Advances in Offshore Seismic Slope Stability: A Case History

摘要

This paper presents a case history showing how the integration of detailed geophysical and geotechnical data and advanced numerical modeling can overcome the limitations of conventional analysis in predicting seismic stability of decpwater slopes. Submarine landslides represent one of the most critical gcohazards for offshore pipelines and deepwater hydrocarbon developments. This is particularly true for seismically active regions where earthquakes are expected to be a triggering mechanism. A typical issue encountered in these cases is the coexistence of several detrimental aspects: poor geomechanical properties of shallow sediments; presence of steep slopes; and/or severe seismic input. The combination of these aspects often makes it difficult to match results of conventional pseudo-static slope stability analysis with field observations. These methods are generally conservative for deepwater conditions and are not able to reproduce observed past failures modes. This case history is of a complex slope system in the Mediterranean Sea. Morphologically the system presents a number of canyons and large-scale landslide features, overlain by a limited number of shallow planar slides. Geochronological testing constrained the large, deep slides to the distant past while confirming the shallow slides as recent phenomena. The use of high quality sampling and advanced laboratory tests provided the necessary input for dynamic nonlinear FEM analyses using OpenSees software. Numerical results based on a set of real time histories confirmed field observations and highlighted the possible formation of seismically triggered shallow slides. The paper describes how geophysical data, accurate soil sampling and advanced laboratory testing together with an advanced numerical model can develop reliable slope stability assessments for projects in difficult environmental conditions.