Research On The Vortex Induced Vibration (VIV) For The Flexible Riser Using Discrete Vortex Method (DVM)

Riser system is an important but weak part of the offshore structure. Under the loading of the ocean current, the vortex induced vibration may occur to the riser due to the vortex shedding which results in the reduction of the service life or the direct damage of the structure. The study on the vortex induced vibration has been a hot and difficult point for the industrial and academic fields for the recent years. Numerical simulations in time domain for the vortex induced vibration of the riser have been carried out using the discrete vortex method (DVM) in conjunction with the finite volume method (FVM), trying to unravel some intriguing phenomena of the vortex induced vibration.Discrete vortex method is one kind of computational fluid dynamics (CFD) methods in the recent years, which has a lot of advantages, such as simple and clear idea, not depending on grid computing and being able to solve the high Reynolds number problem compared to other numerical methods. In this paper, vortex induced vibration for two dimensional flows around circular cylinder was simulated using the discerte vortex method. Based on different restraint patterns for the circular cylinder, the single degree of freedom and two degrees of freedom motion were exerted on circular cylinder and the numerical simulations of the flow around the moving circular cylinder were performed.The simulated results of the flow around the circular cylinder under two different restraints were both compared with the experimental results, which demonstrated very good agreements with each other, revealing that Discrete induced method were very suitable to solve the high Reynolds number problems.The maximum peak of the vibration amplitudes and lock-in phenomenon were captured in the calculation, which was a highlight part in this paper because it was very difficult to be obtained by other numerical methods. The simulated results proved that transverse vibration amplitudes of two degrees of freedom motion were greater than that of one degrees of freedom motion.Owing to the great scale ratio for the riser, three-dimensional simulation for the vortex induced vibration was very difficult to perform under current calculation condition; however, the simulations for the three-dimensional free vibration model were carried out using finite volume method in this paper. By applying the strip theory to the calculation process, the riser model could be simplified into lumped mass system, the two-dimensional hydrodynamics calculation on the transverse section for the control points coupled with solving dynamic response equation of the structure were carried out.For the uniform current cases, the principal vibration modes of the catenary riser went up with the increase of velocity of the current, the votex shelding patterns of 2S and 2P occurred alternately in the wake flow.2S pattern often occurred in the vicinity of vibration nodes in the wake flow,2S pattern often occurred with the incease of the vibration amplitudes. From the comparisons of lift coefficients, due to the fluid-structure interaction, the magnitudes of lift were conspicuously different at different positions for the same riser, which had very close connection with the velocities of flow and vibration modesIn this paper, all the programs were written in FORTRAN language.In a word, this paper aims to inherit and develop a thorough idea and method to simulate the vortex induced vibration of the riser, making some contributions to the oceaning engineering.