A comparison of extreme mooring loads and response of a spar-buoy wind turbine using conditional waves

Abstract

Three-hour random sea-state evaluations for ultimate limit state design of floating offshore wind installations are time-consuming and prevent the use of high-fidelity numerical methods from being implemented. In this work, three conditional waves, Most-Likely Wave (MLW), Most-Likely Response Wave (MLRW) and Conditional Random Response Wave (CRRW), are experimentally investigated as alternative means to generate statistical extreme responses in surge and mooring line loads of a spar-buoy substructure, with shorter time-series duration compared to a 3-hour JONSWAP sea-state. MLW and MLRW are found to generate the greatest mooring line loads, far greater than the statistical expected maximum from a 3-hour random sea-state. However, MLW is uncorrelated with the substructure's linear transfer function of response, and the MLRW includes no memory effects from a random background sea, indicating possible conservatism in the associated load magnitudes. CRRW is a MLRW embedded within a random background and is found to excite a similar mooring load as the 3-hour sea-state, with memory effects included. CRRW could thus be an appropriate method of generating statistically appropriate load responses within a reduced time-series. However, nonlinear effects on the mooring are not accounted for in any of the methods and will be included in future work.