The risks of extreme load extrapolation

Abstract

An important problem in wind turbine design is the prediction of the 50-year load, as set by the IEC 61400-1 Design Load Case (DLC) 1.1. In most cases, designers work with limited simulation budgets and are forced to use extrapolation schemes to obtain the required return level. That this is no easy task is proven by the many studies dedicated to finding the best distribution and fitting method to capture the extreme load behavior as well as possible. However, the issue that is often overlooked is the effect that the sheer uncertainty around the 50-year load has on a design process. In this paper, we use a collection of 96 years' worth of extreme loads to perform a large number of hypothetical design problems. The results show that, even with sample sizes exceeding N = 103 10min extremes, designs are often falsely rejected or falsely accepted based on an over-or underpredicted 50-year load. Therefore, designers are advised to be critical of the outcome of DLC 1.1 and should be prepared to invest in large sample sizes.