Abstract: The riser pipe is the key channel of gas hydrate exploitation in deep water, the force and deformation of the riser are more complicated due to the formation of multiphase internal flow due to the decomposition of gas hydrate in the pipeline, and the riser of gas hydrate exploitation is prone to failure accidents under the comprehensive action of internal and external flow. In order to analyze the mechanical characteristics of hydrate production riser, the decomposition and flow of hydrate in the pipe are considered in the model and introduced into the riser vibration equation. The dynamic model of hydrate riser in marine environment coupled with internal multiphase flow is established. Finite element method is applied to discretize and solve the model. The effects of slurry density, flow rate and outlet back pressure on the mechanical response characteristics of riser in hydrate mining are analyzed by taking solid fluidization test mining technology as an example. The results show that the mass of hydrate production riser decreases and the section tension of the riser increases due to the formation of multiphase flow from hydrate decomposition. The deformation displacement, bending moment and deflection angle of hydrate production riser are all smaller than those of pure liquid riser. The maximum displacement of hydrate production riser without buoyancy block appears below the midpoint of the riser, and the bending moment and deflection angle at the bottom of the riser are the largest. The displacement, bending moment and deflection angle of hydrate production riser increase with the increase of drilling fluid density, slurry discharge and outlet return pressure of riser. Appropriate increase of the tension at the top of the riser for hydrate exploitation or configure action of the buoyancy block, reduction of the drilling fluid density and displacement, and reduction of the outlet pressure of the riser are conducive to reducing the hydrate production riser bending deformation. The research results can provide theoretical guidance for protecting the safety of hydrate production riser.

Key words: natural gas hydrate, multiple phase flow, hydrate production riser, effective tension, mechanical characteristics