Structural Reliability Aspects in Design of Wind Turbines

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 ~ 3 10-5 or β = 4.0 • unmanned: annual PF ~ 5 10-4 or β = 3.3 • IEC 61400-1: land-based wind turbines • annual PF ~ 10-3 or β = 3.0 • IEC 61400-3: offshore wind turbines • annual PF ~ 2 10-4 or β = 3.5 • Observation of failure rates for wind turbines 1984 - 2000 • Failure of blades: approx. 2 10-3 per year (decreasing) • Wind turbine collapse: approx. 0.8 10-3 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 Wind turbine: • 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 = 2 10-4 – 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) > 0 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: 10-15% of energy cost price • Offshore: 25-30% 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 Offshore: • Weather windows • Availability of transport and equipment • Transport time Key aspects: • Availability • Reliability • Maintenance costs • Energy production (benefits) 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 • At present: • mainly based on experience • Future: • risk based – lifetime cost-benefit analyses