A design wave method for the prediction of extreme combined wind-wave loads on floating wind turbines

The design wave approach has been used in the past to evaluate short-term extreme wave loads on offshore structures, effectively reducing the required test duration compared to the irregular wave Monte Carlo approach. The method uses short-duration wave episodes specifically designed to excite extreme loads. Also known as response-conditioned waves (RCW), they are used in a multi-fidelity framework, where the load statistics are first estimated using an efficient low-fidelity surrogate response model. Then, high-fidelity tests, such as experiments or CFD, are performed only for the short-duration episodes and are used to update the initial prediction. In this work, the tower base bending moment on a flexible, spar-type floating wind turbine (FWT) is investigated. First, a wave-only configuration is considered, and the approach is applied to predict the FWT response in different sea states. Comparison with irregular wave Monte Carlo results shows that the design wave approach captures the wave-frequency response nonlinearities effectively, but fails to capture high-frequency hydroelastic effects, with the underestimation reaching approximately 10% for the most extreme case. A Weibull distribution-fitting technique is also proposed, which provides an analytical response distribution, based on a limited set of high-fidelity design wave tests. For combined wind-wave loads, the response-conditioning procedure is coupled with OpenFAST , permitting the identification of design wave episodes under steady wind conditions. Finally, these wave episodes are used to predict the combined wave- and wind-induced bending moment statistics under both steady and turbulent wind conditions, yielding results consistent with experimental Monte Carlo tests.