Statistical extrapolation methods and empirical formulae for estimating extreme loads on operating wind turbine towers

Abstract

The wind turbine tower is designed against extreme load during power production with the return period of 50 years. The statistical extrapolation method is widely used to estimate the extreme load of wind turbine tower and is recommended by the IEC Standard 61400–1. However, various statistical extrapolation methods were used in previous studies and their accuracy and efficiency have not been evaluated comprehensively, which is investigated in this research. The validation procedure is assessed by simulating the one-year load of the reference wind turbine. It is found the fitting before aggregation methods with peak loads extracted by block maxima or peak over threshold techniques and three-parameter Weibull fitting function have the best accuracy and robustness. 60 samples for each wind speed are enough and the data below rated wind speed can be neglected for the optimal statistical extrapolation methods. In addition, an empirical formula for the extreme load estimation of wind turbine tower is proposed considering both the mean and fluctuating moment caused by the thrust force on the rotor, drag forces on the tower and nacelle and the gravities of nacelle and rotor. The extrapolation factor is modelled based on the estimated extreme load by the statistical extrapolation method. The proposed empirical formula shows good accuracy for both short-term and long-term extreme loads of wind turbine tower at various wind conditions.