Research On The Characteristics Of New Deep-Water Multi-Column FDPSO Vortex-Induced Motions

With the increase of offshore oil and gas exploitation, the Vortex-Induced Motion(VIM) problem of offshore platform columns has become a rising concern. Based on domestic and foreign research, theoretical analysis, numerical simulation and model test in wave basin are used in this paper to research the flow and VIM characteristics in different flow incoming angles and velocity and analysis the forces, movement amplitude, spectral characteristics and motion trace about a new deep-water multi-column FDPSO.First, the three-dimensional single-cylinder and variable section column numerical model are built based on the CFD software FLUENT, conducts a single column of three-dimensional of flow around numerical simulation under different flow velocity by detached eddy simulation method and analyses the flow field feature around the two cylinders, cylinder surface pressure distribution and the differences between lift and drag coefficients to simulate VIM combined with UDF on this basis. The study finds that at high Reynolds number flow around a single column, the flow field showed a strong three-dimensional features, lift and drag coefficients around a cylinder were greater than the flow of the variable section column; variable section column and cylinder appear a lock phenomenon when the reduced velocity is in the range of 6 to 9, at the same time, variable section column flow equilibrium position increases along with the increasing of reduced velocity; the motion trace of cylinder and variable section column appear typical "8" shape; variable section column wake mode isn’t obvious at low reduced velocity, it can be broadly summarized as 2P mode at high reduced velocity.Secondly, platform test model based on the scale of 1:80 is made to conduct model test at Shanghai Jiaotong University ocean basin and fully taking into account the impact of the mooring system for the platform VIM, breaking the limitation that experimental research related to VIM always made in towing tank. At the same time, FDPSO overall numerical models is established on the basis of the single column numerical simulation and carry out a three-dimensional flow around platform and the VIM numerical simulation. The flow around platform and vortex-induced motion characteristics in different flow directions and speeds are studied comparatively. It could be found that the vortices are mainly concentrated in two columns around the downstream, and the existence of complex interference phenomena; multiple peaks appear in the result of overall platform lift coefficient spectrum analysis, which reflects the complex three-dimensional characteristics; the flow approaching angle of 0° or 45°, the average drag coefficient and lift coefficient of platform are maintained in a relatively range; the frequency of sway movement of platform not appear typical "lock" phenomenon, but has remained in the range of frequencies close to the natural sway; at the same flow approaching angle, the equilibrium position and the maximum amplitude of longitudinal movement increases along with increasing of the reduced velocity; the motion trace of platform is not showing the typical "8" shape or "banana" shape, sway dominant position at lower flow rates, stream-wise oscillate at high flow rates has become couldn’t be ignored.Again, based on the model test, the characteristics of motion response and mooring line tension of the multi-column FDPSO are studied in this paper. The numerical simulation is carried on with the Sesam software from the DNV. The HydorD module is employed to analyse the hydrodynamic characteristics of the main body of the FDPSO. And the DeepC module is employed to get the coupled results of the body and the mooring system. Results of the numercal simulation are compared with those of model test of the free decay, white noise and the two combined environmental conditions of wind, waves and currents. The results show that in the working and survival condition, the platform’s heave, surge, sway and other motions are less than the maximum amplitude of allowables; numerical model is reliable and can predict the hydrodynamic characteristics of FDPSO; the motion response of the new FDPSO performs well, and mooring system layout is reasonable.Finally, the main work and research content are concluded, and developing trend for vortex-induced motion studies is put forward. Some suggestions of VIM are proposed in the end.