Fatigue damage characteristics of a semisubmersible-type floating offshore wind turbine at tower base

Abstract

Fully coupled analysis of a semisubmersible-type floating offshore wind turbine is carried out for the fatigue damage investigation at tower base by using the code FAST. By combining the axial force and the bending moment, the stress around the circumference can be calculated. This study investigates the effect of second order wave force, wind load, mooring model, and tower elasticity on the stress response at tower base. There are three peaks in the stress amplitude spectrum, corresponding to pitch resonance, wave frequency, and first-order natural mode of the tower, respectively. The first and third peaks are mainly induced by wind load. The effect of the second order wave force and mooring model on the stress seems to be neglectable based on the numerical results. By using the rainflow counting method, the fatigue damage is calculated. The obtained fatigue damage under several environmental conditions indicates that the misalignment and coupling effect of wind and wave loads may significantly affect the total fatigue damage. The wind and wave induced responses are calculated separately (uncoupled method). Next, two superposition methods for fatigue damage calculation are analyzed: fatigue damage superposition and stress superposition. Compared with the fully coupled analysis considering wind and wave together, the former gives lower prediction but the latter seems to provide acceptable results. Finally, considering the long-term wind and wave distribution, the fatigue damage under different environmental conditions is calculated, and the fatigue damage obtained from stress superposition method shows reasonable agreement with that obtained from a fully coupled analysis.