Time domain analysis procedures for fatigue assessment of a semi-submersible wind turbine

Abstract

Long term time domain analysis of the nominal stress for fatigue as- sessment of the tower and platform members of a three-column semi- submersible was performed by fully coupled time domain analyses in Simo- Ri ex-AeroDyn. By combining the nominal stress ranges with stress con- centration factors, hot spot stresses for fatigue damage calculation can be obtained. The aim of the study was to investigate the necessary simulation duration, number of random realisations and bin sizes for the discretisa- tion of the joint wind and wave distribution. A total of 2316 3-hour time domain simulations, were performed. In mild sea states with wind speeds between 7 and 9 m/s, the tower and pontoon experienced high fatigue damage due to resonance in the rst bending frequency of the tower from the tower wake blade passing frequency (3P). Important fatigue e ects seemed to be captured by 1 hour simulations, and the sensitivity to number of random realisations was low when run- ning simulations of more than one hour. Fatigue damage for the tower base converged faster with simulation duration and number of random realisations than it did for the platform members. Bin sizes of 2 m/s for wind, 1 s for wave periods and 1 m for wave heights seemed to give acceptable estimates of total fatigue damage. It is, however, important that wind speeds that give coinciding 3P and tower resonance are included and that wave periods that give the largest pitch motion are included in the analysis.