Motion characteristics of a barge-type floating vertical-axis wind turbine with moonpools

The studies on floating offshore wind turbines have been increasing over recent decades due to the growing investments of industries in marine renewable energy. Compared with horizontal-axis wind turbines, research on vertical-axis wind turbines is relatively few, though the latter may have advantages over the former in some aspects, for example, potentially lower cost of energy. In this paper, the characteristics of the motion responses of a barge-type floating system with four moonpools and a vertical-axis wind turbine are experimentally and numerically investigated. The focus is on the gyroscopic effects of turbine rotations on the motion responses of the floating system under different wave conditions. Physical model tests are conducted in a wave tank, where a 2-MW-class turbine is modelled in the 1:100 scale. It is found that the first-order sway and roll motion amplitudes near the resonant frequencies are reduced by the turbine rotations, whereas the first-order heave motion of the floating system is hardly influenced. The second-order motion responses and mooring tether tensions may be significantly amplified with the increasing rotating speed of turbine. The maximum ratio of the second-order response amplitude of roll motion to the first-order roll amplitude is up to 17% in the experiments of this study. A radiation and diffraction code based on linear potential flow theory is used to predict the motion responses and mooring forces. The comparisons with experimental data suggest that the viscous damping of water resonance in moonpools and the gyroscopic effects of turbine rotations should be taken into account in predictions.