When an offshore platform is exposed to a steady current, alternating vortices are shed from its aft at a regular period, creating oscillatory hydrodynamic forces. Under these forces, the platform experiences vortex-induced motions (VIM) close to its natural period. VIM still poses a design challenge for new deep-water platform moorings and risers. VIM imparts cyclic fatigue loads on the riser’s hang off and touchdown points and on the mooring chains — and an accurate prediction of VIM is important in estimating and allocating fatigue damage budgets to the risers and mooring chains.

High-performance computing numerical simulation tools are used to simulate the VIM response of a 1:50 model scale semisubmersible with round columns. Computational fluid dynamics (CFD) coupled to a structural model is used to predict the VIM response for a range of flow conditions and headings, including wave action. The simulation results are compared side-by-side to the measured response from model scale physical measurements and good agreement instills confidence towards full-scale simulations. The simulation results confirm that inclusion of wave action, in the form of a background sea state, results in diminished VIM.

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