Motion damping of floating offshore wind turbines using porous materials

Motion damping of a TLP floating offshore wind turbine using porous materials

Ed Mackay – University of Exeter

A key challenge for developing cost-competitive floating offshore wind is the efficient design of stable platforms. Large platform motions lead to reduced energy yield and increased fatigue loads on the turbine. Adding a porous outer layer to a floating platform has the potential to reduce platform motions without significant increase in size and cost. This poster presents the results of scaled model tests of a tension leg platform (TLP) for a floating wind turbine, comprising a central solid cylinder with a porous outer cylinder. Tests were conducted with outer cylinders with porosities of 0%, 15% and 30% and are compared to a base case with noouter cylinder. For each configuration, the total mass and centre of mass are kept constant to allow consistent comparison. It is shown that for the cases with a solid outer cylinder the surge motion resonance is shifted to a lower frequency due to the entrained mass of water inside and increased added mass of the outer cylinder. Increasing the porosity of the outer cylinder is shown to increase the frequency of the resonant response, bringing the resonant frequency closer to that of the basecase with no outer cylinder. Increasing the porosity of the outer cylinder also reduces the amplitude of the resonant response, with a 20% reduction for the 15% porosity case and a 40% reduction forthe 30% porosity case. The scaled test results are compared to predictions from an iterative boundary element method (BEM) model and shown to give good agreement. The numerical model indicates that the inclusion of the porous outer layer increases the excitation forces at lower frequencies compared to the basecase. However, the porous outer cylinder significantly increases the damping at low frequencies, where the radiation damping is low, leading to a lower motion response.