Vortex-Induced Vibration Response And Damage Analysis Of Riser In Deep Water

For the requirement of oil/gas resources in deep water, research of risers become more and more difficult but important in deep water regions. Because of the complex marine environment, response and damage rate of risers connecting to the floating units are strongly influenced by vortex-induced vibration (VIV). However, visualizing the vortex pattern in the wake of a cylinder is a difficult and time consuming task.A large number of VIV prediction models have demonstrated that transverse vibration plays main role in the whole phenomenon, while longitudinal vibration of cylinders could be ignored or restricted. Therefore, a number analysis have been proposed to predict the modes, excited regions, hydrodynamic performance, response and damage rate focusing on the single-degree-of-freedom vibration. The main contents and results include:First, the mode patterns of a cylinder in deep water have been theoretically predicted and visualized. The distinction between cable model and beam model, as used here, is that cable model does not have significant bending rigidity. Based on the separation of variables method and WKB approximate method, natural frequencies and mode shapes of two cylinders were studied, and the analytical results have a good agreement with the results calculated by Shear?.Second, the excited modes and excited regions were analyzed. It is necessary to predict the "power-in" regions in travelling wave environments. By comparing mode natural frequencies with the maximum and minimum excitation frequencies, potential excited modes could be obtained. Main excited mode & were decided by identifying a cutoff coefficient. Next, in terms of nodes, the length of the power-in regions, for all excited modes, was calculated.Third, the hydrodynamics of the vortices were calculated. VIV of risers in current was considered as an energy balanced problem:excitation forces in the power-in regions add an equal amount of energy to the system as is dissipated by damping forces outside those regions and structure modal damping. Therefore, an iteration calculation was performed for the calculation of amplitude with the lift force and damping updating.Finally, on the basis of cumulative fatigue damage theory, response and damage rate of VIV are determined. And the parameter analysis of VIV damage on a riser were carried out under conditions of different current velocities, inner fluid, outside diameters and cutoff coefficients.