Vortex-induced vibration analysis of variable-diameter air-lift risers for deep-sea mining

Air-lift lifting is a promising technology for deep-sea mining due to its simple structure, low maintenance, and good adaptability to mineral particles. This paper first develops a one-dimensional steady-state multiphase flow model for variable-diameter air-lift risers and validates it against experimental data. A time-domain vortex-induced vibration (VIV) analysis model is then established by coupling the multiphase flow solver with a phase synchronization-based hydrodynamic force model, and systematic numerical simulations are carried out for a 5000 m deep-sea mining riser. The results indicate that the stepped diameter geometry introduces significant axial non-uniformity in both structural and flow parameters. Compared with constant-diameter risers, variable-diameter designs produce more uneven amplitude distributions and clear multi-modal vibration characteristics. Increasing gas injection rate alters the riser's equivalent dynamic properties and may potentially induce mode transitions, while injection depth has limited impact on the overall VIV response. The diameter variation scale is identified as the dominant factor governing vibration amplitude, dominant frequency, and energy concentration. These conclusions provide theoretical support for the design and safe operation of air-lift risers in deep-sea mining.