Vortex-Induced Motions (VIM) is a key issue for deep draft column stabilized floaters (DDCSF). An example of DDCSF is the concept design called Paired-Column semi-submersible (PC-Semi) with a pair of columns instead of single column at four corners of the platforms. The complex multi-column design involves several extra design parameters such as paired-column gaps and column cross-section areas than conventional semi-submersibles. These parameters can be tuned to mitigate dynamic response to waves and currents. However, it is expensive to perform parametric study of geometric variations by means of experiments.
In the present work, CFD method is utilized to investigate the VIM characteristics for different geometrical variations of the PC-Semi. An in-house CFD code naoe-FOAM-SJTU, which is developed on top of the open source framework OpenFOAM, is used for all numerical simulations. Delayed detached-eddy simulation (DDES) is used for turbulence modeling with massively separated flow. A geometry model scaled at 1:54 from MARIN is select as the baseline model for parametric study. VIM simulations of the baseline model is firstly carried out and compared with experiments to validate the current CFD code. After that, VIM characteristics of paired-column gaps geometrical variations are numerically investigated. Motions of the platform and hydrodynamic forces on columns are compared and analyzed. The ability of CFD method in optimizing geometric design parameters on mitigating VIM response for deep-draft semi-submersibles is demonstrated.