FOWT Modeling and Validation in Extreme Conditions

As the global energy sector transitions towards sustainable sources, floating offshore wind turbines (FOWTs) have emerged as a promising solution for harnessing wind power in deep-water locations. Achieving accurate, reliable, and efficient numerical models for the design and analysis of FOWTs is paramount. This study investigates how the simplifying approximated assumption of uncertain key parameters affects floater motions and mooring line tensions under irregular sea states. The turbine considered is the 5 MW reference model defined by the U.S. National Renewable Energy Laboratory (NREL), supported by the OC4 DeepCwind semisubmersible substructure. The deployed mooring system was priorly optimized for the chosen site off the coast of Lampedusa, Italy. In this paper, a novel two-stage optimization procedure is introduced to calibrate the simplified global linear and quadratic damping parameters, which have been deployed in a hydrodynamic load model developed in OrcaFlex® commercial software to simulate the load effects. The 100-year return period Ultimate Limit State (ULS) event, derived from the environmental conditions of the ERA5 database, is considered for model validation. The numerical predictions show good qualitative agreement with experimental data in capturing resonant frequencies and mooring line tension responses. Nonetheless, further refinement, such as introducing Morison element-based local hydrodynamic damping or leveraging high-fidelity computational Fluid Dynamics (CFD) simulations, would improve amplitude predictions.