The impact of hydrate dissociation coupled with seismic ground motion on the stability of submarine slopes

• Main Author: Lemon, Alexandra Margaret Jamesina
• Other Authors: Lovell, Michael
• Published: University of Leicester 2012
• Subjects: 551.468
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.551929

Gas hydrates are natural ice-like solids formed from the combination of gas and water molecules. Hydrates could be considered a potential geohazard in the marine environment due to their possible inuence on seaoor stability in environments where social infrastructure such as telecommunication/IT cables is present. If hydrates dissociate there will be a reduction in sediment shear strength and an increased susceptibility to induced soft sediment deformation. This research aims to analyse the impact of hydrate dissociation coupled with seismic ground motion on the stability of submarine slopes as described by the Factor of Safety within the sediment column. A simple one-dimensional numerical model is developed that simulates heat flow and hydrate dissociation within the sediments. The dissociation model considers many competing factors such as seawater depth, increasing seabed temperatures, the natural geothermal gradient, the thermal properties of sediments, the quantity of gas hydrate and the rate of heat owing into and out of the hydrate during dissociation. The model also takes into account the changing salinity of the surrounding pore space fluids as the hydrate forms or dissociates. Estimates of pressure change during dissociation are derived from first principles based on conservation laws and the ideal gas law. The model of gas hydrate dissociation is used to inform estimates of shear strength applied to a slope stability model of continental slope sediments under earthquake loading. A sand-rich sequence is modeled. Although, during dissociation, pore-water pressure within the hydrate/sediment mix was found to increase by approximately 10%, this resulted in a reduction of effective stress to almost zero within the shallower sediments, causing slope instability in the depth range 3-7 mbsf in water depths of 600m and an increase in the ground wave acceleration by an average of 0:01m². The greatest reduction in the factor of safety (0:08) and the greatest increase in acceleration (0:016753m²) was shown to occur 3mbsf. It was determined that the dissociation of hydrate without drainage increases the likelihood of slope failure. This in turn leads to instability in the sediments on slopes as shallow as 1.32 deg in the model. These model results are compatible with previous studies of the shallow AFEN slide, NW Scotland.