Extreme response estimation of offshore wind turbines with an extended contour-line method

Abstract

The contour-line method is a simplified approach for finding the long-term extreme values by a small number of short-term analyses. It is assumed the response with a return period of T years occurs during a sea-state with a shorter or equal return period. This provides good extreme response estimates for structures with a response behaviour monotonically increasing with the severity of the sea-state. For an offshore wind turbine, the power production stops at a certain wind speed to reduce wear, changing the dynamics of the system dramatically, from wind to wave load dominated. Also, there is a non-negligible probability for the turbine being unable to operate although the wind speeds would suggest so, due to e.g. grid- or mechanical failure. Therefore, the original contour-line method for extreme response estimation is not applicable without some modifications to account for changing dynamics. In the present work, it is suggested to treat the operational and non-operational conditions as two sub-populations and estimate the T-year response in each population by a standard contour-line method. Next, the results from each sub-population are combined in order to estimate the total T-year response in consistent manner.