Coupled time domain analyses of a semi-submersible wind turbine are performed with the intention to study motions affecting fatigue damage at the base of the tower. The software applied is SIMO/RIFLEX with the extension TDHmill, which gives the wind thrust force and gyro moment on the wind turbine as point loads in the tower top. Short term environmental conditions are chosen from a joint wind and wave distribution for a site in the Northern North Sea. Variance spectra, mean value, standard deviation, kurtosis, skewness and Vanmarcke’s bandwidth parameter are calculated for stresses at the base of the tower. Damage is calculated for each short term condition by two methods; rainflow counting and narrow band approximation. The accuracy of narrow band approximation estimates for fatigue are examined for the structure in question. Time domain simulations are carried out for different sea states and fatigue damage is calculated for each case. Simulations show that turbulent wind dominates the response at low wind speeds and the response spectral density function tends to be very wide-banded. For wave dominated response, spectra have lower bandwidth, and narrow banded approximation for fatigue damage gives estimates 20–50% above rainflow counted damage.

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