The vortex-induced motions (VIM) of semi-submersibles depends on the shape of the submerged structure, small changes in column geometries may have a significant impact on the VIM response of the platform. The study on the effect of column rounded ratios on the VIM of semi-submersible platforms has important guiding significance for the design of platforms. In the present study, numerical simulations of the VIM characteristics of a semi-submersible platform with different column comer radius at 45° current heading have been carried out by naoe-FOAM-SJTU. SST-DDES was used for modeling the massively separated turbulent flows. The displacements and hydrodynamic forces information in three DOFs including sway, surge and yaw motion are obtained. Based on the advantages of CFD method, the mechanism of the influence of the column rounded comer on the VIM of the semi-submersible platform is studied. Vortex shedding patterns and other flow field information are given for further analysis.
As the increase of the depth of offshore oil and gas exploitation, semi-submersible platforms have been widely used in the field of ocean engineering because of their strong loading capacity. The heave motion performance is improved by increasing the platform draft. However, the Vortex-Induced Motions (VIM) response increases significantly because the increase of the effective excitation length of columns will lead to higher fluctuating pressure caused by the vortex shedding. As a kind of multi-column floating platform, the interaction of vortex shedding between the multiple columns makes the VIMs of the semi-submersible platforms more complex than those of the single-column floating platforms such as Spars. The VIM not only affect the operation of platforms, but also severely reduce the fatigue lives of risers and mooring systems. (Volnei et al, 2009)
Experimental works on the VIM of semi-submersible platforms were carried out in recent years. (Waals et al, 2007; Magee et al, 2011; Goncalves et al, 2012; Liu et al, 2016). With the deep development of studies, many researchers transferred focus on the geometric parameters of the platforms effect on the VIM of the semi-submersible platforms. Studies have shown that the VIM response depends on the shape of the submerged structure and the columns have the most significant impact. (Gonçalves et al, 2015) At present, most of the columns of the semisubmersible platforms are square section with corner rounding, but the studies on the effect of the column rounded corner on the response of the platform are still very few. The study on the effect of column rounded corner on the VIMs of semi-submersibles platform has important guiding significance for the design of platforms.