This paper 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 compared to a base case with no outer 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 base case with no outer cylinder. Increasing the porosity of the outer cylinder also reduces the amplitude of the resonant response. The use of a porous outer layer increases the quadratic drag on the body and significantly reduces the low frequency resonant response where the radiation damping is low.
A key challenge for developing cost-competitive floating offshore wind is the e cient design of stable platforms. Large platform motions can 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.
Porous structures are commonly used in fixed and floating breakwaters to dissipate wave energy and reduce wave disturbance (e.g. Huang et al, 2011; Dai et al, 2018). The use of porous structures has also been investigated for motion damping and load reduction on fixed offshore structures (e.g. Molin, 1990; Molin, 2011; Park et al, 2014) and floating structures (e.g. Downie et al, 2000a,b; Williams et al, 2000; Lee & Kerr, 2002; Park et al, 2013; Vijay & Sahoo, 2018).