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Time-domain Fatigue Response and Reliability Analysis of Offshore Wind Turbines with
Emphasis on Gear Components
Dr. Wenbin Dong
Det Norske Veritas (DNV)
CeSOS Highlights and AMOS Visions Conference27-29th May 2013
Education and work experience 2000-2004: Naval Architecture and Ocean Engineering (Tianjin University
of China , Bachelor )
2002-2004: Finance Management (Tianjin University of China , Minor Bachelor )
2004-2007: Naval Architecture and Ocean Engineering (Tianjin University of China , Master)
2008-2012: Centre for Ships and Ocean Structures-CeSOS (Norwegian University of Science and Technology-NTNU, Ph.D) Supervisor: Prof. Torgeir Moan, Adj. Assoc. Prof. Zhen GaoResearch Topic: Reliability of offshore wind turbinesDefense: 09.12.2012
2012 - : Structural Engineer in Det Norske Veritas (DNV)
2007-2008: Project Officer (Maritime Research Centre-MRC, NanyangTechnology University of Singapore-NTU)
Outline
Background
Objectives and scope
Gear contact fatigue response and reliability analysis
Conclusions
Outline
Background
Objectives and scope
Gear contact fatigue response and reliability analysis
Conclusions
Background Climate change and atmospheric pollution are greatly influenced by theenergy sector.
Wind is one of the most rapidly growing renewable energy sources, as it isclean and environmentally friendly.
Compared to land-based wind energy, there is more available space, morestable and higher wind speed, and less visual disturbance and noise foroffshore wind energy.
Offshore wind turbines (OWTs) are subjected to environmental loads due towaves, current and wind, as well as the effect of corrosion from salt water,and in some cases floating ice; Moreover, they are less accessible than landbased turbines. The operation of OWTs is more expensive than work on land.
The reliability of OWTs is crucial. Fatigue is one of the design drivingcriteria
Background Up to now, offshore fixed wind turbines with monopile, gravity-based and tripod foundations are mainly used for shallow water depths of 20-30 m.
Research work is ongoing for larger water depths (>40 m, Jacket, Semi-submersible, Spar, TLP, etc. ) In the midlevel water depth (40-100 m), jacket structures are competitive and usually used.
Outline
Background
Objectives and scope
Gear contact fatigue response and reliability analysis
Conclusions
The main objective of this study is to investigate simplified reliability-basedfatigue design methods for key mechanical components of OWTs by evaluatingload effects by time-domain simulations.
Objectives and scope
The emphasis is on reliability-based gear contact fatigue analysis of thedrive train of an on-land wind turbine under dynamic wind loads.
The main tasks of this study are to:
• Perform gear contact fatigue analysis of the drive train for anon-land wind turbine under dynamic conditions, with emphasison subsurface pitting.
• Present a framework for reliability-based fatigue designmethod of gears in the drive train with emphasis onsurface/subsurface pitting.
Outline
Background
Objectives and scope
Gear contact fatigue response and reliability analysis
Conclusions
Time-domain based simulation model of 750 kW land-based wind turbine
Gear contact fatigue analysis of a wind turbine drive train under dynamic conditions
Reliability-based gear contact fatigue analysis - a framework
Gear contact fatigue response and reliability analysis
Time-domain based simulation model of 750 kW land-based wind turbine
Gear contact fatigue analysis of a wind turbine drive train under dynamic conditions
Reliability-based gear contact fatigue analysis - a framework
Gear contact fatigue response and reliability analysis
Time domain based simulation model of 750 kW land-based wind turbine
General Description of the Wind Turbine
General Design Operation
Blades
NREL 750kw land-based wind turbine model(SIMPACK 8904)
Model information-wind turbine:
Time domain based simulation model of 750 kW land-based wind turbine
Main shaft
Main bearing
Gearbox
Generator
GRC Drive train configuration
750kw gearbox model in SIMPACK
Model information-drive train:
Time domain based simulation model of 750 kW land-based wind turbine
(Haines and Ollerton,1963; Kalker,1990)
• Gear pair Component Force Element FE:225 in SIMPACK;
• The slicing method is used for helical gears;
Helical gear wheel consisting of several thin independent spur gear teeth. (Flodin and Andersson, 2001)
Gearbox model in SIMPACK
Planet gear
Sun gear(output) Ring gear
Model information-Gearbox:
Time domain based simulation model of 750 kW land-based wind turbine
Global response analysisin FAST
Time series of Torque
tooth -1
tooth +1
tooth 0
De-coupled analysis method:
Time domain based simulation model of 750 kW land-based wind turbine
Global model withoutgearbox
Global model with gearbox
Main purpose:
(1) Check the effects of gearboxon the Torque calculation;
(2) Check the effects of global model on the contact forcecalculation.
Comparison between coupled and decoupled analysis method:
Time domain based simulation model of 750 kW land-based wind turbine
Comparison of the mean value and the standard deviation
(More details: Dong, et al., energies 2012; Int. J. Fatigue2013).
Post-processing of SIMPACK results
Time domain based simulation model of 750 kW land-based wind turbine
Post processing of gear contact forces
Gear contact fatigue response and reliability analysis Time-domain based simulation model of 750 kW land-based wind
turbine
Gear contact fatigue analysis of a wind turbine drive train under dynamic conditions
Reliability-based gear contact fatigue analysis - a framework
Gear contact fatigue analysis of a wind turbine drive train under dynamic conditions
• Crack initiation:
Predictive models to estimate service lives
The first stage of the pitting process
Surface roughness, surface damage, lubrication Single crystal materials
Polycrystalline materials
• Present study: Crack initiation is neglected
The initial crack size
Gear contact fatigue analysis of a wind turbine drive train under dynamic conditions
• Deterministic model for crack propagation:
Linear elastic fracture mechanics (LEFM) model is used:
_
m
II effda C KdN
, :C m material parameters, which can be determined by experiments;
:a half crack length;
:N cycle numbers;
_ :II effK effective Mode II (shear) stress intensity factor range ;
For subsurface initiated pitting, the driving force for crack propagation in rollingcontact has been postulated to depend on hardness and the maximum shear stress,which was shown in experiments (Jiang et al., 1993; Chen et al., 1988).
Gear contact fatigue analysis of a wind turbine drive train under dynamic conditions
A new equation for predicting the gear contact fatigue life :
maxp Maximum contact pressure range (MPa)
cHV Core hardness (HV)
sHV Surface hardness (HV)
effz Effective case depth (mm)
a Crack length(mm)
tK
U
maxp Equivalent Maximum contact pressure range (MPa)
Surface hardness (HV)
Account for the matrix microstructureThe coefficient of friction of the contact surfacesThe porosity fraction
A factor related to crack closure
(a) Contact model of two gear flanks and(b) Equivalent model of two cylinders
(Glodez, et al.,1997)
Schematic of crack propagation
0 2 max max
1, , , , , , , ,
ca
p m m ma a t c s eff
N daC G a K HV HV z p U a p
max
1
max max max max0
m
mpp p f p d p
(More details: Dong, et al., Int. J. Fatigue 2013)
Gear contact fatigue analysis of a wind turbine drive train under dynamic conditions
• Mean wind speed :wU 4-24m/s with an increment of 2m/s;
• The simulation time for each wind speed is taken as 700s, and the first 100’s results arethrown away.
Four different GG models
Long-term fitting results of the maximum contactpressure range generated in the representativetooth of the sun gear for case 1.
Point 0
Environmental conditions and long-term fitting results
Gear contact fatigue analysis of a wind turbine drive train under dynamic conditions
Gear contact fatigue analysis results
Practical cycle numbers based on time domain simulation
Estimated pitting service lives of the sun gear and the planet gears
Gear contact fatigue response and reliability analysis Time-domain based simulation model of 750 kW land-based wind
turbine
Gear contact fatigue analysis of a wind turbine drive train under dynamic conditions
Reliability-based gear contact fatigue analysis - a framework
Reliability-based gear contact fatigue analysis-a frame work
Based on the deterministic model for crack propagation, the following limitstate function can be obtained, If the two-parameter Weibull function is used tofit the long-term probability distribution of :
0
0 02 max
1 1, , , , , , , ,
cam
c m ma a t c s eff
mg a a da C T ABG a K HV HV z p U a
0
02 max
1 1, , , , , , , ,
cam m m mload dyn gearm m
a a t c s eff
mg da C T ABG a K HV HV z p U a
Limit state function
Uncertainty treatment
(More details: Dong, et al., ICOSSAR2013)
Reliability-based gear contact fatigue analysis-a frame work
Gear contact fatigue reliability analysis
planet gearplanet gear
sun gear sun gear
Outline
Background
Objectives and scope
Gear contact fatigue response and reliability analysis
Conclusions
ConclusionsThe main conclusions of this study are summarized as follows:
A new simplified predictive subsurface pitting model for estimating service lives of gearsunder dynamic conditions is presented, and verified by comparison with publishedexperimental evidence.
A two-parameter Weibull function, the generalized gamma function and the three-parameter Weibull function could be used to fit the long-term probability distribution of thegear contact pressures under dynamic conditions.
A framework for reliability-based probabilistic fatigue design of gears for onshore andoffshore wind turbines is suggested. Its application is presented through examples
The environmental conditions have a big effect on gear contact fatigue life, whichshould be considered in the real gear design for wind turbines, especially for offshoreapplications.
Reference
Flodin A, Andersson S. A simplified model for wear prediction in helical gears. Wear 2001;241:285–92.Jiang B, Zheng X, Wang M. Calculation for rolling contact fatigue life and strength of case-hardened gear materials by computer. J Test Eval 1993;21(1):9–13.Chen Q, Leng X, Shao E. Influence of microstructure and residual stress on the stages of case crushing. Wear 1988;122:45–55.Glodez S, Winter H, Stuwe HP. A fracture mechanics model for the wear of gear flanks by pitting. Wear 1997;208:177–83.
Dong WB, Xing YH, Moan T. Time domain modeling and analysis of dynamic gear contact force in wind turbine gearbox with respect to fatigue assessment. Energies 2012;5(11):4350–71.
Haines, D.J.; Ollerton, E. Contact stress distributions on elliptical contact surfaces subjected to radial and tangential forces. Proc. Inst. Mech. Eng. 1963, 177, 95–114.
Kaller, J.J. Three-Dimensional Elastic Bodies in Rolling Contact; Kluwer Academic Publishing: Dordrecht, The Netherlands, 1990.
Dong, W.B.; Xing, Y.H.; Moan, T.; Gao, Z. Time domain based gear contact fatigue analysis of awind turbine drivetrain under dynamic conditions. Int. J. Fatigue 2013; 48: 133–146.
Dong, W.B.; Moan, T.; Gao, Z. Reliability-based gear contact fatigue analysis for wind turbines under stochastic dynamic conditions. 11th International Conference on Structural Safety & Reliability (ICOSSAR 2013). New York, USA. June 2013.