Proper performance of structures requires among other things that its failure probability is sufficiently small. This would imply design for survival in extreme conditions. The failure of a system can occur when the ultimate strength is exceeded (Ultimate Limit State) or fatigue limit (Fatigue Limit State) is passed. The focus in this paper is on the determination of extreme responses for ULS design checks. The present paper deals with coupled wave and wind induced motion and structural response in harsh condition up to 14.4 (m) significant wave height and 49 (m/sec) 10-min average wind speed (at top of tower, 90 m) for a parked floating wind turbine. In survival condition the wind induced resonant responses (mainly platform pitch resonance) are dominant. Due to platform resonant motion responses, the structural responses are close to Gaussian. The dynamic structural responses show that the process is wide banded. The critical structural responses are determined by coupled aero-hydro-elastic time domain simulation. Based on different simulations (20 1-hour, 20 2-hours, 20 3-hours and 20 5-hours) the mean up-crossing rate has been found in order to predict the extreme structural responses. The most probable maximum of the bending moment and the bending moment having up-crossing rate of 10−4 are found to be close in the present research. The minimum total simulation time in order to get accurate results is highly correlated to the needed up-crossing rate. The 1-hour and 2-hours original values cannot provide any information for 10−4 up-crossing rate. Comparison of different simulation periods shows that the 20 1-hour simulations can be used in order to investigate the 3-hours extreme bending moment if the proper extrapolation of up-crossing rate used.

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