ANALYSIS OF VORTEX-INDUCED VIBRATION FOR A CANTILEVER RISER IN A DEEP-SEA MINING SYSTEM

Different from the traditional marine riser, the vertical lifting pipeline in the deep-sea mining system can be regarded as a flexible cantilever riser with an unconstrained bottom end. Likewise, problems in terms of vortex-induced vibrations (VIVs) and flexible deformations can be encountered during operation. In this paper, a quasi-three-dimensional time-domain numerical model coupled with the discrete vortex method (DVM) and finite element method (FEM) is employed in the time domain. Systematic simulations have been carried out to investigate the VIVs of a cantilever riser under different current speeds. The results indicate that, for a cantilever riser, the transverse vibration mode number rises with increasing the reduced velocity. In a certain range of reduced velocities, the dominant vibration modes remain unchanged. When the modal transition occurs, the corresponding vibration amplitudes can abruptly drop. However, when the new high-order mode is excited, vibration amplitudes of the riser again gradually increase with increasing the incoming velocities. In the same vibration mode, the root-mean-squared values for the bottom displacements of the riser linearly rise with the reduced velocities. When vibration mode changes, a jump phenomenon for the dominant vibration frequencies can be observed. Especially, the present work discusses the vibration responses of the cantilever riser in the three-order dominant mode. It can be found that the unconstrained bottom end of the riser exhibits relatively large vibration energy. The standing wave characteristics of the vibration amplitudes gradually enhance with the increase of the reduced velocities. The VIV response characteristics of a two-ends hinged riser and a cantilever riser are compared in this investigation, both of which exhibit the same variation tendency in terms of amplitude and dominant vibration frequency.