Heave motion response of a floating offshore wind turbine with damping plate

Abstract

A rigid damping plate attached to the bottom of a cylinder has a distinct advantage in reducing the motion response of a floating circular cylinder by increasing the added mass and the damping coefficient. The added mass plays an important role in determining the location of the resonant frequency, and also the radiation damping reduces the motion amplitude at resonance. Furthermore, the porous holes of the permeable damping plate induce flow separation and vortices agitation, causing the increase of the viscous damping and energy loss. To obtain better heave characteristics, the present research proposes dual damping plates: a porous upper-damping plate attached to the side wall of the cylinder and a rigid lower-damping plate to the bottom. Analytical and experimental studies are carried out to investigate the heave motion response of the cylinder according to the characteristics of the damping plate such as depth ratio, diameter ratio, and porosity. The analytical method by using Matched Eigenfunction Expansion Method (MEEM) has been developed for the heave motion analysis of the floating circular cylinder with the single rigid or dual (a rigid and a porous) damping plates in the context of linear potential theory and Darcy's law (the normal velocity of fluid passing through a thin porous disk is linearly proportional to the pressure difference across it). To apply the MEEM, the fluid domain is divided into three regions, and both the diffraction and the radiation potential in each region are expressed by the Fourier Bessel series. The unknown coefficients in each region are determined by applying the continuity of the pressure and the normal velocity at the matching boundaries. With the assumption of an inviscid fluid in the potential theory, a heave viscous damping is calculated by the non-dimensional damping coefficient obtained from a heave free decay test. In order to confirm the analytical solutions, experiments have been conducted in 2-D wave flume for the test in regular waves, and in large scaled wave flume for the test in irregular waves, with varying wave conditions. The analytical results are in good agreement with the experimental results in both regular and random waves, and the heave motion response of the cylinder is decreased drastically in the heave resonance region by the proposed dual damping plates. In the application study of the damping plate to the Floating Offshore Wind Turbine (FOWT), the NREL offshore 5MW baseline wind turbine and spar type floating support structure are adopted to perform computational simulations using FAST code. The hydrodynamic input data of the floating support platform with a single rigid and dual (a rigid and a porous) damping plates for HydroDyn, hydrodynamics module of FAST code, is calculated by the MEEM. The heave motion and the variation of the axial forces of the FOWT are considerably decreased at around the heave resonant frequency by the damping plate. It would be a great advantage to extend the fatigue life of the FOWT. In addition, it shifts the heave natural frequency to the lower frequency region due to the increase of the added mass. However, the decrease in the heave motion of the FOWT hardly affects the power generation.