The determination of the extreme loads on wind turbines – some practical issues

Abstract

The probabilistic method commonly applied to arrive at the ultimate loading is as follows: for several different mean wind speeds load simulations are performed. For each mean wind speed a conditional distribution can be fitted to the load maxima for that particular wind speed. The overall distribution of the load response is obtained by a weighted average of these conditional distributions taking into account the probability of occurrence of the wind speed bins (Weibull). Two practical issues are addressed: • The plotting positions • The averaging over the mean wind speeds The plotting positions (for the m-th ranked value out of N) are unique and given by: m/(N +1) . This means that the plotting positions do not depend on the particular application and/or the anticipated distribution function. The maximum of the 50 year estimates based on the exceedance probability Q short (LlUi) is an upper bound of the long term 50 year load value L50. A lower bound for L50 is given by the maximum of the estimates based on the relative exceedance probability Rshort (LlUi)=Qshort (LlUi) ni; with ni the fraction of time for wind speed bin Ui. In the situation that load data of just 1 wind speed bin is available it is in general not possible to determine L50. In case it is assumed that the considered wind speed bin governs the load, a good estimate (lower bound) of L50 is obtained by considering Rshort. If it is assumed that the load distributions of the other wind speed bins are about the same, a good estimate is obtained by considering Qshort (upper bound).