of the
rotor (1P) Blade passing frequency
(3P)
Introduction Approach Modeling Results Conclusions and recommendations
17
ω
Energy
Energy
ω
ω
Energy
Content Introduction Approach Modeling Results Conclusions and recommendations
18
Introduction Approach Modeling Results Conclusions and recommendations
19
fwind
fwav
e
Dynamic analysis
fwav
e
Quasi-static post-processing analyses
Quasi-static post-processing analyses
Introduction Approach Modeling Results Conclusions and recommendations
19
fwind
fwav
e
fwave g
Quasi-staticForce controlledDynamic
analysis
g
fwind
fwav
e
fwav
e
Quasi-static post-processing analyses
Introduction Approach Modeling Results Conclusions and recommendations
19
fwind
fwav
e
fwave g
Quasi-static
fwaveub
Force controlled
Quasi-static
Dynamic analysis
g
fwind
fwind
fwav
e
fwav
e
ub
Displacement controlled
fwav
e
Accuracy of quasi-static post-processing
Introduction Approach Modeling Results Conclusions and recommendations
20
ωfree
Elastic energy in the foundation structure
En
erg
y [ J
]
Excitation frequency [Hz]
En
erg
y [ J
]
Accuracy of quasi-static post-processing
Introduction Approach Modeling Results Conclusions and recommendations
20
ωfree
ωfree ωfixed
Elastic energy in the foundation structure
En
erg
y [ J
]E
nerg
y [ J
]
Excitation frequency [Hz]
Expansion of reduced response
Response detailed foundation model obtained by expanding the reduced response of the foundation
Only accurate if model converges spectrally and spatially
Introduction Approach Modeling Results Conclusions and recommendations
21
fwav
e
fwav
e
~
fwin
d
fwav
e
~Expansion
Dynamic analysis
Spectral convergence
Introduction Approach Modeling Results Conclusions and recommendations
22
Relative difference eigenfrequencies of reduced OWT model
Frequency [Hz]
Spectral convergence
Introduction Approach Modeling Results Conclusions and recommendations
22
Relative difference eigenfrequencies of reduced OWT model
Frequency [Hz]
Introduction Approach Modeling Results Conclusions and recommendations
23
Relative energy difference of expanded response
Expansion of reduced response
Excitation frequency [Hz]
Introduction Approach Modeling Results Conclusions and recommendations
23
Residual correction
Expansion of reduced response
Excitation frequency [Hz]
Relative energy difference of expanded response
Expansion of reduced response
Introduction Approach Modeling Results Conclusions and recommendations
23
Residual correction
Excitation frequency [Hz]
Relative energy difference of expanded response
Post-processing analysis with reduced foundation in complete OWT model
Introduction Approach Modeling Results Conclusions and recommendations
24
fwave
fwave
fwind
fwave~~
Dynamic analysis
Post-processing analysis with reduced foundation in complete OWT model
Introduction Approach Modeling Results Conclusions and recommendations
24
fwave g
Force controlledDynamic and Quasi-
staticg
fwind
~fwave
fwave
fwave
fwind
fwave~~
Dynamic analysis
Post-processing analysis with reduced foundation in complete OWT model
Introduction Approach Modeling Results Conclusions and recommendations
24
fwave g
Force controlled
fwaveub
Dynamic and Quasi-static
Displacement controlledDynamic and Quasi-
static
g
fwind
fwind
ub
~fwave
~fwave
Dynamic analysis
fwave
fwave
fwind
fwave~~
Post-processing analysis with reduced foundation in complete OWT model
Introduction Approach Modeling Results Conclusions and recommendations
24
fwave g
fwaveub
g
fwind
fwind
ub
~fwave
~fwave
Force controlledDynamic and Quasi-
static
Displacement controlledDynamic and Quasi-
static
Guyan reduction Craig-Bampton reduction Augmented Craig-Bampton
reduction
fwave
fwave
fwind
fwave~~
Dynamic analysis
Post-processing analysis with reduced foundation in complete OWT model
Introduction Approach Modeling Results Conclusions and recommendations
24
Guyan reduction Craig-Bampton reduction Augmented Craig-Bampton
reduction
fwave g
fwaveub
g
fwind
fwind
ub
~fwave
~fwave
Force controlledDynamic and Quasi-
static
Displacement controlledDynamic and Quasi-
static
fwave
fwave
fwind
fwave~~
Dynamic analysis
Post-processing analysis with Craig-Bampton reduced foundation in OWT model
Introduction Approach Modeling Results Conclusions and recommendations
25
ωfree ωfixed
Relative energy difference with respect to exact solution
Excitation frequency [Hz]
Post-processing analysis with Craig-Bampton reduced foundation in OWT model
Introduction Approach Modeling Results Conclusions and recommendations
25
ωfree ωfixed
Quasi-static post-processing inaccurate ωfree and ωfixed within excitation spectrum
Dynamic post-processing accurate CB reduced model spectrally converged Internal dynamics included
Relative energy difference with respect to exact solution
Excitation frequency [Hz]
Post-processing analysis with Craig-Bampton reduced foundation in OWT model
Introduction Approach Modeling Results Conclusions and recommendations
25
ωfree ωfixed
Quasi-static post-processing inaccurate ωfree and ωfixed within excitation spectrum
Dynamic post-processing accurate CB reduced model converges spectrally Internal dynamics included
Relative energy difference with respect to exact solution
Excitation frequency [Hz]
Fatigue damage - Jacket
Introduction Approach Modeling Results Conclusions and recommendations
26
Relative damage difference with respect to exact damage
Expansion
Quasi-static Force controlled
Dynamic Force controlled
Quasi-static Displacement controlled
Dynamic Displacement controlled
Fatigue damage - Jacket
Introduction Approach Modeling Results Conclusions and recommendations
26
Relative damage difference with respect to exact damage
Content Introduction Approach Modeling Results Conclusions and recommendations
27
Conclusions Following aspects tend to influence the accuracy of
the calculation procedures: The characteristics of the structure
First fixed and free interface eigenfrequency Qs FC significantly underestimates fatigue damage for jacket
Use of a reduced foundation model in complete OWT model Spectral and spatial convergence Residual correction improves accuracy fatigue damage results
Post-processing method Dynamic post-processing provides accurate fatigue damage
results despite errors in interface loads/displacements
Introduction Approach Modeling Results Conclusions and recommendations
28
Conclusions Following aspects tend to influence the accuracy of
the calculation procedures: The characteristics of the structure
First fixed and free interface eigenfrequency Qs FC significantly underestimates fatigue damage for jacket
Use of a reduced foundation model in complete OWT model Spectral and spatial convergence Residual correction improves accuracy fatigue damage results
Post-processing method Dynamic post-processing provides accurate fatigue damage
results despite errors in interface loads/displacements
Introduction Approach Modeling Results Conclusions and recommendations
28
Conclusions Following aspects tend to influence the accuracy of
the calculation procedures: The characteristics of the structure
First fixed and free interface eigenfrequency Qs FC significantly underestimates fatigue damage for jacket
Use of a reduced foundation model in complete OWT model Spectral and spatial convergence Residual correction improves accuracy fatigue damage results
Post-processing method Dynamic post-processing provides accurate fatigue damage
results despite errors in interface loads/displacements
Introduction Approach Modeling Results Conclusions and recommendations
28
Conclusions Following aspects tend to influence the accuracy of
the calculation procedures: The characteristics of the structure
First fixed and free interface eigenfrequency Qs FC significantly underestimates fatigue damage for jacket
Use of a reduced foundation model in complete OWT model Spectral and spatial convergence Residual correction improves accuracy fatigue damage results
Post-processing method Dynamic post-processing provides accurate fatigue damage
results despite use of reduced foundation
Introduction Approach Modeling Results Conclusions and recommendations
28
Recommendations Apply the different calculation procedures in
BHawC with different load cases
Introduction Approach Modeling Results Conclusions and recommendations
29
Recommendations Apply the different calculation procedures in
BHawC with different load cases Set up clear guidelines for spatial
convergence Error estimation methods
Introduction Approach Modeling Results Conclusions and recommendations
29
Recommendations Apply the different calculation procedures in
BHawC with different load cases Set up clear guidelines for spatial
convergence Error estimation methods
Determine an efficient and accurate calculation procedure for more complex models
Introduction Approach Modeling Results Conclusions and recommendations
29
Recommendations Apply the different calculation procedures in
BHawC with different load cases Set up clear guidelines for spatial
convergence Error estimation methods
Determine an efficient and accurate calculation procedure for more complex models
Validate results with real OWTs and loads
Introduction Approach Modeling Results Conclusions and recommendations
29
Thank you for your attention
Levelized Cost of Electricity
Onshore LCoE
Offshore LCoE
Cost of electricityEEX Leipzig
100
10
1
4
1980 1990 2000 2010 2020 2030
€/kWh
Fatigue damage computation
Response Stresses SN-curveFatigue damage
Force versus displacement controlled
Force controlled approach
Displacement controlled approach
fwave g
fwaveub
Relative energy difference quasi-static analysis
Interface loads - Monopile
Interface loads - Jacket
Guyan reduced jacket in complete OWT model
Augmented Craig-Bampton reduction1. External load represented by a spatial and temporal part
2. Quasi-static response and orthogonalize w.r.t. fixed interface vibration modes
3. Orthonormalize w.r.t. each other
4. Construct reduction basis
Augmented Craig-Bampton reduced jacket in complete OWT model
Facts wind energy
Wind turbine Household
Power capacity 3 MW
Energy production 6 – 7,5 GWh per year Serves ± 2000
households
Average household 2,2 persons
Energy usage 3500 kWh per year
Requirement for calculation procedures
Detailed foundation in OWT model
Reduced foundation in OWT model
Expansion If spectrally and spatially converged
Force controlled
Dynamic If spectrally and spatially converged
Quasi-static If ωfree >> max(ωext) If spectrally and spatially converged If ωfree >> max(ωext)
Displacement controlled
Dynamic If spectrally and spatially converged
Quasi-static If ωfree >> max(ωext) If spectrally and spatially converged If ωfree >> max(ωext)
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