Submarine landslides are one of the major hazards for offshore pipelines. Progressive differential ground movements caused by earthquakes can initiate run-out/debris flows that can impact and damage pipelines. Hence, both the stability of submarine slopes caused by earthquakes and the potential run-out distances must be assessed. This is particularly important for deepwater pipelines, whose routes often cross areas that are prone to landslide and debris flow. This paper presents an overview of slope stability and run-out assessment for offshore pipelines using both analytical and numerical methods. Analytical slope stability assessment is based on guidelines for seismic design and assessment of natural gas and liquid hydrocarbon pipelines, and SLOPE/W and QUAKE/W are used for the numerical assessments. The paper provides a review of run-out assessments in literature and summarises the best methodology through a case study.
Submarine landslides, which can be triggered by many factors such as shallow gas release and seismic events, pose a threat to offshore pipelines, and the outcome of such event could be catastrophic. Progressive differential ground movements, such as those caused by landslides, earthquakes and runout/debris flows, can cause pipeline deformations that may impact serviceability of pipelines. Hence, it is essential to assess the stability of submarine slopes and study the risk associated for pipelines stability. There are mainly two types of slope failures usually associated with submarine slopes undergoing earthquake shaking: coherent failure and disruptive failure. Coherent failure is where the soil moves as a single solid mass (Figure 1a), and disruptive failure is where the soil loses the majority of its strength and flows as individual particles (Figure 1b). Hence the assessments need to be conducted firstly to identify the stability of slopes during shaking and then to assess the expected run-out.