Abstract
Second-order potential flow models were found to under-predict the low-frequency surge and pitch responses of the floating DeepCwind semi-submersible platform as concluded from the OC6 Phase 1A study. Subsequently, a component level approach was undertaken to examine the hydrodynamic loads on the individual parts of the semi-submersible in isolation using bichromatic wave in the OC6 Phase 1B study. Therefore, a direct assessment of the difference frequency wave loading could be obtained which further enabled numerical validation using the experimental dataset arising from the OC6 Phase 1B study. In this paper, a numerical approach based on Computational Fluid Dynamics (CFD) using StarCCM+ was used to simulate some of the OC6 Phase 1B experimental test cases. Fixed platform scenarios were simulated and compared with the experimental results in terms of surge force and pitch moment. Relatively good agreement could be achieved using the numerical model. Sensitivity studies such as the choice of laminar or turbulence flow model and domain length were carried out to establish robustness in the simulated results. The CFD results established the importance of considering up to 3rd order wave force/moment and viscous effects. The same semi-submersible platform was then made to be freely-floating with soft mooring, and additional bichromatic cases were simulated to analyse and compare the resultant low-frequency and wave-induced motion responses. Significant responses at the difference frequency could be excited due to non-linear interactions of the two wave frequency components.
