Abstract
The first tower bending frequency of a floating wind turbine (FWT) can be significantly higher in water than if the substructure were clamped on land. The paper presents a simplified two-degree-of-freedom analytical model of the coupled floater-tower motions describing this phenomenon. In spite of its simplicity, the model conveniently explains coupling effects, the relationship between wet and dry eigenfrequencies and mode shapes, and highlights the driving parameters for such couplings. For common semi-submersible wind turbine designs, the coupling between tower deflection and rigid floater motions has little effect on the near-rigid-body modes of motion, but a significant effect on the modes involving tower deflection. The corresponding wet mode shape typically involves an out-of-phase motion between the floater and the tower, and the natural frequency predicted by the model increases compared to a floater resting on land. Another important finding for the prediction of fatigue damage is that the damping of tower vibrations can be significantly increased due to the coupling. Energy is transferred from the tower vibration mode to the near-rigid-body mode and dissipated by hydrodynamic damping. The main driving parameter for coupled vibrations is the ratio between the inertia of the turbine compared to the inertia and added mass of the floater. Coupling effects are stronger for heavy turbines mounted on light floaters and must therefore be considered carefully in future floating wind turbine concepts with increasing power ratings and optimized floaters. The limitations of the simplified model are discussed by comparing its predictions against a linear finite element analysis that captures deformation of the tower and against experimental results.