Probabilistic slope stability analysis has been performed for the West Nile Delta (WND) deepwater area, offshore Egypt. It has been used to minimise geohazard risk for the WND subsea development. The analysis was premised on earthquakes being the predominant trigger of slope instability, exacerbated in the past by excess pore pressures generated by relatively rapid sediment accumulation. Earthquake-induced slope displacements calculated using the Newmark method were mapped to a pseudostatic safety factor to facilitate area-wide characterisation of slope failure probability. The annual probability of slope failure was assessed across the WND development area within ArcGIS, incorporating mapped variable geological controls (e.g. soil strength, slope angle) as inputs. Failure probabilities were spatially generated using the concept of slope facets, defined as areas of seabed with similar expected susceptibility to slope failure, based on geomorphological examination.

1. Introduction

BP Egypt and its equity partners, RWE Dea Group and Egyptian Natural Gas Holding Company (EGAS), have embarked on a programme of subsea natural gas developments in the West Nile Delta (WND) deepwater area. A systematic method has been adopted to mitigate geohazard risk in the area, using a multi-disciplinary team approach to risk identification and quantification (described in Evans et al., 2007; Evans, 2011). The various geohazards identified have been highlighted by Moore et al. (2007), foremost of which is the potential for slope instability. This failure type has been the subject of both an expert judgment assessment and probabilistic stability analysis (PSA) to best constrain the associated risk. The former was conducted in multi-disciplinary workshops, based primarily on historical event frequency and informed by variations in environmental controls (including sedimentation rate and related pore pressures). The latter, and the subject of this paper, is considered a geotechnical assessment of slope failure probability.

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