Determination of Strength Parameters of Soil Samples Recovered from Eastern Nankai Trough for Seafloor Stability Study

In Japan, the interest in methane hydrate is increasing rapidly owing to increasing recognition of its potential as a next-generation energy resource that can replace conventional fossil fuels. To produce methane gas safely and minimize the associated environmental damage, we need to address many wide-ranging environmental issues. One such issue entails assessing seafloor stability during methane gas production. Methane hydrate binds the sand grains that make up the strata under the seafloor. It has been suggested that methane production may lead to seafloor deformation because the strata become unstable following the removal of methane hydrate. The geotechnical properties of ground have significant effects on its deformation behavior, but deep seafloors have not been thoroughly investigated yet.

The world’s first offshore test gas production from methane hydrate was conducted in the eastern Nankai Trough. We present geotechnical properties of the samples recovered from the gas production site; these properties were determined by means of laboratory tests. Soil index tests, consolidation tests, K₀ consolidated undrained triaxial compression/extension tests and direct box shear tests were conducted for obtaining the geotechnical parameters necessary for deep seafloor stability analysis. The strength parameters corresponding to peak and residual states were determined by the reversal direct box shear tests.

This study is supported by the Research Consortium for Methane Hydrate Resources in Japan. We wish to express our appreciation to the MH21 Research Consortium and Japan Oil, Gas and Metals National Corporation for their permission to use the laboratory test data.