CFD Study of Fully Coupled Mooring and Riser Effects on Vortex-Induced Motion of Semi-Submersible

Vortex Induced Motion (VIM) of column-stabilized floaters has created substantial design and operational challenges to the mooring and risers. VIM design criteria are typically obtained from a towing tank model test. But in the model test, moorings and risers are not physically modeled and the lateral stiffness from moorings and risers is modeled simply with linear springs. Recent field measurements of semi-submersibles and truss spars indicate that the overly-simplified mooring and riser setup in model test could lead to overly-conservative VIM design criteria. In this study, CFD simulations were employed to investigate the effects of actual mooring and riser configurations on VIM response of deep draft semi, such as asymmetric restoring and hydrodynamic damping. Descriptions of the CFD and mooring and riser modeling and comparisons of the VIM response with the simplified setups are presented, along with a discussion on the implications of the mooring and riser simplification in the model test are presented.