Frequency Versus Time Domain Fatigue Analysis of a Semi-Submersible Wind Turbine Tower
The current paper addresses a study of a semi-submersible wind turbine, where tower base bending moments and short term tower fatigue damage was estimated by a frequency domain method assuming two-dimensional platform motions. Both a rigid structure assumption and a generalised degree of freedom model for including the first flexible mode of the turbine were investigated. First, response wind- and wave loads were considered separately, then superposition was used to find the response to combined wind and wave loading. The bending moments and fatigue damage obtained by these methods were compared to results from a fully coupled, non-linear time domain analyses. In this study a three column, catenary moored semi-submersible with the NREL 5MW turbine mounted on one of the columns was modelled. The time domain simulation tool used was Simo-Riflex-AeroDyn from Marintek and CeSOS. The frequency domain method accounting for a flexible turbine gave a good representation of the tower base bending moment histories for the moderate sea states in these analyses, also for the combined wind and wave analyses. The frequency domain fatigue damage predictions were underestimated by 0–60%, most likely due to the exponential relationship between damage and stress amplitude.