The Paired-Column Semi-Submersible platform(PC Semi) is a deep-draft semi-submersible (DDS) designed by Houston Offshore Engineering (HOE). It has 8 columns instead of 4 columns compared with conventional DDS. In this paper, the VIM characteristics of PC Semi was numerically investigated by our in-house CFD solver naoe-FOAM-SJTU. Drag test and free-decay tests was carried out first, then VIM tests at different reduced velocities were performed. Shear-stress transport based delayed detached-eddy simulation (SST-DDES) was used for modeling the massively separated turbulent flows. To illustrate the benefit of SST-DDES model, an extra unsteady Reynolds-Average Navier-Stokes (URANS) simulation was computed for drag test. Drag, flow fields and vortical structures were compared with those obtained by SST-DDES. For VIM tests, the dynamic overset grid technique was applied to handle the motions of platform. Results showed that the current CFD approach is applicable and reliable for VIM and can be an alternative for model testing.
The origins of semi-submersible floating production systems can date back to 1960s. Till today semis are widely installed in offshore oil and gas fields all over the world. In deep water oil drilling production environment, semis are often favored for their low dynamic response to waves in surge and sway. While for conventional semi, large wave induced heave motion is always a great challenge to the strength and fatigue of its riser and mooring system. To overcome this issue and reduce heave motion, one common improvement is to increase the draft of platform. Deep draft semi-submersible (DDS) is of course such an attractive solution and preferred by offshore industry. The deep draft hull forms can mitigate large heave motion. However, it introduces a new issue, the vortex-induced motions (VIM). VIM is actually the result of transverse fluctuating pressure caused by periodic vortex shedding on long vertical hull of platforms. The periodic sway motions will also accelerate the fatigue failure for riser and mooring system.