Extrapolation of Extreme Response for Wind Turbines based

Extrapolation of Extreme Response for Wind Turbines based on Field Measurements Authors: Henrik Stensgaard Toft, Aalborg University, Denmark John Dalsgaard Sørensen, Aalborg University / Risø-DTU, Denmark

Contents • • • Introduction Load Extrapolation based on Extreme Response in Storms Numerical Example Conclusion Future Work

Introduction The extreme load can occur in two situations: • Standstill position – The wind turbine is parked and behave like a “normal” civil engineering structure (wind speed > 25 m/s). • Operational condition – The wind turbine is producing power and behave like a machine (wind speed 3 -25 m/s). www. vestas. com

Introduction The extreme load during operation is dependent on: • • • The mean wind speed. The turbulence intensity. The type and settings of the control system. Tower Mudline Moment Pitch controlled wind turbine Flap Bending Moment Pitch controlled wind turbine

Introduction IEC 61400 -1: Extreme load determined from 10 min. simulations of the response during operation over the range of significant wind speeds. The method is based on the following assumptions: • • Local extremes in the response are independent. Individual 10 min. time series are independent. In the present paper are these assumptions investigated. www. vestas. com

Introduction • In the present paper is a new method for estimation of the extreme load presented. • By comparing the extreme load determine by this method and the method in IEC 61400 -1 can the assumptions about independence be studied. • The new method uses a long measured time series which rarely is available – Therefore is the new method only used for validation of the existing method.

Load Extrapolation based on Extreme Response in Storms The new method for estimating the extreme load is based on: • • Existing methods for estimating e. g. the extreme wave height. Assumption about independent storms. www. vestas. com

Load Extrapolation based on Extreme Response in Storms • A storm wind speed Ustorm should be defined so extreme loads occur for mean wind speeds above Ustorm.

Load Extrapolation based on Extreme Response in Storms • Ustorm is defined from the nominal wind speed Unom (typically 10 -15 m/s) and the standard deviation for the turbulence 1. • Independence of storms: • Storms are combined if the mean wind speed between them not are below a percentile of Ustorm. • Storms are separated a minimum number of hours.

Load Extrapolation based on Extreme Response in Storms 1. The measured time series is divided into independent storms. 2. For each storm is the extreme response extracted. 3. To the 25 -30 largest extreme responses is a distribution function fitted. 4. The characteristic load is calculated for the probability:

Numerical Example • • Stall controlled onshore wind turbine. • Dataset for mean wind speed measured over 4 years. • Site: Less severe than class III in IEC 61400 -1. Turbulence intensity Iref = 0. 12. Dataset for response measured over 62 days in winter and spring. (52 days of complete measurements)

Numerical Example Investigation of storm definition. • Storms are combined if the mean wind speed between them not are below 80% of Ustorm. • • Storms are separated at least 2 hours. Higher nominal wind speed leads to less and shorter storms. Nominal wind speed Unom Storm wind speed Ustorm Storms per year Length of storm Ts 14 m/s 9. 5 m/s 202 8. 0 h 15 m/s 10. 2 m/s 165 7. 4 h 16 m/s 11. 0 m/s 135 6. 7 h

Numerical Example Load extrapolation according to IEC 61400 -1. • • Extremes extracted by the Peak Over Threshold method. • Normalized characteristic loads. Characteristic loads calculated with/without statistical uncertainty (Hessian matrix). Threshold Characteristic load without stat. unc. with stat. unc. 1. 4 1. 000 1. 037 2. 0 1. 041 1. 091

Numerical Example Load extrapolation based on extreme response in storms. • • • 25 largest extremes from independent storms are used. Measured time series are short – 52 days of complete measurements Choice of distribution function has a significant influence on the results. Distribution Characteristic load without stat. unc. with stat. unc. Weibull 1. 106 1. 348 Gumbel 1. 303 1. 401

Conclusion • • • New method for calculation of the extreme response is proposed. • Based on a stall controlled wind turbine and a short measured time series. Definition of a storm is proposed. Higher characteristic load using the new method which could indicate that the 10 min. time series or local extremes are dependent. www. vestas. com

Future Work • Refinement of the storm definition based on: • Longer measured time series. • More severe sites. • Estimation of the “correct” long-term distribution for the extreme response: • Weibull • Gumbel • Etc. • Comparison of the new method and IEC 61400 -1 for a long measured time series.

Extrapolation of Extreme Response for Wind Turbines based on Field Measurements Authors: Henrik Stensgaard Toft, Aalborg University, Denmark John Dalsgaard Sørensen, Aalborg University / Risø-DTU, Denmark