Investigation of CFD versus Potential-Flow Methods for Hydrodynamic Loads on a Floating Offshore Wind Turbine

Development of floating offshore wind turbines (FOWTs) has picked up great pace in the last five-ten years. Commercial projects such as Hywind Scotland, Hywind Tampen and others in planning and development demonstrate a shift of industry perception making it relevant to understand the behavior of these complex systems. In this paper, the focus is on the hydrodynamic behavior of the floating substructure (floater) for FOWTs. Generally, Potential-Flow (PF) methods are used to calculate the hydrodynamic loads on the floater with a Morison Element (ME) model to approximate the drag (viscous) forces empirically. Computational Fluid Dynamics (CFD) methods, on the other hand, inherently account for viscous effects. Here, a diffraction analysis for a semi-sub FOWT is performed using the open-source CFD platform OpenFOAM (OF) in regular waves and results compared against PF method. The effect of varying wave height and wave heading for the floater is examined. The influence of a non-zero pitch angle in operating condition for FOWTs on wave-drift loads is investigated using CFD and compared against PF results. The utility of CFD in the analysis of FOWTs is assessed and possible shortcomings in industry-standard PF methods are discussed. The present work represents a stepping-stone towards future work planned for developing a fully coupled aero-hydrodynamic CFD simulation.