Roughness Effects on the VIM Response of Deep-Draft Semi-Submersible Platforms

摘要

Experiments on the Vortex-Induced Motions (VIM) acting on deepdraftSemi-Submersible (SS) platforms were carried out in a towingtank in order to investigate the roughness effects on the surface of twodifferent 1:100-scale models. Both models were built based on thetypical SS geometry with four columns and each model with the samearrangement of four pontoons in closed configuration. The models wereelastically supported by means of a set of linear springs, which alsoprovided low structural damping, and then submitted to a range oftowing velocities in the reduced velocity range from 4 up to 25covering the Reynolds numbers from 7,000 up to 80,000. Threedifferent levels of roughness were tested: 1) bare models; 2)intermediate level of roughness and 3) high level of roughness. For theSS with rounded square columns, no significant differences wereobserved in terms of the VIM response For the SS platform withcircular columns, some difference on the transverse and yaw motionswere observed. These results seem to confirm two important aspects:the important role played by the separation point near the roundedcorners of the rounded-square columns and the roughness effect on theboundary layers of the circular columns at the Reynolds numbers justbelow the critical regime. The results presented herein are part of amore complete campaign of experiments carried out within the VIMJIP which is run by MARIN and USP. Also within this campaign, theeffect of column design on VIM response was investigated using thesame models. Those results can be found in a complementary paper inthis conference.