An experimental study on vortex-induced motions (VIM) of a deep-draft semi-submersible (DDS) was carried out in a towing tank, with the aim to investigate the VIM effects on the overall hydrodynamics of the structure. In order to study the fluid physics associated with VIM of the DDS, a comprehensive numerical simulation was conducted to examine the characteristics of vortex shedding processes and their interactions due to multiple cylindrical columns. The experimental measurements were obtained for horizontal plane motions including transverse, in-line and yaw motions as well as drag and lift forces on the structure. Spectral analysis was further carried out based on the recorded force time history. These data were subsequently used to validate the numerical model. Detailed numerical results on the vortex flow characteristics revealed that during the “lock-in”, the vortex shedding processes of the upstream columns enhance the vortex shedding processes of the downstream columns leading to the rapid increase of the magnitude of VIM. In addition to the experimental measurements, for the two uniform flow incidences (0° and 45°) investigated, comprehensive numerical data of the parametric study on the VIM characteristics at a wide range of current strength will also serve as quality benchmarks for future study and provide guidance for practical design

A numerical study on the flow over a deep-draft semi-submersible (DDS) for both stationary and vortex-induced motions (VIM) was carried out using the computational fluid dynamics (CFD), with the aim to investigate the overall hydrodynamics of the structure. In order to study the fluid physics associated with VIM, a comprehensive numerical simulation was conducted to examine the characteristics of vortex formations, shedding processes and especially their interactions due to the multiple cylindrical columns. In addition to the vortex shedding characteristics, the drag and lift forces on each member of the overall structure were calculated. It is revealed that under 45 degree incidence, the transverse forces induced by the portside and starboard side columns are the dominant excitation forces responsible to VIM while the horizontal member - pontoons restraining VIM. In addition, the hysteresis phenomenon observed between the force and motion domains - the peak lift force occurs slightly earlier than the peak transverse motion is mainly due to the vortices shed from the upstream column move back to impinge on one of the side columns after impinging on the other side column and the symmetrical strong vortices which shed from the side columns

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Vortex-Induced-Motions (VIM) is a cyclic rigid body motion that is induced by vortex shedding of a large sized floating structure (Spar, Semi-Submersible and Tension-Leg Platform) in deep water. Since the potential impact of VIM on fatigue life of mooring and riser systems can be critical, the study of the VIM phenomenon has drawn considerable attention among offshore engineering community. Despite the effort, there is still lack of understanding the complex fluid-structure interaction phenomenon. To date, it is very much relying on the simplified empirical approach in practical design. Most of the Semi-Submersibles consist of four rounded-cornered rectangular columns. To investigate the flow interaction between each columns can reveal some physics behind the VIM. In this study, flow past four stationary rounded-corner rectangular columns are tested in the circulating channel and numerically simulated by the Star-CCM+ CFD package. Forces on the columns and the flow characteristics behind the structures are present in this study. A 2-D particle imaging velocimetry (PIV) technic has been adopted in present study to obtain the flow characteristics. The aim of this study is to investigate the interactions between four square columns.