The changes in the mechanical properties of gas hydrate-bearing sediments (GHBS) induced by gas hydrate (GH) dissociation are essential to the evaluation of GH exploration and stratum instabilities. Previous studies present substantial mechanical data and constitutive models for GHBS at a given GH saturation under the non-dissociated condition. In this paper, GHBS was formed by the gas saturated method, GH was dissociated by depressurization until the GH saturation reached different dissociation degrees. The stress–strain curves were measured using triaxial tests at a same pore gas pressure and different confining pressures. The results show that the shear strength decreases progressively by 30%–90% of the initial value with GH dissociation, and the modulus decreases by 50% –75%. Simplified relationships for the modulus, cohesion, and internal friction angle with GH dissociated saturation were presented.

The solid–liquid two-phase flow with coarse particles is ubiquitous in natural phenomena and engineering practice, which is characterized by coarse particles, high particle concentration, and large particle size distribution. In this work, the numerical models describing two-phase flows are reviewed, which given that the Eulerian–Lagrangian method is applicable in this work. Then, some modified models are proposed for the situation where the conventional Eulerian–Lagrangian method is not applicable to deal with coarse particles. The continuous phase equations of liquid are solved based on the finite volume method. The pressure implicit with splitting of operators algorithm for solving the Navier–Stokes (N–S) equations of the pseudo-single-phase flow, considering phase fraction and momentum exchange source term, is proposed. The discrete coarse particle is tracked in the Lagrangian method. A virtual mass distribution function is proposed for calculating coarse particle volume fraction. A weighted function method relating to the particle size is given for the interpolation between the Eulerian and Lagrangian fields. The barycentric coordinates are introduced into the particle localization. All the modified models are algorithmically implanted in the open-source field operation and manipulation (OpenFOAM) as a new solver named coarse discrete particle method FOAM (CoarseDPMFoam). Subsequently, the applicability of the numerical simulation method is verified by some typical test cases. The proposed numerical simulation method provides new ideas and methods for the mechanism investigation and engineering application of the two-phase flow with coarse particles.

Solid–liquid two-phase flow with dense coarse particles in the pipeline exists widely in energy and resources engineering, especially in the prospective hydraulic lifting scenario of deep-sea mining. In this paper, the coarse particles' transportation behavior in a vertical pipe is investigated based on an optimized Eulerian–Lagrangian method proposed in our previous work. The coarse particle transportation in the vertical pipe is characterized as multi-processes and multi-physical effects and is not fully captured due to the limitations on experimental apparatus, experimental measurement techniques, and numerical simulation techniques. Little research has been done on the local characteristics of the flow and their effect on the hydraulic lifting performance. The characteristics of the internal flow field, particle retention, and hydraulic lifting efficiency are disturbed under the condition of the continuous supply with the constant inlet two-phase concentration and velocity. The simulation results illustrate that particle transportation process can be roughly divided into three stages, single phase flow stage, mixing stage, and stable transportation stage, and shows a dilute-dense non-continuous flow pattern due to particle retention. Based on parametric studies, the retention phenomenon is alleviated by the increase in the water inlet velocity and almost disappears at approximately 12 000 of the Reynolds number, where the flow pattern transition occurs. Finally, the hydraulic lifting performance, such as a critical lifting condition and the friction loss, is analyzed and the transition of flow pattern is discussed.

Summary: Gas hydrate‐bearing sediments (GHBSs) have been considered as a potential energy resource. In this paper, the mechanical properties of GHBS are firstly investigated by the integrated test apparatus for synthesis of GHBS using silty sand as skeleton. Triaxial tests indicate an obvious transition of stress‐strain relationship from strain hardening under low hydrate saturation and strain softening under high hydrate saturation. The hypoplastic models coupled with Drucker‐Prager criterion and the Mohr‐Coulomb criterion are proposed to analyze the stress‐strain relationship of GHBS with considering the effective porosity because of the hydrate filling in the pores of GHBS. The strain hardening and softening behaviors are well predicted with less material parameters compared with the classical models. Compared with the test results, the proposed hypoplastic models are verified to be capable of capturing the salient features of the mechanical behaviors of GHBS under the conditions of little temperature change and no hydrate dissociation.