Philippe Jeanjean

Measurements and modeling of the shallow pore pressure regime at the Sigsbee Escarpment Successful prediction of overpressure and ground-truthing with borehold measurements

Editors note: The material in this paper was prepared and presented (OTC Paper 15201) at the 2003 Offshore Technology Conference, 5–8 May in Houston, Texas, U.S., and is published with permission. Shallow overpressures and shallow water flow (SWF) are significant hazards to deepwater drilling and facilities. In this paper we demonstrate how a combination of geomorphology, hydrogeology, and seismic interpretation has been successfully used in infer shallow overpressures, to develop a testable hypothesis of pore pressure distribution, and to delineate where to collect in-situ data. In-situ pore pressure measurements provide ground-truth data that can validate the model and be used to evaluate the hazards that internally driven failure might pose for facilities development. Several slope failures on the Sigsbee Escarpment in the Mad Dog and Atlantis field areas show a geomorphology distinct from “top-down” slope failures. Specifically, these slumps have steep, amphitheater-shaped headscarps, shallowly sloping bases, sharp inflection points with the surrounding seafloor at both the top and bottom of the headscarp, a linear trend in map view, and linear side walls. These geomorphic characteristics suggest that the slumps are formed by internally driven failure, and that the slumps grow retrogressively by headward migration. Within the Mad Dog and Atlantis field areas, Mad Dog Slump 8 and Atlantis Slump A are the best examples of this morphology; in this paper we will focus our attention on Mad Dog Slump 8 due to its proximity to planned facilities. The types of slope failures discussed in this paper are found on many margins worldwide. Their characteristic morphology indicates that the primary mechanism of slope failure is due to internal forcing (overpressure, weakening), rather than by downslope erosional flow. Internally driven slope failures can provide clues about the shallow overpressure regime, and provide exploration targets. Because the ocean is at constant head, and …

Reliability of Gulf of Mexico Deepwater Slopes

This paper describes the methodology used by BP on two deepwater projects to assess the reliability level of submarine slopes in the deepwater Gulf of Mexico environment. The paper describes the necessary steps in the analyses: establishing a geological model, properly characterizing the slope geophysically and geotechnically, carrying out deterministic slope stability analyses, performing a thorough review of potential triggering mechanisms, assessing how the slope safety factor will evolve over time, and estimating the annual probability of failure of a given slope. Possible means of estimating annual probabilities of failures from a FORM analysis are presented. The paper establishes that, even if a FORM analysis gives a relatively high probability of failure, the annual probability of failure can be several orders of magnitude lower if the trigger mechanisms are non-random and slow acting.Copyright