BP America commissioned a wide range of geological, geophysical and geotechnical studies into the status of the Sigsbee Escarpment which is located in deep water of the Gulf of Mexico. Within the broader programme were studies on the geotechnical and slope stability conditions in the area of the escarpment coinciding with the Mardi Gras pipeline crossing and at the Mad Dog 2 development area.
GCG, in collaboration with Imperial College, were commissioned to perform numerical analyses using the finite element code ICFEP to assess the stability of the Mardi Gras slopes and the potential impact that delayed progressive failure processes might have at the Mad Dog 2 location. This required a review of the geological history at both sites to develop an appropriate geological model, selection of appropriate soil models and the derivation of the associated soil parameters through numerical calibration. In order to recreate the geological conditions in the analyses, 600000 years of the history of the submarine slopes were simulated to incorporate processes including basal uplift driven by salt diapirs, deposition of clay sediments and previous slope failure. The results of the analyses showed that strain softening due to swelling and residual fabric formation reduces the slope stability considerably through processes that develop over hundreds to thousands of years. Furthermore, the numerical analyses provided many other insights into the key geotechnical processes at both sites that are likely to affect and control slope stability, and the mechanism of any possible failure.