A Safety Evaluation For Submarine Slope Instability Of The Northern South China Sea Due To Gas Hydrate Dissociation

As a significant new-energy resource, Gas hydrates are crystalline solids that form from mixtures of water and natural gases such as methane and ethane.In submarine environments, gas hydrates developed underneath the gently sloping continental margin where dissociation can have the effect of reducing the seafloor stability and increasing the probability for gas blowouts. Evidence available suggests a link between hydrate instability and occurrence of landslides on the continental margin. The dissociation of gas hydrates causes a large quantity of gas and water to be released into the soil sediment. If there are no pathways in which the excess water and gas can escape,then process will be essentially undrained and excess pore pressures will be generated.This melting process causes a loss of strength through both a loss of sediment cementation and an increase in pore pressure.The loss of sediment cementation is an obvious drop in sediment strength similar to the melting of ice-laden sediments.Theoretical analyses have also shown that it will cause an increase in pore pressure and a reduction in effective stress and thus result in loss of soil strength due to dissociation of gas hydrate.Seafloor slopes of 5°and less should be stable on the continental margin, yet many landslide scars are present. A likely mechanism for initiation of landsliding involves a breakdown of hydrates at the base of the hydrate layer.The effect would be a change from a semi-cemented zone to one that is gas-charged and has little strength, thus facilitating sliding.The cause of the breakdown might be a reduction in pressure on the hydrates due to a sea-level drop, numerous case studies have been investigated showing a possible connection between slope failures and the presence of gas hydrates.To simulate the slope failure of Shenhu area in the northern South China Sea due to gas hydrate dissociation,a numerical analysis has been undertaken. The dissociation of gas hydrates can stem from any change in the pressure and temperature regime in the hydrate stability zone and results in a significant volume change. This paper is to show the influence of gas hydrate dissociation on the stability of submarine slopes. In submarine slopes, the significant volume change occurring during hydrate dissociation combined with the low permeability sediments or hydrate seals result in rapid buildup of excess pressures, which may induce instability. This work is based on High-resolution multi-channel seismic data and geological samples from CNOOC and GMGS,which suggest that the geological structure on the continental slope of the northern South China Sea is favorable for the formation of gas hydrates.this work take Shenhu area as the study case in that Sediments samples containing gas hydrate was colleted in May 2007.this study is relevant to any area where gas hydrates are present and their dissociation affects the behaviour, which encompasses a large area of the world's oceans.