Optimal threshold selection in the POT method for extreme value prediction of the dynamic responses of a Spar-type floating wind turbine

This paper concerns the calculation of the short term extreme structural responses of a Spar-type floating wind turbine by applying the peak-over-threshold (POT) method. A new approach is proposed for finding an optimum threshold value by incorporating a declustering algorithm into the POT method. To select the clusters and check the Poisson character of the number of clusters we use the concept of a dispersion index. Then, the method of maximum product of spacing is utilized to estimate the parameters in the Generalized Pareto distribution of the largest values in all the selected clusters over the optimum threshold value. As examples of calculations, optimum threshold values of the out-of-plane and in-plane blade root bending moments and the tower - Spar interface bending moments of the NREL 5-MW OC3-Hywind floating wind turbine have been obtained by utilizing the new approach. The extreme response probability plot based on an optimum threshold value has been compared with the probability plot based on an empirical threshold value, and the accuracy and efficiency of the new approach have been convincingly validated. Goodness-of-fits plots have also been included in this article for testing the accuracy of the proposed new approach. Finally, in order to make a fair comparison, we have also extrapolated the short-term extreme values using the different POT methods and compared them with the reference values obtained directly from a large number of time-domain simulations.