Numerical Study On Vortex Induced Yaw Motions Of Deep Draft Semi-Submersibles

As the production of ocean resources gradually extends to the deep sea,the semi-submersible platform has become one of the most popular platforms due to its excellent sport performance and economical advantage.However,the problem of vortex induced motions(VIM)in which the largescale reciprocating movement in the transverse and yaw directions will occur due to the alternating vortex shedding of the columns is also serious for semi-submersible.VIM will cause fatigue damage to the mooring system.At present,the research on VIM still focused on the transverse motions while the vortex induced yaw motions about multi-column floating platform has gain little attention.Therefore,in this paper a numerical study has been carried out on the vortex induced yaw motions of the four-column deep draft semi-submersible platform to further improve the theoretical mechanism of the vortex-induced motion of the multi-column floating platform.The numerical method used in this paper is the self-developed solver vim-FOAM-SJTU which is based on the open source platform Open FOAM.The solver includes a turbulence simulation module based on the SSTDDES method,a motion solution module based on the six-degree-offreedom theory of ocean engineering,a spring mooring simulation module,and the moving grid technology,which can handle complex large-separation flows and three-dimensional motion problems as well as accurately simulate the vortex-induced motion of offshore platforms.First of all,in order to figure out the vortex regime of each column at different current headings,a numerical study of the fixed turbulence characteristics of semi-submersible platforms at the current headings of 0and 45 degree is carried out.By comparing the moment results,it is found that the vortex mode of the platform at 0 current heading can provide initial excitation for the yaw motion,while the vortex mode of the 45 one does not meet the condition.Furthermore,numerical research on vortex-induced motion of semisubmersible platforms with the same current headings are conducted over a range of current velocities.By comparing the transverse motions and yaw motions,it is found that when the vortex frequency of each column is equal to the sway natural frequency and the phase of the vortex is consistent,the transverse motion response of platform is the largest.When the drag phases between the symmetrical columns along the flow axis are exactly opposite,which can be called the “transfer of the drag center”,the response of the yaw motion reaches the peak.Compared with 45 current heading,the platform of 0 current heading has a pair more of columns that are symmetrical along the flow direction.Therefore,the moment due to the “transfer of the drag center” is greater,and therefore the amplitude of the yaw motions is also larger.Finally,the effect of motion of different degrees of freedom(DOF)on the yaw motions for semi-submersible platforms are also studied.By comparing the results of three combinations of DOFs,it is found that within the present measured speed range,the effect of transverse motion on the yaw motion is dominant.