Nonlinear short term extreme response of spar type floating offshore wind turbines

In this work, the dynamic characteristics of spar based floating 5 MW offshore wind turbine (OWT) are studied under operational and survival conditions for offshore Indian conditions. The spar with OWT is installed in 320 m water depth. The OWT is subjected to combined wind and wave loads according to irregular Pierson-Moskowitz spectrum. After obtaining nonlinear spar platform responses, 3-h short term extreme motions are calculated which are useful for the design of an OWT. Extreme values provide realistic estimates to the associated risk of an event since the tail region governs the largest/smallest events or even the events over a threshold in a sample. In this paper, the extreme values are obtained by fitting the peaks in the tail regime using a Weibull-distribution. While one obtains Gaussian responses under the survival loads, the operational conditions have non-Gaussian responses and also larger than survival ones. The results show one should exercise caution for generating the extreme values for non-Gaussian responses and do a proper sensitivity analysis with respect to samples used for generation of extremes.