The vortex-induced motion (VIM) of deep draft semi-submersibles (DDS) could have critical impact on fatigue life of mooring and riser systems. In this paper, VIM of a DDS at different current incident angles is simulated using Shear Stress Transport (SST) based delayed detached-eddy simulation (DDES) model. The numerical simulations are carried out by the in-house CFD solver naoe-FOAM-SJTU which is developed on the open source platform OpenFOAM. The nominal response of sway motion is presented and compared with experimental data. For different current incident angles, time histories, trajectories and vorticity of the DDS at different reduced velocities are reported. The result shows our CFD solver naoe-FOAM-SJTU is applicable and reliable to study VIM of semi-submersibles.
Deep draft semi-submersibles (DDS) have become a trend in semi-submersible design due to its favorable heave motion characteristics compared to conventional semi-submersibles. As the draft increases, both in-line drag forces and transverse lift forces of a DDS become higher and the vortex-induced motion (VIM) of a DDS has emerged as an important issue. VIM is a complex fluid-structure interaction phenomenon, where vortex shedding occurs on a fixed or floating structure subjected in current, resulting in alternating cross-flow forces on the structure. If the vortex-shedding frequency coincides the platform's natural frequency in calm water, the lock-in phenomenon may occur, resulting in large sway motion amplitudes of the platform. This can have significant effect on the fatigue life of mooring system and risers connected to platform. Thus, it is essential to investigate the VIM of offshore floating structures, such as spar and semi-submersible.
Up to now, some researchers have studied the VIM of different types of semi-submersibles. Previous investigations were mainly carried out by model tests, which were performed in towing tanks. The models in the experiments were generally free to surge, sway and yaw motion. Waals et al. (2007) discussed the effect of mass ratio and draft on VIM of semi-submersible platform, and found that lower mass ratio for semi-submersible may result in larger sway response. The semi-submersible with smaller column height showed much less flow induced transverse and yaw response than that with larger column height. To systematically study the VIM of a semi-submersible platform, a series of model tests were carried out at the Institute of Technological Research (IPT) by Gonçalves et al. (2012, 2013). Effects of current incidence angles, hull appendages, surface waves, external damping and draft conditions were discussed. They found that lock-in phenomenon could occur for reduced velocity (Ur) ranging from 4 to 14 at different current incidence angles. The effect of appurtenances, tow direction and wave action was investigated by Martin and Rijken (2012). They found that semi-submersibles can also exhibit vortex- induced yaw (VIY) response which may be due to shed vortices from upstream columns interacting with downstream columns. The complexity of experimental settings can lead to time consuming to study VIM of a semi-submersibles. Generally, model tests are always based on Froude scaling, which will result in much smaller Reynolds numbers at model scale than those at full scale. In addition, there are limitations in obtaining physical understanding of flow field around semi-submersibles in model tests.
