The Case for Confidently Siting Facilities along the Sigsbee Escarpment in the Southern Green Canyon Area of the Gulf of Mexico: Summary and Conclusions from Integrated Studies
Authors: Jeanjean Philippe , Hill Andrew , Thomson James 
Published: October 2008

This paper summarizes the key conclusions and evidence that enabled BP to confidently site facilities along the Sigsbee Escarpment. The conclusions arise from the results of a series of multidisciplinary studies. The thorough understanding of the following key aspects are clearly documented: geologic environment, current level of stability of the slope, timing of past slope failures, behavior of key past and future debris flows, plausible natural and made-made trigger mechanisms within the life of the field, and annual probability of occurrences of slope instability.

The current high level of stability of key slopes in the vicinity of future facilities, combined with a lack of credible natural trigger mechanisms within the life of field, and low annual probability of occurrences of slope failures led to the conclusion that the risks that geohazards pose to the chosen field architectures for the Mad Dog and Atlantis field were acceptable. Other areas of the field where such conclusions are not valid have also been delineated.

Trigger mechanisms

The crucial question to be answered here is: if the escarpment today is stable, and has been stable for a fair amount of time, (see Young et al, 2003) why would it fail tomorrow? To address this issue, an in-depth review of potential trigger mechanisms was performed. The objective was to identify mechanisms capable of triggering a slope failure, within the lifetime of the field or an engineering time frame. Engineering time frames are usually defined as less than 1,000 years (such as the survival event for design of a floating system) and rarely go beyond 3,000 years (as in the DLE event in earthquake engineering). The review was not aimed at predicting the evolution of the escarpment over a longer, geological, time period.

Because the slope stability process involves soil elements resisting the shear stresses imparted on them by mobilizing the soil shear strength, slope instability can be triggered by increasing the shear stress applied to the soil, decreasing the soil shear strength, or a combination of both processes. Natural trigger mechanisms for submarine slopes have been investigated by many authors (Hampton et al, 1996, Mulder and Cochonat, 1996, Locat and Lee, 2000) and are summarized in Table 1.

The mechanisms in Table 1 involve natural processes that are acting on the slope regardless of human activity in the area. Therefore the activities involved in the development, and production from, the field need to be reviewed to see if they may encourage slope instability. Activities with potential impact on slope stability again can be divided into those that may cause a reduced shear strength in the soil mass, and those that may cause an increase in shear stresses. These activities are presented in Table 2. In listing these potential maninduced trigger mechanisms, it is important to capture all identified mechanisms, regardless of how unlikely they may seem. Their potential relevance should not be pre-judged.