Geotechnical Properties of the Sigsbee Escarpment from Deep Soil Borings

Abstract

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.

Geologic Setting of the Mad Dog Mooring System

“…The shallow sediments along the Lower Continental Slope (northward of the escarpment) are comprised of normally consolidated, hemipelagic sediments (Al-Khafaji et al, 2003). These highly plastic Holocene clays overlie older, stiffer, and lower plasticity Pleistocene clays.…”

Age Dating of Past Slope Failures of the Sigsbee Escarpment within Atlantis and Mad Dog Developments

“…Boring ASB-2 was drilled to a depth of 500 ft (152.4m) close to Core EE-4 and penetrated 10 to 20 ft (3.1 to 6.1m) of normally consolidated clay overlying heavily overconsolidated clays (10). Boring ASB-2 was drilled to a depth of 500 ft (152.4m) close to Core EE-4 and penetrated 10 to 20 ft (3.1 to 6.1m) of normally consolidated clay overlying heavily overconsolidated clays (10).…”

Characterization of Soft Deepwater West Africa Clays: SHANSEP Testing is Not Recommended for Sensitive Structured Clays

“…When the SHANSEP method is used to measure the shear strength of Gulf of Mexico clays, it generally gives good correlation with the strengths obtained from other tests, such as the vane shear test or the in situ CPT test (Quiros et al [8], Al-Khafaji et al [9]). This paper presents the laboratory triaxial tests performed with the procedure mentioned above in order to verify the possibility of applying the NSP concept to soft deepwater GoG clays.…”