Examining the Effects of Scale, Mass Ratios and Column Shapes on the Vortex Induced Motion Response of a Semisubmersible Through CFD Analyses

The phenomenon of vortex induced motions (VIM) of a semisubmersible has been observed in the field and in model tests. The VIM response can be onerous to the fatigue live of SCR’s that are supported from the semisubmersible. Field observations indicate significantly smaller VIM amplitude than what present day model tests predict. The Scruton number is a parameter which affects the magnitude of the VIM response; the Scruton number includes the effect of mass ratio and damping. The effect of damping on VIM response has been examined in model test scale, and this analysis indicated that damping has a minimal effect on the VIM response amplitudes. The effect of the mass ratio and the effect of performing physical experiments at model scale are examined through CFD analyses. The objectives of the CFD analyses are to focus on the VIM phenomenon itself and to compare response magnitudes, while giving less importance to semisubmersible hull details and absolute response magnitudes. Hence, most of the CFD work is done in a 2D environment. Responses for various column cross-sectional shapes are examined; the column cross-sectional shapes include square, rectangular and five-sided. The VIM response is the result of pressures acting on the semisubmersible columns. The CFD analyses provide means to obtain the pressures at various locations at the various columns, and therefore can be used as a way to describe the phenomenon. These pressure data sets are evaluated to show where the majority of the excitation occurs and phase relationships between excitation forces acting on each individual column are presented.