Investigation of the influence of sinusoidal internal waves on a SPAR buoy structure

Offshore wind has a great potential as a competitive source of renewable energy, especially in deep waters where wind speeds are more consistent and fewer restrictions apply for running large wind turbines. Previous analyses of Floating Offshore Wind Turbines (FOWTs) mostly considered obvious sources of loading: surface waves, currents, wind and mooring. However, in some deep-water locations, internal waves can occur and these have been shown to significantly affect floating structures. Since the hydrodynamic response of an FOWT governs the structure's general stability, the aim of this research is to investigate the impact of sinusoidal internal waves on the platform motion of a free-floating SPAR-type cylinder. A potential flow model and Morison's equation are applied numerically to calculate the forces acting on a free-floating cylinder in an oscillating flow. The commercial Finite Element Analysis software OrcaFlex is verified by the potential flow model of the oscillating flow and is then used to mimic sinusoidal internal waves acting on a free-floating cylinder in a stratified flow. Three different internal wave amplitudes and peak velocities are analysed, and the nine resulting cases are investigated for the oscillating and stratified flow each. It has been found that the pitch rotations of the SPAR cylinder were small (< 0.1°) in all cases and can likely be disregarded. The surge displacements of the free-floating cylinder were substantial in both oscillating and stratified flows, with maximum surge magnitudes of 423m and 120m, respectively. Therefore, significant additional mooring loads due to internal waves could be sustained by SPAR-type FOWTs.