Abstract
AbstractGas hydrates can form more or less at the same time as seafloor sediment. They can have the effect of significantly stiffening and strengthening deep‐ocean sediments. Subsequent increases in situ temperature or decreases in pressure may trigger hydrate dissociation, leading to large reductions in the strength and stiffness of the sediment and possible seafloor instability. Gas hydrate dissociation not only removes cementing. It also releases freshwater and significant amounts of trapped gas that are dependent on multiple factors such as type of sediment, available pore space, hydrate morphology, and hydrate saturation. The presence of pock marks in areas of known seabed instability suggests that hydrate dissociation may have been a factor in triggering failure at these locations. Having reviewed the mechanisms by which the strength and stiffness of seabed sediment may be changed during dissociation, this paper reports the results of laboratory testing to evaluate the effects of loss of hydrate cement on strength and stiffness, for a range of sand‐sized materials with differing particle size, specific surface area, and particle shape, using a laboratory gas hydrate triaxial apparatus. The results suggest that both the strength and the stiffness of hydrate‐cemented granular materials are affected significantly by the specific surface available for hydrate cementation and, to a certain extent, by the particle shape. Uniform coarse granular sediments of lower specific surface area can suffer significant loss of stiffness and strength upon hydrate dissociation, changing the sediment from dilative to contractive. Finer‐grained sediments appear less affected by dissociation.
Highlights
Occurring gas hydrates are ice-like compounds that exist only under restricted thermobaric conditions, and where there is a biogenic or thermogenic source of methane
Having reviewed the mechanisms by which the strength and stiffness of seabed sediment may be changed during dissociation, this paper reports the results of laboratory testing to evaluate the effects of loss of hydrate cement on strength and stiffness, for a range of sand-sized materials with differing particle size, specific surface area, and particle shape, using a laboratory gas hydrate triaxial apparatus
Three types of loading tests were performed in the gas hydrate triaxial loading apparatus: 1. The evolution of stiffness with time in hydrate-cemented sands was explored by performing multistage undrained small-strain probing tests (Malandraki & Toll, 2000); 2
Summary
Occurring gas hydrates are ice-like compounds that exist only under restricted thermobaric conditions, and where there is a biogenic or thermogenic source of methane. Gas hydrate dissociation has been suggested as the possible trigger for a number of well-documented massive submarine slides, for example, at Storegga (Kvalstad et al, 2005; Talling et al, 2014), for the North Cascadian Slide (Yelisetti et al, 2014) and the Tuaheni Slope (Mountjoy et al, 2014), and for the AFEN slide (Madhusudhan et al, 2017). Thermobaric conditions at these locations certainly favor the creation of methane hydrates. Knowledge of the amount of strength and stiffness lost during hydrate dissociation will be important when predicting and simulating the triggering and runout of submarine slope failures, and the identification of potential failure surfaces in sediments hosting gas hydrates
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