1. Structural Reliability Aspects in Design of Wind Turbines
John Dalsgaard Sørensen
Aalborg University & Risø National Laboratory
Denmark
Introduction
Failure modes & stochastic models
Reliability analysis & optimal reliability level
Operation & maintenance
Summary

2. Introduction Size / Onshore – offshore Wind turbine / Function
Failure types
Structural reliability:
Blades - glass fiber
Hub - cast steel
Nacelle - cast steel
Tower - steel
Foundation
Optimization:
Reliability-based design
Optimal reliability level / Calibration of partial safety factors
Operation & maintenance

3. Introduction - installed wind power
Source: BTM: 2006

4. Introduction - onshore

5. Introduction - offshore
Nysted 72 Bonus 2,2 MW
Middelgrunden – 20 Bonus 2,0 MW
Horns Rev – 80 Vestas 2,0 MW

6. Example: Vestas V120-4.5 MW Diameter: 120m
Height: site dependent (90 m)
Power: 4.5 MW
Control: Pitch
Weight:
Nacelle: 145 t
Rotor: 75 t (each blade: 13 t)
Tower: 220 t
bottom diameter: 5.5 m
Source: Vestas 2006

7. Example: Vestas V120-4.5 MW Nacelle: Hub (cast iron)
Main frame (cast iron)
Source: Vestas 2006

8. Introduction
Power curve: example
Source: Vestas 2006

9. Introduction Blade flap moment: Stall controlled: Pitch controlled:
Source: Vestas 2006

10. Introduction – Failure types
Gearbox
Generator
Blade pitch mechanism
Yaw mechanism
Main shaft …
Hub: cracks…, collapse
Blades: cracks,…, collapse
Tower: yielding, cracks, corrosion, …, collapse
Foundation: collapse

11. Introduction – Failures
Frederikshavn, Denmark October 26, 2006

12. Introduction – Failure types
Failure Rates and Downtimes
Source: ISET: 2006

13. Structural reliability – limit states
Operational mode:
Standstill (Vhub ≥ 25 m/s)
Tower: steel – STR
Nacelle/hub: cast steel – STR
Foundation: steel / concrete - GEO
Blades: glass fibre – STR
Operation (3 m/s < Vhub < 25 m/s)
Tower: steel – STR, FAT
Nacelle/hub: cast steel – STR, FAT
Foundation: steel / concrete / GEO
Blades: glass fibre – STR, FAT
Failure modes:
STR/GEO: structural / foundation failure - collapse
FAT: fatigue
Wind turbine: machine or ‘building’?

14. Structural reliability – uncertainties
Loads:
Natural randomness of load (wind + wave + ice + current)
Statistical uncertainty - estimation of statistical parameters
Statistical uncertainty - load extrapolation based on simulations
Model uncertainty - load models
Model uncertainty - structural analysis (dynamic and non-linear effects)
Strengths:
Natural randomness of material strengths
Model uncertainty – laboratory tests  real WT
Model uncertainty – resistance models
Single WT / WT in park

15. Structural reliability – Wakes in wind parks
Source: Risø: 2005

16. Structural reliability – Wakes in wind parks
ULS combinations:
Standstill: wind velocity at hub height exceeds 25 m/s → wind turbine parked
Wind load = annual extreme wind load
Operation: wind turbine is in operation and produces electricity
Wind velocity is ≤ 25 m/s at hub height
Maximum wind load:
dependent on the control system and maximum turbulence intensity – often less than 25 m/s
based on simulation of limited number of response realisations and extrapolation
dependent on single / park WT (turbulence)

17. Stochastic model Load effect E depends on: Mean wind speed V
Turbulence σ1
Wind shear
Change in wind direction during gust
Control system fault
Loss of electrical network
Normal shut down
Emergency shut down
Control system

18. Stochastic model – offshore WT
Load combination problems:
Wind, wave, ice and current
Standstill / operation expected value standard deviation
Example: stall WT:
Base shear:
Overturning moment:
Source: Risø: 2003

19. Target reliability index - optimal reliability level
Building codes: e.g. Eurocode EN1990:2002:
annual PF = 10-6 or β = 4.7
Fixed steel offshore structures: e.g. ISO 19902:2004
manned: annual PF ~ or β = 4.0
unmanned: annual PF ~ or β = 3.3
IEC : land-based wind turbines
annual PF ~ 10-3 or β = 3.0
IEC : offshore wind turbines
annual PF ~ or β = 3.5
Observation of failure rates for wind turbines
Failure of blades: approx per year (decreasing)
Wind turbine collapse: approx per year (decreasing)
2.75 MW test wind turbine, Aalborg University, Denmark

20. Optimal reliability level
Offshore wind turbines: probability of human injury is small  reliability level could be assessed by cost-optimization:
Systematical rebuilding in case of failure
No rebuilding in case of failure
Inspection / maintenance included

21. Systematic rebuilding
Optimal design:
Cost-benefit optimization
LQI (Life Quality Index) criterion – less important for (offshore) wind turbines:
(Rackwitz 2001):

22. Example – offshore wind turbine with monopile foundation
2 MW offshore pitch controlled wind turbine with monopile foundation
Tower height h = 63 m
Limit states:
Yielding
Local buckling
Fatigue

23. Example Initial costs: Failure costs: Benefits:
Result: Optimal reliability level: annual PF = – 10-3 corresponding to β = 3.1 – 3.5

24. Risk-based optimal design
Optimal decision ≡ max expected benefits – costs:
B expected benefits
CI structural costs
CF expected failure costs
Basic requirement: B – CI (z) – CF (z) > in optimum

25. Probabilistic design of wind turbines
Stochastic models for loads, strengths and computational models
Reliability analysis of WT limit states (standstill/operation – single/park)
Optimal reliability level
Direct probabilistic design – use of test results / measurements
Reliability-based adjustment of partial factors based on test results
Operation & maintenance

26. Operation & maintenance
Costs to operation and maintenance are large, especially for offshore wind parks
Onshore: % of energy cost price
Offshore: % of energy cost price
Deterioration process always present to some extent
Maintenance should be planned using risk-based methods
Reliability of component or complete wind turbine:
Probability to survive until time t
Availability of component or complete wind turbine:
Probability of function at time t

27. Operation & maintenance
Key aspects:
Availability
Reliability
Maintenance costs
Energy production (benefits)
Offshore:
Weather windows
Availability of transport and equipment
Transport time
Source: ECN: 2006

28. Operation & maintenance
Unplanned (corrective): exchange / repair of failed components (Condition Monitoring) cost: 0,005 – 0,010 €/kWh
Planned (preventive): cost: 0,003 – 0,009 €/kWh
Scheme: inspections, and evt. repair after predefined scheme
Conditioned: monitor condition of system and decide next on evt. repair based on degree of deterioration
→ based on pre-posterior Bayesian decision model

29. Risk-Based Planning of Operation & Maintenance
Theoretical basis:
Bayesian decision theory – pre-posterior formulation:
Optimal decision: Minimum total expected costs in lifetime

30. Operation & maintenance
Time scale for decisions:
Short: minutes
Operation: ex: stop wind turbine if price is too low
Use uncertainty on wind forecasts and price development in decision
Medium: days
Maintenance: ex: Start maintenance operation on offshore wind turbine
Use uncertainty on weather windows (wave height and wind speed)
Long: month/years
Preventive maintenance
Inspection and monitoring planning
Gear boxes, generators, fatigue critical structural details …

31. Summary
Wind turbines: building (standstill) / machine (operation & accidental modes)
Reliability level: lower than civil engineering structures
Design approach:
At present:
LRFD based codes
Future:
direct probabilistic design?
inclusion of test / condition monitoring results on a probabilistic basis
Inspection & maintenance: very important
mainly based on experience
risk based – lifetime cost-benefit analyses