Nonlinear vibration model and characteristics of dual-riser in deep-sea solid fluidization mining under multi-field coupling excitation

The dual-riser system in deep-sea solid fluidization mining faces multi-source excitations related to complex gas-liquid-solid flow with phase change, pipe-in-pipe (PIP) collisions, and platform heave. To address the theoretical gap in multi-field fully coupled time-domain analysis of existing models, a nonlinear fluid-structure interaction (FSI) dynamic model for the dual-riser system in deep-sea hydrate mining is established. A fully-coupled nonlinear FSI model is established for dual-riser systems. This model integrates external vortex-induced vibration (VIV), hydrate dissociation kinetics, and nonlinear PIP contact effects. It is the first model that simultaneously accounts for all these coupled mechanisms in a unified time-domain framework. The model is derived from Hamilton's principle and the conservation laws of multiphase flow, and discretized using the Galerkin weighted residual method. It comprehensively accounts for the geometric nonlinearity of the riser with a large aspect ratio, external vortex-induced vibration, dynamic dissociation and phase change of hydrates, and nonlinear spring-damper contact effects between the inner and outer pipes. The challenge of time-varying system stiffness is overcome through a cooperative solution scheme that combines the incremental Newmark- β method with the Newton-Raphson iteration, enabling accurate time-domain numerical solutions of the governing equations. A coupled vibration simulation experiment of the mining riser under combined internal and external flow excitations was further conducted. The experimentally measured root-mean-square (RMS) displacement profiles were compared with the theoretical model predictions, and the accuracy of the proposed model was thereby validated. The results show that as the external shear flow velocity increases, the in-line displacement of the riser increases significantly. Under high flow velocity, the tension stiffening effect suppresses the cross-flow amplitude in the deep-water region. It also reduces the high-frequency stress in the mid-upper section. The hydrate particle size exhibits a non-monotonic regulation. The in-line deflection is largest for the 5 mm particle size and smallest for the 7 mm case. The 3 mm small particles tend to induce local bending and stress concentration at the deep-water bottom. As the internal flow rate increases, the in-line displacement rises monotonically. The in-line stress is most effectively suppressed at 0.5 m 3 /s. However, a high flow rate of 0.6 m 3 /s disrupts the tension balance at the deep-water bottom. This leads to abnormally high local dynamic stress. The research findings can provide a theoretical basis for structural optimization and instability control of the dual-riser system in deep-sea hydrate mining. They can also provide practical technical guidance for the rational optimization of production parameters and the extension of the riser service life.