Towing Simulations of a Semi-Submersible Floating Wind Platform Using RANS-based CFD

Abstract Two RANS-based CFD approaches were assessed for predicting calm-water towing resistance of the VolturnUS 1:8 semi-submersible floating wind platform. Simulations were performed in FINE™/Marine (ISIS-CFD) using a rigid-lid (RL) single-phase formulation and a free-surface-resolving (FSR) two-phase VOF formulation with quasi-static heave and pitch and prescribed surge. Grid-refinement studies with six near-geometrically similar meshes were conducted with Spalart–Allmaras and SST k – ω models. Pressure resistance contributed 98–99% of the total, confirming bluff-body behaviour. Turbulence-model sensitivity was modest (≤2.5% for RL and ≤0.9% for FSR on the finest meshes). SA provided resistance close to SST while yielding lower discretisation uncertainty, whereas SST exhibited large residuals for the specific dissipation rate. Additional towing-speed cases were compared with published sea-trial data and a DNV-RP-C205 estimate: RL underpredicted resistance, FSR reduced the bias and remained slightly conservative, and DNV-RP-C205 overpredicted even without modelling wave-making. CFD and DNV-RP-C205 followed an approximately quadratic speed trend, whereas the sea-trial data were non-monotonic, suggesting unmodelled external disturbances. Overall, the results support the FSR approach as a practical basis for routine engineering towing-force assessment.