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 do-main 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 in-vestigated. First, response wind- and wave loads were consid-ered 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 re-sults 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 tur-bine gave a good representation of the tower base bending mo-ment 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. NOMENCLATURE β Frequency ratio ∗Address all correspondence to this author. ζ Wave elevation θ Phase angle ξ Damping ratio ψ/ψ ′ ′ Normalised mode shape/2nd z-derivative ω Angular frequency ω0 Lowest flexible eigenfrequency A Ndo f xNdo f added mass matrix B Ndo f xNdo f damping matrix C Ndo f xNdo f restoring matrix F 1xNdo f complex force vector M Ndo f xNdo f mass matrix x 1xNdo f platform motion vector A Rotor area c ∗ Generalised dampin