Nonlinear behavior of 3D mining risers induced by vortex-shedding and gas–liquid–solid three-phase internal flow in deep-sea natural gas hydrate extraction

During the process of hydrate lifting, the riser is influenced by the combination of internal gas–liquid–solid three-phase flow, external ocean loads, and its large aspect ratio, making it susceptible to complex nonlinear flow-induced vibration failures. By utilizing the finite element method, Hamiltonian principle, and energy method, a vibration model for deep-sea hydrate exploitation risers is established, which considers the combined effects of the riser's large aspect ratio, vortex-induced vibration, gas–liquid–solid three-phase flow vibration, dynamic decomposition of hydrate, and heave motion of the offshore platform. The experimental device for simulating the nonlinear vibration of a mining riser under internal and external flow excitation has been developed, and a vibration simulation experiment for gas–liquid–solid three-phase flow in the riser has been conducted. The correctness of the model is verified by comparing the experimental test results with the theoretical model calculation results. On this basis, the influence of external environmental parameters and multiphase flow parameters in the riser on the nonlinear behavior of longitudinal vibration is described by the phase trajectory and the Poincaré mapping. The results show that the increase in shear flow velocity, platform depth period, and inflow displacement, cause the nonlinear characteristics of the vertical vibration of the riser to be constantly enhanced. Consequently, the vibration behavior becomes more complex and unpredictable. When the increase output volume is 0.4 m³/s and the particle size is 9 mm, the longitudinal vibration of the entire riser exhibits quasi-periodic motion, and the vibration state is the most stable. When the riser is in the resonance state, its stability of longitudinal vibration is enhanced, and it exhibits strong resistance to external small disturbances. However, there is a significant energy accumulation in the maximum displacement region of the riser, which results in increased local stress and significantly raises the risk of fatigue damage. The research results provide a theoretically sound guidance for designing and practically sound approach for effectively improving the service life of mining riser in deep-sea natural gas hydrate extraction.