A Study On Numerical And Experimetal Investigation Into Vortex-Induced Vibration Of Marine Risers

Risers are widely used in the offshore industry to convey fluids, such as oil and gas, from the seabed to the platform. They are slender structures exposed to complex ocean conditions. As the fluid passes risers, the well-known vortex shedding is observed, resulting in the fluctuating forces on the structures and finally inducing the vibrations of the structures in both cross-flow and in-line directions. This phenomenon is commonly called as vortex-induced vibration (VIV). When the natural frequency of the riser is close to the vortex shedding frequency, large amplitude oscillations is observed in both directions. This phenomenon is usually called lock-in. Vortex-induced vibration is one of the most important factors accounted for the fatigue damage of risers in deep water. The dynamic effects of long flexible risers under vortex-induced vibration become of increasing concern.In order to investigate the physical mechanism of vortex-induced vibration, a Computational Fluid Dynamics (CFD) model based on Navier-Stokes equations under the Arbitrary Lagrangian-Eulerian (ALE) reference coordinate system was established to simulate the forced vibration circular cylinder. In this study, the ratio of displacement amplitude to cylinder diameter is from 0.2 to 0.6, and the frequency ratio (the oscillating frequency of the cylinder and vortex shedding frequency) ranges from 0.4 to 1.6. The numerical results show that when the circular cylinder vibrates at the amplitude of 0.6 D, the vortex structure in the wake of the circular cylinder is 2P mode. Otherwise, the 2S mode can be observed when the circular cylinder oscillates at rather smaller amplitude. According to the numerical results, the fluid forces in phase with acceleration and velocity of the circular cylinder versus reduced velocity are also calculated in this study.Based on the previous forced oscillating circular cylinder model, a self-excited model is developed. The circular cylinder is free to vibrate in both the cross-flow and in-line directions. For the large mass ratio m*=10.0, the numerical results present the initial branch, upper branch and lower branch for the cross-flow displacement. The relationship between the different branches and the vortex shedding mode is examined. Employing the self-excited model, the influence of mass ratio and damping ratio on the vortex-induced vibration was studied. The numerical results indicate that the decreasing in mass ratio results in the increase of the lock-in range. However, the damping ratio cannot change the essence of the dynamic system.Though many efforts have been made to research the physical mechanism of vortex-induced vibration with CFD method, it is far from the practical applications due to its huge time consuming. According to the theory of SHEAR7, a mathematical model used to evaluate the dynamic response and fatigue damage of deepwater risers under vortex-induced vibration was developed.In order to examine the VIV response of a long flexible riser oscillating at rather higher mode and providing the validation data for the empirical model, Laboratory tests were conducted to investigate the multi-mode dynamic responses of riser model subjected to steady uniform flow. The widely used semi-empirical methods in predicting VIV of deepwater risers, such as SHEAR7 and VIVANA, mainly rely on the observations and experimental results of a forced oscillating rigid cylinder under steady currents. Therefore, these empirical models are not able give exact prediction to the dynamic response of the long flexible risers involving higher mode and multi-mode. In this work, by using the available displacements data of VIV experiment as the input data, the time history of total hydrodynamic forces exerted along the axis of the riser model is obtained by the finite element analysis method. The total hydrodynamic forces are further decomposed into a component in phase with velocity (fluid exciting fluid coefficients) and a component in phase with acceleration (added mass coefficients). The results associated with both single mode and multi-mode responses are presented in this work. The hydrodynamic coefficients considering the higher order modes are addressed in this work.Employing the empirical model developed in this work, the influences of the top tension, the distribution of incident current velocity along the axis of the riser, outer diameter, inner diameter and wall thickness of risers on the dynamic response were investigated. For the problems of deepwater risers equipped with buoyancy modules, the influence of the buoyancy modules on the dynamic VIV response of the riser is studied. The numerical predictions indicate that the unreasonable arrangement of the buoyancy modules may lead to serious fatigue damage of the riser. As for the continuous paving buoyancy modules, the 40%-60% coverage rates are suggested. Otherwise, the space ratio of the buoyancy modules to bare riser should be 1:2 for the alternant paving buoyancy modules used according to the numerical results of this work.