Pseudo-coupled approach to fatigue assessment for semi-submersible type floating offshore wind turbines

Recent studies and developments in the wind industry indicate a trend of floating offshore wind turbine (FOWT) designs with larger power ratings. The fatigue damage and cost of maintaining the hull structure become more critical for the asset integrity as the turbine size grows larger, which exacerbates the urgency to incorporate fatigue life estimation early in the design spiral for proper asset optimisation. Fatigue calculations for FOWTs are complex and computationally more expensive than for most of the other floating systems deployed in use of offshore industry. To enable fatigue prediction in design development, this paper introduces a pseudo-coupled analysis approach to reduce the computational expense by the order of one-hundredth whilst avoiding a sig-nificant compromise in the accuracy. The results are benchmarked against the state-of-the-art fully-coupled approach, and a good agreement is observed. A preliminary design spiral for VolturnUS-S FOWT is established through a Monte Carlo simulation, accounting for fatigue along with other design criteria. The results show that fatigue is sensitive to even minor changes in the design parameters. The fatigue damage can be three times more or less for similar size and shape FOWTs which proves that fatigue prediction for FOWTs at initial design development is crucial.