Numerical Modelling of Submarine Landslides and Their Consequences on Offshore Infrastructure

Abstract

Submarine landslides are caused by underwater slope instabilities in natural water reservoirs, from rivers and lakes to seas and oceans. While they may occur on very gentle slopes (e.g., slopes of less than 1°), they can displace more than 3500 km3 of soil at a run-out distance of up to 3000 km. Therefore, submarine landslides are the underlying cause of some of the most devastating natural disasters. They may cause the disappearance of entire regions along the shoreline, destroy the offshore infrastructure, and generate tsunami waves with disastrous onshore consequences. Recent improvements in mapping the sea floor have revealed that submarine landslides are much more common than had previously been thought. There is an evidence of such landslides along the Israeli continental shelf and slope. It seems like the landslides were caused by strong earthquakes. Israel is located along the Dead Sea Fault, and as a consequence, is subjected to its expected moderate to strong earthquakes. This implies that the Israeli shoreline and continental shelf might be exposed to submarine landslides caused by earthquake ground shaking, and therefore to tsunami events that might cause fatalities and financial damage. Furthermore, the already existing, and the future planned gas transportation infrastructure along the continental slope, requires a better understanding of the processes that result in subsea landslides in order to improve our ability to predict, assess, and mitigate their potential hazards. In the past the detection of submarine landslides has halted infrastructure developments as the risks were unknown and difficult to quantify. Unfortunately, laboratory and field studies cannot reproduce realistic submarine landslides. Numerical studies that will allow us to understand when these landslides occur, and their consequences on buried infrastructure as well as their potential as tsunami generators are critical to advancing our understanding in this topic. Computer simulations, unlike the laboratory experiments, enable us to simulate full-scale events and specific set-ups that are impossible to build in the laboratory (e.g., materials with limiting properties, movements with extreme velocities, or large dimensions). Moreover, computer simulations usually cost much less than laboratory experiments and they are able to reproduce much more detailed and versatile phenomena. While some important field data may be missing or impossible to collect, by changing possible geometric conditions (e.g., bathymetry, potential weak zones) and soil mechanical properties, it is possible to assess the upper and the lower limits of potential consequences of the landslide and possible hazards to submarine infrastructure. Moreover, by means of parametric study it is possible to define how important each missing parameter is. The process of landslide evolution involves large deformations and movements of the soil. In such problems, the traditional Lagrangian finite element approach – in which the nodes are fixed within the material and elements deform as the material deforms – suffers from excessive element distortions. Several advanced continuum approaches to treat large deformation problems in soil mechanics exist, which treat large deformations with a certain degree of success and have advantages and drawbacks regarding particular applications. In the proposed work, the computational framework adopts an Eulerian approach. In the Eulerian approach the material is not limited by the amount of deformation and free to move through the mesh, which is fixed in space. Consequently, the problem of excessive element distortions becomes irrelevant. The Eulerian approach in case of extremely large soil movements in large-scale problems appears to be more computationally efficient and robust than other approaches. An Eulerian mesh naturally allows shear band propagation in arbitrary directions (due to the stress state) without the need to model its trajectory apriori, and it can capture all the phenomena that accompany the landslide evolution. Another advantage of the Eulerian framework is that it is widely available within commercial codes, such as Abaqus, Ansys, LS-Dyna and etc. accessible to engineers. Moreover, Abaqus software for example, can easily adapt various enhancements and modifications by means of user subroutines and Python codes. We demonstrate the framework on several examples of landslide events, including impact on offshore infrastructure.

AAPG Datapages/Search and Discovery Article #90341 ©2019 AAPG Geoscience Technology Workshop, Exploration and Development of Siliciclastic and Carbonate Reservoirs in the Eastern Mediterranean, Tel Aviv, Israel, February 26-27, 2019