CFD Investigation on Scale and Damping Effects for Vortex Induced Motions of a Semi-Submersible Floater

Available field measurements of semisubmersibles indicate that the Vortex Induced Motion (VIM) response is typically found to be smaller than what is predicted in model tests. Reductions of as much as 50% seem to be occurring in the field compared to the response found in model tests. Several variations in the model test setup have been made in the past to achieve a better agreement between field observations and model test results, but no conclusive results have been obtained so far to explain the reason for those differences.MARIN and the University of Sao Paulo (USP) initiated the VIM JIP with the primary objective to enhance the understanding of possible differences between field measurements and model test observations. In this JIP the effects of waves, current inflow conditions, external damping, mass ratios and Reynolds number are investigated using CFD and model tests. Some of these effects are examined using CFD, some experimentally and some by both CFD and model tests.In this paper the CFD results are presented to examine the effects due to Reynolds number and external damping. The basic step of the JIP is the comparison of the model-scale CFD results to experimental results. Extensive verification steps are undertaken to improve the confidence in both model-scale and full-scale CFD results. The model-scale CFD results show VIM response very similar to behavior observed in model tests. Then, the CFD model is applied to full-scale Reynolds number to investigate the effects of Reynolds number on the VIM response. Furthermore, the effects of external damping are examined as well, both at model-scale and full-scale Reynolds number.The CFD simulations at full-scale Reynolds number presented in this paper show a shift in the response curve to lower reduced velocities, but no significant reduction in normalized VIM amplitude is observed. Thus, Reynolds scale effects seem not to be the main reason for the difference between model tests and field observations. However, a significant reduction of VIM amplitude is observed when increasing the external damping in the CFD calculations.According to these observations the importance of external damping on the VIM response is identified. Further investigation is recommended to assess the damping from riser and mooring configurations which occurs at prototype semisubmersibles, so that the correct VIM response can be considered in the design phase.

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