Abstract: The decomposition of natural gas hydrates can induce the instability of submarine slopes and damages to submarine engineering infrastructures. Accordingly the stability assessment of the submarine slope is of great significance to the site selection and safe operation of the submarine engineering infrastructures. Based on the engineering geological characteristics of the hydrate-enriched area in the Shenhu offshore area in the northern South China Sea, we use the finite element strength reduction method to analyze the effects of the slope geometry, soil strength changes, and hydrate reservoir characteristics on the stability of the submarine slope before and after hydrate decomposition. The results show that, without the consideration of the hydrate decomposition, the stability of the submarine slope is mainly controlled by the slope of the slope gradient and the soil strength, and it is mainly manifested as a shallow landslide. However, when the decomposition of hydrate is considered, the reduction in the strength of the hydrate-bearing layer will affect the overall stability of the slope, but the position of the most dangerous sliding surface under the same overburden condition is greatly affected by the depth of the hydrate-bearing layer. A critical burial depth that is controlled by the terrain geometry and the strength of the overlying soil exists. In addition, when the burial depth is greater than the critical burial depth, the hydrate decomposition has little effect on the stability of the slope, and the most dangerous sliding surface is located in the upper shallow layer, which manifests as a shallow surface failure. When the burial depth is less than the critical burial depth, the most dangerous sliding surface passes through the hydrate-bearing layer, which shows a deep landslide. Finally, according to the buried depth conditions of the hydrate-bearing layer in our current model, the safety factor of the deep sliding surface after hydrate decomposition is still larger than that of the shallow layer, indicating the submarine landslide hazards in this area are mainly shallow landslides.

Key words: gas hydrate, stability, submarine landslide, strength reduction, Shenhu offshore area