Quantifying the Effect of Two-Dimensional Wave Spectra on Floating Offshore Wind Turbine Fatigue

Abstract This paper explores the quantitative effect of applying two-dimensional wave spectra to describe the wave process in a fatigue analysis of a floating offshore wind turbine. The analysis is performed with a fully coupled dynamic model of a 15 MW floating wind turbine on a three-column semisubmersible substructure. The turbine is defined with a stiff-stiff tower configuration and a semi-taut mooring system. A floater-specific turbine control system is used for accurate simulation of the turbine response to dynamic wind loading and floater motion. The system has been analyzed for severe environmental conditions of the North Sea. The NORA3 long-term wind and wave hindcast, which is of high quality and available in different levels of detail and granularity, is available in this region. Based on this, several simulations of the fatigue load cases are performed, using the same wind field, but comparing the influence from the two-dimensional spectra with two-partition short-crested sea states, on tower and mooring line fatigue damage. The results show that, for the selected North Sea location, using a full 2D spectral approach may increase the estimated fatigue life of both the mooring line top chains as well as the tower base by up to 25%. This introduces significant potential steel savings for the system, which on a wind park level may equate to important reductions in capital expenditure.