Vortex Induced Motion of a Dry Tree Semisubmersible

The main challenge in the hydrodynamic design of a dry tree semisubmersible is in limiting its motion responses particularly heave motions to enable the use of riser tensioners. Deep draft semisubmersibles have low heave motions but are more susceptible to vortex induced motions (VIM) due to high slenderness ratios of the columns. A novel in-house developed semisubmersible design named the Heave and VIM Suppressed (HVS) semisubmersible has been designed to possess low VIM and low heave responses required for dry tree applications. A case study of the feasibility of a dry tree HVS semisubmersible in South East Asian environment has been published separately [1]. This paper presents the VIM performance of the same hull, estimated using model testing and Computational Fluid Dynamics (CFD) analysis. From the model tests, VIM suppression is observed in the HVS semisubmersible due to the presence of the column steps. CFD simulations of the model tests show results comparable to the measured data for the HVS semisubmersible. Additional CFD analysis is performed to account for the external damping effect of the mooring lines and risers on the VIM performance of the HVS semisubmersible. This paper together with the earlier publication [1] shows the robustness of the HVS semisubmersible design concept in addressing both the heave and VIM issues in semisubmersibles for dry tree applications.