Abstract
Second-order wave loads on floating offshore wind turbine (FOWT) may excite severe resonance at the eigenfrequencies of the floating system and cause fatigue damage. The hydrodynamic coefficients including first- and second-order wave force transfer functions are calculated, and fully coupled time-domain simulation by FAST is conducted for selected environmental conditions. Each load case is simulated without second-order loads, and with difference-frequency full quadratic transfer functions (QTFs), sum-frequency full QTFs, and complete second-order solution. The effect of second-order difference- and sum-frequency wave loads on the dynamic behaviors of the 10 MW semi-submersible FOWT is discussed. Subsequently, the effect of second-order wave loads on the structural fatigue behavior of the 10 MW and 5 MW semi-submersible FOWTs is investigated and compared by computing ultimate and damage-equivalent loads (DELs). The results show that ignoring second-order difference-frequency wave loads may underestimate the low-frequency responses, and second-order sum-frequency wave loads significantly affect the structural dynamics of the 10 MW FOWT. In addition, the effect of second-order wave loads on the fatigue damage of the 5 MW and 10 MW semi-submersible FOWTs are significantly different.
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