CFD Study of Deep Draft SemiSubmersible VIM

In deepwater oil and gas development, a stable floating platform is critical to the safe and reliable operation of the topside production/drilling facilities. The motion characteristics of the platform are a significant factor in the service life of the mooring system and risers connected to the platform. Among different floating platform configurations, deep-draft semisubmersible (DDS) concepts are known to exhibit better vertical motion characteristics in wave environments than conventional semisubmersibles because of the smaller wave exciting forces on the pontoon structure. In a DDS design, vortex-induced motion (VIM) stemming from vortex-induced lift force remains one of the challenges in offshore engineering. Compared to a single column floating platform such as a spar, a semisubmersible attracts a more complex VIM phenomenon due to its multi-column configuration. Because of the elongated submerged column and enlarged projected area to the current, both in-line drag forces and transverse vortex-induced lift forces are larger, resulting in increased in-line and transverse horizontal motions in a current environment. In this study, an overset grid Computational Fluid Dynamics (CFD) approach is adopted on DDS to facilitate the computation of an arbitrary large VIM displacement. Due to the high grid resolution requirement for vortex shedding flow simulation, the best practice O-type grid topology with grid points located up to the near wall boundary sub-layer level is used around the semisubmersible hull. The results show the calculated drag forces are in good agreement with measurement data. To further consider the deep draft semi-submersible VIM, an appropriate spring-type constraint is added based on a real application case of a mooring line system. Systematic CFD simulations show there are complex phenomena for VIM responses due to different inflow current speed and heading angle relative to the DDS. In addition to the common large VIM lock-in motion at the DDS natural sway frequency, simulations also show a combined motion of galloping and VIM that manifests as transverse motion beyond the natural sway frequency range. Spectra analysis also is performed for the VIM time history to provide more comprehensive information, which is useful for the fatigue life estimate for mooring lines and risers.