Long-term creep deformation mode evolution of hydrate-bearing submarine slopes under different dissociation degrees

Conventional elastoplastic models fail to characterize the time-dependent creep behavior of deep-sea natural gas hydrate (NGH) reservoirs, and cannot predict the progressive instability of submarine slopes on decadal timescales. This study adopts a fractal derivative creep damage model to establish a two-dimensional numerical slope model of the Shenhu area, South China Sea. 30-year long-term creep simulations are performed under multiple dissociation gradients, and the Horizontal-to-Vertical Displacement Ratio (HVDR) is proposed to quantify deformation mode evolution. The results indicate that hydrate saturation mainly governs the viscoelastic coefficient while exerting a relatively minor influence on the fractal order, which greatly simplifies model parameter calibration. Slope deformation transitions from vertical settlement-dominated to reservoir-parallel shear slip with rising dissociation degree. Under deep-sea high pressure, long-term deformation is dominated by viscous creep flow with suppressed plastic shear failure, presenting a distinctive feature of “significant deformation without plastic zone penetration”. This study clarifies two distinct deformation mechanisms of hydrate-bearing slopes, namely strength-induced instability and creep flow. The findings provide a theoretical reference for the safe exploitation of deep-sea natural gas hydrates and the long-term stability evaluation of submarine slopes.