Research On Turbulence Flow And Its Development Around A Circular Cylinder And Vortex Induced Vibration Of Marine Riser

In the oil and gas industry where the hydrocarbon drilling exploration and floating production activities move progressively towards deepwater areas with depths greater than1000meter or beyond, vortex induced vibration (VIV) of such key structures as drilling/production risers have become the subject of increasingly intense research investigation. In China, the key of ocean exploitation has turned into the South China Sea. Riser is the necessary device to connect the floater with oil well. Nevertheless, the prediction of deepwater riser VIV is very challenging owing to the fact that the incident flows are practically non-uniform and the associated fluid-structure interaction phenomena are highly complex. These result in a nonlinear coupled system which depends on several physical and mechanical parameters. Large amplitude oscillations occur when the vortex shedding and the structural vibration frequencies coincide, a condition referred to as ’lock-in’. The lock-in condition can occur over a range of oncoming flow velocities and the vortex shedding frequency can be driven relatively far from the Strouhal frequency which leads to increase in hydrodynamic loading and reduction in service life due to fatigue.This study contains three parts:the first one is investigation of scale effect for turbulence flow around a circular cylinder and development of turbulence flow around circular cylinder; the second one is numerical pridiction of vortex induced vibration of elastically supported rigid circular cylinder with low mass damping in a fluid flow and the last one is vortex induced dynamic response of marine riser under true ocean environment.The structures of this thesis are listed as follows:By analysis the state of art of VIV, the engineering sense of present study is proofed. Conclusions are made of current research status from the aspects of experimental fluid dynamics, computational fluid dynamics and empirical method. The basic theory of VIV and associated parameters are introduced.Numerical computation of flow parameters around a two-dimensional circular cylinder within Reynolds numbers range from103to107is accomplished. Velocity field, vorticity field, the fluctuation of turbulence flow and its development downstreams are discussed and comparisons of characteristics of turbulence flow at different scales are also made. In order to investigate the scale effect of turbulent flow around a circular cylinder, two items of simlarity criterion based on turbulent kinetic and turbulent dissipation rate which are associated with the fluctuation characteristics of turbulence wake are deduced by the equation analysis of Reynolds-Averaged Navier-Stokes equations (RANS). The result indicates that the fluctuations of turbulence flow along the center line in the wake of circular cylinder can never be changed with the increasing Reynolds numbers when Re≥3×106. Then characteristics of turbulence field when Re≥3×106are discussed specially and laws of fluctuations of turbulence downstreams are given.RNG k-ε and SST k-co turbulence models are adopted to predict transverse vortex induced vibration of elastically supported rigid cylinder with low mass damping in a fluid flow. By comparing the peak amplitude, response frequency and hydrodynamic coefficients as well as wake modes of three different response branches at two different turbulence models, analysis of differences between two turbulence models are presented. Results indicate there exisit significant differences between two turbulence models, and SST k-co model is better than RNG k-ε model on the whole. By comparing turbulence fields and developments of three different response branches with static cylinder at coequal Reynolds number, some differences are observed. Pressure coefficient on the wall, shear stress coefficient on the wall, mean streamwise velocity, streamwise velocity fluctuation, cross-flow velocity fluctuation and Reynolds shear stress of three branches are different from each other and from those of static cylinder.An investigation on the nonlinear dynamic response and vortex-induced vibration of marine riser subjected to waves and currents is performed. Three dimensional governing equations which considered nonlinearities including the geometrical nonlinearities, fluid dynamic and fluid-structure coupling nonlinearities are given. The in-line force is solved by Morison equation under combined waves, currents and platform movement while cross-flow force is solved by wake oscillator model which considered variation of added mass coefficient. The nonlinear governing equations are solved by Updated Lagrangian method(UL) and Newton-Raphson interation method in time domain. The influence of basic parameters on the dynamic response and vortex-induced vibration of marine riser were investigated. This research provides a basic foundation for practical design and theoretical analysis of marine riser. Since the vortex shedding frequency varies with flow velocity, a depth varying flow past a flexible cylinder will result in multi-frequency excitation. Depending on the range of velocity covered by the flow profile, many vibration modes of the structure can be excited. Understanding the vibration mechanisms of the structure in those cases is not a simple task. Based on this model, multi-mode excitation of riser pipeline in linear sheared flow, the effect of shear parameter on vortex induced vibration and dynamic response of riser pipeline are discussed, and the different responses in uniform and linearly sheared currents are compared. The results indicate that shear parameter has a significant effect on vortex induced vibration and dynamic response of riser pipeline. The multi-mode response of riser pipeline in linearly sheared flow differs from that in uniform current.