Numerical Investigation On The Vortex-induced Vibration Of A Flexible Slender Riser With Internal Flow

As oil and gas passages in drilling and exploitation, marine risers have been widely used in many modern offshore engineering structures. They generally connect to the universal joints at lower ends and join to the sliding joints in platform or to the FSOPs directly at upper ends. Besides the fluid transferred inside with both high speed and pressure, risers are also subjected to the wave and current loads. When current passes by circular cylinders,risers for example, vortices shedding from each side of the structures alternatively, which generate periodic hydrodynamic forces acting on the risers and thus lead to vortex-induced vibration(VIV). For vibrating risers(bending vibration) with internal flow, the centrifugal force and the Coriolis force, imposed on the internal fluid, will lead to secondary flows superimposing on the Hagen-Poiseuille flow and thus generate helical flows in some conditions. Due to the changes of inner flow field, the body forces induced is also changed,which leads to additional vibrations of the risers. Apparently, the VIV of flexible slender risers with internal flow is a complicated FSI problem with a high nonlinearity.Although the research on VIV of risers initiated in decades, but research on VIV of risers with internal flow is limited. Based on the immersed boundary method(IBM), a parallel code, named as CgLES IBM, has been developed to be a FSI solver, combining with an implicit dynamic structural solver, X-code, to simulate the deformation and motion of risers. The contents and main conclusions achieved in this paper include:1. The simulations of cross-flow(CF) vibration a flexible riser were carried out. The VIV of a flexible riser with the first three vibration modes was investigated and the effects of the internal flow, the mass ratio, et al. were taken into account. The results include: the responses of a low mass ratio riser are more sensitive to the internal flow, the internal flow reduces the intrinsic frequency of the riser and the amplitude of CF vibration.2. The simulations of 2DOF vibration of a flexible riser were carried out. The VIV of a flexible riser with the first three vibration modes were investigated and the effects of the internal flow, the mass ratio, et al. were taken into account. By comparing to the riser with CF vibration only, it was found that the in-line(IL) bending vibration amplifies the CF vibration amplitude and the internal flow increases the mode number participating in vibration.3. The analysis of near-wake of a flexible riser was carried out. The near-wake manifests a 3D characteristic, i.e., the standing wave vibration mode and the parallel interwoven wake were observed in all the simulation cases in this paper. In addition, the internal flow modifies the wake characteristic length in somehow.4. The secondary flow of the internal flow is investigated. The secondary flow, in the form of one or more pairs of stream-wise vortices, was found to be pronounced in the internal flow in some conditions. The number and position of the vortex pairs change periodically with the movement status of the riser undergoing VIV, while the 3D structures of stream-wise vortices alter significantly when the vortex number increases. In addition,the periodicity and 3D characteristic of stream-wise vortex were explained by using the theory of flow instability.