Investigation On Characteristics Of Flow Around The Deep-sea Floating Platform And Vortex-induced Motion At High Reynolds Numbers

The demand for oil and gas increases in the country with the development of economy in society.For satisfying this demand,the oil and gas explorations in the shallow water have been transferred to the deep water.The deep-sea floating platforms,such as Spar,Semi-submersible and Tension Leg Platfrom,have been applied widely in the oil and gas productions and explorations in the deep water.To ensure the operation safety of productions and explorations in the deep water,the design draft of platform has been increased.The increase of draft leads to the growth of the immersed area and height of the column,and hence stimulate the vortex-induced motion(VIM)of platform with large amplitudes in the horizontal plane.The occurance of VIM not only will shorten the fatigue life of mooring system,but also can impact the operation safety of platform due to the damage of mooring system.Therefore,it is of scientific and practical significance to investigate the characterisitics of flow around the deep-sea platform and its VIM.In view of the above situations,in this paper,the flow around single cylinder,two cylinders and TLP,and the characteristics of hydrodynamics are investigated by numerical simulations and experiments.The characteristics of VIM for the multi-column deep-sea platform are studied by simualtions.The theoretical model based on the coupled wake osillators is build for predicting the VIM responses of a floating platform.These studies in this paper can lay a foundation for the studies on the VIM characteristics and predicted methods for VIM in full scale.The main research contents are as follows:The flows around the single and two cylinders at high Reynolds numbers are investigated by numerical simulations based on the improved delay detached eddy simuations(IDDES).The flow patterns and hydrodynamic behaviors of the cylinders with different cross-section shape(ie.,circular cylinder,square cylinder and square cylinder with round corner).The flow around the two tandem cylinders in the turbulent separation is mainly analyzed.The hydrodynamic behaviors of the upstream and downstream cylinders and the flow interference between cylinders are discussed.The results show that the turbulent separation induced that the separation point on the upstream cylinder moves backward,and the reattachment point on the downstream cylinder moves foreward.At the supercritical Reynolds numbers,the drag and lift forces coefficients of the upstream cylinder keep a stable value with the increase of spacing ratio between the two cylinders,while those of the downstream cylinder jump at the critical spacing.It is also found that the fluctuating lift forces on the downstream cylinder are larger than those on the upstream cylinder.The experimental method is developed to measure the hydrodynamic loads on the deep-sea platform at high Reynolds numbers based on a high-speed cavitation water tunnel.The mean and fluctuating hydrodynamic coefficients of the TLP at high Reynolds numbers are obtained and analyzed.The change laws of the streamwise force,transverse force and yaw moment with Reynolds number and column height are discussed for different current incidence angles,and the correlations between forces and moments are presented.The flow pattern around the TLP and the forces on each column at high Reynolds numbers are also investigated by numerical simulation.The correlations between forces on each column and the platform are examined.The results show that comparing the results at the subcritical Reynolds numbers,at the supercritical Reynolds numbers,the fluctuating transverse forces and yaw moment coefficients of the TLP are less for all the current incidence angles.In addition,the fluctuating yaw moments are dominated by the fluctuating transverse forces,and are little affected by the fluctuating streamwise forces.Furthermore,the numerical results illustrated that for 0? current incidence,the fluctuating transverse forces on the downstream columns are larger than those on the upstream columns in different Reynolds number regimes.Based on the CFD method with IDDES,the numerical simulations on the VIM for the deep-sea multi-column platform are performed.The effects of grid parameters,time steps and iterations on the computed results are discussed.The applicability of different DES method to the simulations on the VIM is also examined.In addition,the transverse and yaw motion response amplitudes for the platform,motion trajectory curves and motion frequency distributions are studied for different current incidence angles and reduced velocities.The flow patterns and physical mechanism in VIM are investigated in the lock-in region.The results show that the computed results using the IDDES agree better with the experimental data,compared with those using the DDES.The maximum transverse motion response occurs for 22.5? current incidence,while the maximum yaw motion response appears for 0? current incidence.For different current incidence angle,the differences in VIM amplitudes,lock-in region and motion trajectories are presented,because of the various of separation patterns on the column and the interferences among the columns.In order to predict theoretically the VIM responses for the deep-sea floating platform with linear and non-linear stiffness,a phenomenological theoretical model coupled the inline and transvserse motion of a single column platform is build,based on the wake oscillators.The characteristics of VIM and hydrodynamic forces coefficients with non-linear stiffness is analyzed using the model.Moreover,the results with different aspect ratios of the column is also presented.The results show that the predicted VIM responses using the phenomenological theoretical model has a good agreement with the experimental data in the lock-in region.Compared with the results with linear stiffness,in the non-linear stiffness constraint,the in-line and transverse motion amplitudes are reduced,and this reduction amplifies with the increase of reduced velocity.In addition,in the non-linear stiffness constraint,the in-line and transverse force coefficients decrease,due to the increased added fluid mass.It is also observed that the non-linear stiffness in the transverse direction can mainly responsible for the reduction in in-line and transverse motion responses.