Experimental and numerically obtained low-frequency radiation characteristics of the OC5-DeepCwind semisubmersible

Abstract

Added mass and damping play a significant role in accurate prediction of floating wind turbine (FWT) motions, especially near the resonance frequencies. This paper investigates the still-water hydrodynamic characteristics of a semi-submersible FWT around the natural periods of surge, heave and pitch motion. A higher-fidelity tool (Computational Fluid Dynamics, CFD) based on OpenFOAM is employed in the numerical computations. The tool is validated against experimental measurements (decay tests and forced surge motions) and then applied to investigate the hydrodynamic characteristics of the whole floater and each column at different amplitudes of forced motions. The heave and pitch decay match well with the experimental measurements, whereas the CFD simulations underestimate the damping in the surge decay. However, better agreement is obtained between measured and numerically estimated surge force in the forced oscillations in surge. Furthermore, the added mass derived from the CFD simulation is around 12% larger than that estimated by the potential flow theory, except the estimated heave added mass under the largest heave motion (up to 35% larger). This additional added mass in the CFD simulations is due to the viscous effects. The damping shows a small dependence on the oscillation period and a larger dependence on the oscillation amplitude within the tested period range. At these frequencies, radiation damping is completely negligible compared to the viscous damping due to vortex shedding, and the accuracy of Morison's drag forces in capturing the viscous damping is sensitive to the drag coefficient.