Research On Vnrtex-induced Motion Of Semi-submersible Platform Based On Overlapping Grid Method

It can be seen from the research that the traditional semi-submersible platform faces significant heave response in deep sea environment,which can seriously affect the efficiency and safety of drilling operations.In order to improve the vertical motion performance of the platform,one of the most effective approaches is to increase the draft of the platform.However,due to the augment of the draft and the existence of the deep water column structure,the amplitude of the Vortex Induced Motions(VIM)response becomes more obvious,which is receiving more attention.The VIM problem belongs to one of the international frontier basic science issues.Compared with the VIM of the Spar platform,the VIM of the deep-draft semi-submersible platform is more complex due to the mutual interference of the multi-column wake.First,both the commercial CFD software STAR-CCM+ and the self-developed unstructured grid Navier-Stokes(N-S)equation solver are used to simulate the flow around the cylinder.The results are in good agreement with the benchmark,which has verified the correctness of the technical route based on STAR-CCM+ software and the accuracy of the self-developed solver.On the basis of the numerical simulation of flow around a cylinder,the dynamic overlapping grid technique of STAR-CCM+ is adopted to simulate the lateral forced vibration of the two-dimensional cylinder.In the calculation,by selecting different frequency ratios and dimensionless amplitudes,changes of the vortex shedding mode from 2S to 2P are accurately captured,and the influence of the cylinder vibration on vortex shedding is also analyzed.Second,in order to simplify the mooring system into the mass-spring system,this paper presents a modified spring stiffness method which combines the theoretical formula with the free attenuation numerical test.The results show that the modified spring stiffness can provide the accurate natural frequency for the vibration system,which validates the feasibility of the method.On the basis of the modified method,the two-dimensional cylinder VIM numerical simulation is carried out.The track of the cylinder VIM is consistent with the classical "8" shape.The effect and the mechanism of the locking phenomenon are clarified,and the influence of pre-tension on the motion trajectory is discussed.In order to compare the characteristics of the VIM response of the 2-dimensional cylinder with the square column(e.g.motion trajectory),the flow around square column and the VIM are simulated under the low Reynolds number(Re=150,200 and250).The results show that trajectory shapes of the two types of structures are roughly the same,but the VIM disturbance degree of the square column is obviously higher than that of the cylinder.In addition,the VIM of square column of the inlet flow angle of 0° and 45° are analyzed and discussed.The results indicate that the disturbance degree of the working condition of 45° is significantly higher than that of 0°.However,when the vortex excitation locking occurs,the disturbance degree of VIM is remarkably reduced,the vortex shedding mode changes from P+S to 2S.Finally,according to the deep-draft semi-submersible platform model towing test,the numerical simulation of the static flow around a single square column and VIM is carried out with different reduced velocity.The results are similar to that in laminar flow,but with increased disturbance.On the basis of the above study,the VIM of 2D simplified semi-submersible platform model with different inlet angle and reduced velocity are analyzed in RANS method.The results show that the variation trend of the VIM amplitude at low reduced velocity is consistent with the model test.The VIM of3 D semi-submersible platform are analyzed in DES method.The results are basicaly in good agreement with the model test.