Numerical Study Of Column Rounded Corner Effects On Vortex-Induced Motions Of Semi-Submersibles

As the increase of the depth of offshore oil and gas exploitation,The traditional fixed platform is difficult to meet the actual requirements of the project due to the constraints of its construction conditions.Under this background,the concept of floating platform came up,semi-submersible platforms have been widely used in the field of ocean engineering because of their strong loading capacity.Studies have shown that the heave motion performance can be improved by increasing the platform draft.But it leads to another problem,which is,compared with traditional semi-submersible,the Vortex-Induced Motions(VIM)response increases significantly.The VIM of semi-submersible platform is caused by periodic fluctuating pressure of column caused by alternating vortex shedding,the interaction of vortex shedding between the multiple columns makes the VIMs of the semi-submersible platforms more complex than those of the single-column floating platforms.Therefore,columns are the most important structure that affects the response characteristics of the VIM of semi-submersible platform.At present,most of the columns are square section with corner rounding,but the studies on the effect of the column rounded corner on the response of the platform are still very few.The study on the effect of column rounded corner on the VIMs of semi-submersibles platform has important guiding significance for the design of platforms.In this thesis,numerical simulation have been carried out by an in-house CFD code vim-FOAM-SJTU,which is developed on the opensource platfom OpenFOAM.Firstly,numerical simulations of flow past and VIM of a semi-submersible platform with square columns are carried out under two current headings.The rationality of grid size and time step is proved by convergence verification,and the reliability of the solver and the numerical method is verified by comparing with the experimental results.On this basis,three other semi-submersible platforms with different column corner radius are selected to carry out numerical simulation of flow past them.The circumferential distribution of pressure coefficient on the surface of column,the fluid force of column and platform and the frequency of vortex shedding have been studied to summarize rules of the effect of corner radius,the average flow velocity near the wake of column and vorticity images are shown to study the effect on flow field.Three other semi-submersible platforms with different column corner radius were simulated.The motion response,trajectory and fluid forces are presented to summarize the rules of column rounded corner effect on VIMs of semi-submersibles.Selecting the typical reduced velocity,the detailed flow field have been studied to explain the mechanism of the effect of column corner radius on the VIM of semi-submersible platform.The results show that the change of column corner radius will affect the position of flow separation point,thus affecting the width and length of the recirculation region at the rear of the column,resulting in the change of pressure distribution of flow field and the platform surface,the interaction of vortices shedding between the columns are also been influenced,resulting in the difference of hydrodynamic characteristics of the platform,and thus affecting the motion of the platform.VIM is a complex fluid-structure coupling phenomenon,and different displacement responses of the platform will have different effects on the flow field.