Numerical analysis of vortex-induced vibration of deep-sea mining riser with auxiliary pipes based on discrete vortex method

Air injection and discharge pipes play an essential role in the deep-sea mining airlift process, and their influence on the flow field and riser vortex-induced vibration (VIV) is not fully investigated. In this study, the discrete vortex method is modified to incorporate the influence of multiple solid domains, and the flow field evolution in the presence of auxiliary pipes is simulated. Based on the assumption that the vortex structure is constant within a certain spanwise length, an analytical model is established by strip theory and a weakly coupled fluid-structure interaction approach to investigate riser VIV in the cross-flow direction. The results show that the auxiliary pipes will interfere with the vortex shedding of the main riser and inhibit the formation of a stable wake pattern, which leads to a reduction in both the amplitude and frequency of the lift force. The VIV exhibits a mixed behavior of standing and travelling waves, and multi-mode responses induced by temporal drift of the frequency can be observed. In addition, the auxiliary pipes suppress the VIV by disturbing vortex shedding and increasing the bending stiffness of the riser. Moreover, the effects of inflow angle, current velocity, and vessel navigational motion on the VIV response and power region distribution are investigated. Specifically, a 45° inflow angle provides optimal VIV suppression and leads to the narrowest power-in region, while vessel motion in either the upstream or downstream direction aggravates VIV and significantly alters the power region distribution.