Predicting short term extreme response of spar offshore floating wind turbine

In this paper, the short term extreme response of spar offshore 5MW NREL benchmark wind turbine is predicted. The spar is installed in a water depth of 320 m. The coupled wind and wave analysis is performed by coupling aerodynamic software FAST (Jonkman and Bull Jr, 2005) and hydrodynamic software ANSYS-AQWA(2010). The global time domain responses of the spar type OWT is calculated for 600 s. The wave spectrum has significant wave height 6m and peak spectral period 10s and follows Pierson-Moskowitz spectrum. For safe operation, the structures should survive against different environmental conditions. The OWT can fail either in the operational regime or in the harsh environmental conditions. Therefore the dynamic simulation is carried out for two wind speeds, i.e., one in operational (hub height wind speed as 11.5 m/s) regime and another in idling condition (30 m/s) using the above wave parameters. Monte Carlo method is used to simulate the OWT responses in irregular wave loading condition. After obtaining the time-domain nonlinear responses, the 3-hr short term extreme responses are obtained which are useful for design of OWT. The 3-hr extreme response is obtained using Global Maxima Method (GMM). In Global Maxima Method, one maximum is taken from each time series and the maxima are fitted to two Generalized Extreme Value distributions, viz., Gumbel and Weibull distribution. The results show that Weibull fit is on a conservative side than Gumbel fit. Since the calculation of extremes is dependent on time domain simulations, a comparative study is also done for 100 and 20 samples so as to understand the sensitivity of extreme values due to lower sample size.