Gas Hydrate Occurrences and Seafloor Deformation: Investigation of Strain-softening of Gas-hydrate Bearing Sediments and Its Consequence in Terms of Submarine Slope Instabilities
Authors: Sultan Nabil , Garziglia Sebastien , Colliat Jean-Louis 
Published: May 2011

Published laboratory geotechnical data by Masui and co-authors showed that increase in gas hydrates content tend to increase the peak shear strength and stimulate strain softening of the host sediment. Therefore, development of shear strains may lead to an important degradation of the shear strength (strain softening). In the present work, the strain softening of gas hydrate-bearing sediments was implemented in a 3D slope stability model (SAMU-3D). This was done by adding to the classical limit analysis method a shear strain field compatibility equivalent to the velocity field compatibility. Examples of slope failures related to strain softening behavior documented in the literature were used to test the model formulation.

The developed model was then used to assess the stability of a steep flank of a shale-cored anticline in the eastern part of the offshore Niger delta. Numerical modeling showed that the formation of gas hydrates in the shallow sedimentary layers could considerably affect the factor of safety of the studied slope. The present work showed clearly that the strain-softening behavior of gas hydrate-bearing sediments has relevance for the stability of submarine slopes.

Introduction

Over the last 3 decades cases of submarine slope failures have been reported worldwide in areas where gas hydrate occurrence was proved or suspected. McIver (1981) was amongst the first authors to speculate about the possible link between gas hydrate and submarine slope failures. In the McIver's conceptual model, the excess pore pressure induced by hydrate dissociation and the decrease in sediment shear strength (loss of hydrate playing the role of cementing agent between sediment grains) are the two key factors increasing the susceptibility of a slope to failure. The causal factor of the hydrate dissociation in the McIver (1981) model is the continuous sedimentation, which induces the upward migration of the base of the Gas Hydrate Stability Zone (GHSZ). Afterwards, Kvenvolden (1994) has stated that an upward movement of the bottom of the GHSZ due to an increase of bottom water temperature may accelerate the process of slope failures associated to hydrate dissociation. Different authors have later developed several other hypotheses and theories supposing all that gas hydrate dissociation generated by pressure and/or temperature changes may lead to important excess pore pressure and lead to sediment deformations and slope instabilities (Kayen and Lee, 1991; Kvenvolden, 1999; Paull et al., 1996 and 2000). Although several authors have raised serious concerns regarding possible connections between gas hydrate melting and submarine slope failures, none conclusively demonstrate that (Grozic, 2010).

On the other hand, recent laboratory geotechnical data have clearly shown that the presence of gas hydrates may lead to higher peak strength of the host sediment and promote strain softening, that is, a post-peak decrease in strength with increasing strains (Masui et al., 2005, 2006, 2008). The experimental data presented by Masui et al. (2005) suggest that at large strains, the strength of gas hydrate-bearing sediments may reach a remolded value as low as that of their fully water saturated equivalents (i.e. similar void ratio without hydrates). To the authors' knowledge no published studies have yet investigated the potential impact of the strain softening behavior of gas hydrate-bearing sediments on submarine slope stability. The present work attempts to address theoretically this important question by implementing the strain-softening behavior of gas hydrate bearing-sediments in the SAMU-3D (Sultan et al. 2007a) software which is a three dimensional slope stability analysis model based on the limit analysis method. The developed software will then be used to assess the slope stability of the steepest flank of a shale-cored anticline forming a dome-shaped bathymetric high in the eastern part of the offshore Niger Delta.