QUANTITATIVE ASSESSMENT OF SUBMARINE SLOPE STABILITY

R. L. Kleinberg
Schlumberger-Doll Research, Ridgefield, Connecticut

The geological record contains evidence of submarine landslides associated with gas hydrate deposits. Landslide events are often cited as a means by which considerable quantities of methane could be rapidly transported to the atmosphere, triggering or reinforcing global warming episodes. Moreover, it is frequently noted that mechanically unstable natural gas hydrate deposits may pose hazards to man-made deep water structures such as boreholes, wellheads, and pipelines. The mechanical properties of sediments containing gas hydrate have been the subject of a limited number of geophysical, laboratory, and well logging studies, but a comprehensive theory of the effect of gas hydrates on seafloor stability has yet to emerge.

Submarine gas hydrate deposits are found on continental slopes, where average grades are only a few degrees. Basic fracture mechanics theory suggests that low angle failures cannot result from slow changes of earth stresses or mechanical properties. Low angle failures, including those found in the geological record, can only result from (1) large transient stresses, such as those associated with earthquakes, (2) sudden changes in material properties, such as might be associated with rapid decomposition of gas hydrate, or (3) sudden changes of pore pressure, which might be due to hydrate decomposition or migration of gas from other sources.

Understanding the sources of seafloor instability requires a quantitative understanding of the mechanical properties of the sediments. The two most important properties are the friction angle and the cohesion. Because measurements of sediments containing hydrate have been limited, literature data on ordinary marine sediments and permafrost have been used to estimate these properties.

Friction Angle: Friction angle is relatively high for earth materials with a high proportion of sand, and decreases as silt and clay content, and porosity, increases. For marine sediments with porosities greater than 30% the friction angle is in the range of 5° to 15°. Surprisingly, the freezing of soils results in little or no change of the friction angle. 

Cohesion: The cohesion can be estimated from the compressional and shear wave speeds through correlations among the unconfined compressive strength, the static Young's modulus, and the dynamic Young's modulus. The wave speeds are available from well logs. The wave speeds can also be modeled, based on the observation that hydrate has a pore-filling growth habit that partially supports the solid frame. Numerical computations (J. Dai) have been performed for hydrate being formed (sediment, water, and hydrate present) and decomposed (sediment, water, hydrate and gas present). Cohesion during hydrate creation and decomposition is hysteretic.

A numerical example appropriate to a typical marine sediment demonstrates the importance of using realistic values for stresses and material properties.