Hydro-elastic coupling effect on the dynamic global response of a spar-type floating offshore wind turbine

Abstract. Designing floating wind turbine systems requires integrated load assessments (ILA) using fully coupled hydro-servo-aero-elastic models. Although potential flow models are commonly employed for floater hydrodynamics in mooring design and floating offshore wind turbines movement estimation, such models usually assume a rigid body floater. This assumption can significantly impact tower eigenfrequency calculations, especially for large floaters. This study demonstrates these impacts using in-situ sensor data from the Zefyros 2.3 MW spar wind turbine. We detail the methodology used to accurately determine tower eigenfrequency. A rigid floater without added mass resulted in a 37 % error compared to measured modes. Incorporating floater flexibility and added mass reduced this error to 5 %, and further to 3 % with blade flexibility. The observed eigenfrequency discrepancies necessitate modifications to the hydro-servo-aero-elastic model to align with the detailed finite element hydro-structure model eigenfrequencies. We present potential model adjustments and discuss their impacts. After implementing one model modification, we present the results and illustrate the updated model validation process.