CFD Study of Deep Draft SemiSubmersible VIM

Abstract

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.

Hydrodynamics Around a Deep-Draft Semi-Submersible With Various Corner Shapes

“…Most of the corner shape studies were focused on the 0 degree incidence. However, for square section shaped multi-columns structures, the most severe transverse motion which was induced by vortex shedding occurred at 45 degree incidence which was confirmed in both filed measurements and model tests [6][7][8][9][10][11][12]. Thus, the present numerical investigation about corner shape effects is focused on 45 degree incidence.…”

Numerical Simulation on a Deep-Draft Semi-Submersible

“…In addition to that, the three-dimensional effects from a span wise-cross flow direction could also be eliminated with these computational domain settings. This was shown by the computational domain settings used in Lee et al (2014), which were 6Bl × 4.5Bl × 2.8Bt and 5Bl × 4Bl × 2.2Bt. Settings that were considered large enough to do the elimination were 27Bl × 18Bl × 6.5Bt and 11Bl ×6Bl × 3Bt, respectively.…”

Computational Fluid Dynamics Analysis of Rigs-to-Reefs (R2R) Jacket Structures

“…The main characteristic of the measured pressure distributions is that the pressure at the rear side of the cylinder (wake region) is always negative. This is due to the separation of the boundary layer and reverse flow region [6]. The negative values cause downward lift coefficient of downforce, which results in a negative value as mentioned by Dandan et al, [7].…”