Geotechnical Properties of the Sigsbee Escarpment from Deep Soil Borings
Authors: Al-Khafaji Z.A. , Young A.G. , DeGroff W. , Humphrey G.D. 
Published: October 2008

The engineering properties of soils are used in the design of various foundation systems to support and anchor offshore oil and gas platforms. Engineering soil properties are also used for detailed geohazard studies. Considering the significance of the facilities located in offshore deepwater environments and the complexity of the geological environments in which their foundations are installed, extensive geotechnical siteinvestigations are performed using many tools and specialized tests.

This paper presents the wide variations of the soil properties along the Sigsbee Escarpment. The geologic and geotechnical data resulted in dividing the seafloor into three geologic provinces; the Lower Continental Slope, the Sigsbee Escarpment and the Upper Continental Rise. The geotechnical properties of the normally consolidated and overconsolidated clay soils encountered within these three geological provinces are presented and compared with those observed at other locations in the deepwater GOM. In particular, the interpreted stress history and the normalized shear strength properties are presented and discussed.

Introduction

Understanding the geotechnical properties of the various soils encountered at complex geological settings in deepwater regions is essential to plan appropriate geotechnical site investigation and laboratory testing programs. Also, documentation of the geotechnical properties of the soils encountered at complex geological provinces is key to improve the evaluation process for future developments.

Extensive field geotechnical investigations, including recovery of soil samples using jumbo piston cores and deep soil borings as well as performance of in situ tests, have been conducted at two of BP's deepwater prospects (Mad Dog and Atlantis). As shown in the regional rendering presented in Figure 1, both prospects are located along the Sigsbee Escarpment, in the Green Canyon Area of the Gulf of Mexico.

This paper discusses the geotechnical properties of the normally consolidated and the overconsolidated soils encountered at the locations of four soil borings drilled at the Mad Dog Prospect in Blocks 782 and 826 and four soil borings drilled at the Atlantis Prospect in Block 743 of the Green Canyon Area in the Gulf of Mexico.

Geologic Setting

Mad Dog Prospect

The Mad Dog Prospect is located along the Sigsbee Escarpment in the Green Canyon Area at the southern extent of the northern Gulf of Mexico Continental Slope. The Escarpment represents a complex topographic and geologic feature involving steep slopes, faults, and slumps. The upward and lateral movement of underlying salt resulted in the seaward movement of sediments and subsequent deformation and over-steepening of the slope. The steep slope angles lead to slope instability producing gravity driven slumps. The isometric view presented in Figure 2a reveals a total of eleven slump features that have been mapped in the Mad Dog Prospect area.

Atlantis Prospect

The Atlantis Development is also located along the Sigsbee Escarpment in the Green Canyon Area at the southern extent of the northern Gulf of Mexico Continental Slope. The Escarpment represents a complex topographic and geologic feature involving steep slopes, faults, and slumps. The vertical and horizontal movement of the underlying salt nappes resulted in the deformation of the overlying sediments and over-steepening of the slope.