Deterministic Slope Stability Analyses of the Sigsbee Escarpment
Authors: Nowacki Fritz , Solhjell Egil , Farrokh Nadim Farrokh , Liedtke Eric , Andersen Knut H. , Andresen Lars 
Published: October 2008

The paper describes the slope stability analyses carried out for BP's planned developments of the Mad Dog and Atlantis prospects. Both prospects are located along the Sigsbee Escarpment in areas where extensive slumping has taken place. The approach was based on combining geological, geophysical and geotechnical data from field and laboratory testing to develop models for slope stability evaluation. Examples of results are presented, including a brief discussion on how sensitive the results are to the variables in the analytical models.

The examples demonstrate how an interdisciplinary approach to the geological and geotechnical modeling renders possible extrapolation and interpolation of geotechnical soil data, and that slopes of heights up to 150 - 230 m and slope angles of more than 22º have safety factors in the range 1.1 to 1.5 based on best estimate of soil parameters.

Introduction

Slope instability is a direct potential threat to sub sea infrastructure and is therefore a key activity in most geohazard studies. The offshore developments in the vicinity of the Sigsbee Escarpment are no exceptions and slope stability analyses are a fundamental input to the risk evaluation of Mad Dog and Atlantis prospects. A retrogressive failure mechanism may cause bearing capacity problems for installations placed uphill of the critical slope. Installations placed downhill from the slope may be hit by suspension or debris flows. Submarine mass transport may take place over surprisingly long distances. It is also well known that submarine slides have initiated tsunamis with devastating effects on adjacent coastal areas. Slope stability analyses are therefore a key activity in most geohazard studies.

This paper describes the approach used to model slope stability for the planned development of Mad Dog and Atlantis. The work was an integrated part of a team effort which covered a variety of geological, geophysical and geotechnical subjects. This paper belongs to a suite of papers dealing with these aspects of which many form important input to the analyses discussed herein.

Both prospect areas studied are located at the main geological feature in the area, the Sigsbee Escarpment, at a distance of about 25 km between each other. Although there are differences in the geological characteristics, there are also important similarities which led to a similar analytical approach for assessing stability at the two sites.

The aim of the deterministic slope stability evaluations presented in this paper, was to:provide a best estimate safety factor, defined as the ratio between the average shear strength and shear stress along the critical shear surfaceestablish benchmark results for the probabilistic analysesin combination with probabilistic analyses, provide an understanding of how much safety factors increase as the shear surface moves away from the edge of the escarpment. Best estimated ground conditions and parameters were applied for the deterministic calculations of the slope stability. The uncertainties involved were dealt with in the probabilistic analyses and evaluations described in Ref. (1).