Experimental Study on the Vortex-Induced Motions (VIM) of a Semi-Submersible Floater in Waves

Abstract

Recent studies suggest that the vortex-induced motions (VIM) of a semi-submersible found in model tests over-predicts the response in the field, which may lead to an over conservative design of the mooring and riser system. Within the Vortex Induced Motion Joint Industry Project (VIM JIP), run by MARIN and University of São Paulo (USP), possible reasons for this over-prediction are investigated using model tests and CFD [1–6]. A model test campaign was carried out at MARIN to test different candidates that might explain the observed differences. The results obtained with an air bearing setup regarding damping, mass ratio, draft variations and sinusoidal tow velocities, have been published elsewhere [6]. The present publication focuses on the influence of waves on the VIM response. The model was a generic bare hull semi submersible with four rounded square columns at scale 56.5. A simplified mooring system consisting of four springs was designed to match the desired natural period in the sway and yaw direction. The model was towed at different velocities, corresponding to the range of reduced velocities where the highest response is expected. A VIM tow test campaign was carried out in calm water as a benchmark. The model was then tested at 7 different wave-current conditions, and the results are compared with the benchmark case. The results suggest that two factors are important for VIM response in waves: the wave height, and the relation between wave and current direction. Comparing to calm water conditions, a reduction of 15% on the peak nominal response was found for a smaller sea state (HS = 2m, TP = 10s), however with a higher significant wave height (HS = 4m, TP = 10s) the peak nominal response was reduced by 30%. Depending on the combination of current-wave direction, the influence of the same sea state (HS = 4m, TP = 10s) on VIM response can be negligible (transverse seas) or result in a 30% reduction of the peak nominal response for collinear sea and current. This is a relevant finding since most research on the topic has focused on collinear conditions, and VIM tests in waves with transverse or oblique conditions are rare [7–9]. Comparing the calm water VIM response obtained with the air bearing setup, published in [6], with the soft mooring configuration reported here, the latter shows a generally smaller response, with a narrower lock in region. Nevertheless the peak response is found to be similar for both experimental setups.