Hydrodynamic analysis and validation of the floating DeepCwind semi-submersible under 3-h irregular wave with the HOS and CFD coupling method

Abstract

Recent studies, such as the Offshore Code Comparison, Collaboration, Continued, with Correlation (OC5) led by NREL, have identified significant under-prediction of low-frequency pitch and surge responses in the DeepCwind semi-submersible platform when comparing model test results with predictions from engineering tools like OpenFAST, which rely on potential flow theory assumptions for hydrodynamics. To address this mismatch, a newer experimental campaign was conducted as part of the OC6 Phase 1A study. In this work, a coupling numerical approach was employed to replicate various scenarios of the floating DeepCwind semi-submersible, including decay, regular, and 3-h irregular waves, which were explored during the OC6 Phase 1A model testing. The numerical technique involved utilizing far-field nonlinear waves simulated using the High-Order Spectral (HOS) method and applying them to a near-field Computational Fluid Dynamics (CFD) domain. This allowed for the solution of wave-structure interactions using the overset mesh capabilities of StarCCM+. The critical hydrodynamic responses of the freely floating DeepCwind semi-submersible were modeled and validated against the model test results. The results obtained from the coupling approach were then presented and compared with simulations using OpenFAST and results from other researchers. The coupling strategy demonstrated improved agreement, especially for low-frequency responses in surge and pitch motion, under the 3-h irregular wave conditions, compared to OpenFAST simulations. While the validation investigation showed overall success in capturing low-frequency and wave-frequency surge and heave motion, there was an underestimation of low-frequency pitch motion in the TCOMS coupling tool. This underprediction can be attributed, at least in part, to the effect of wave overtopping on the rear columns, which resulted in deviations from the experimental pitch motion, particularly during extreme events.