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.
To quantify the effects of methane gas on mechanical properties of soft marine clay, an exhaustive laboratory testing program was developed using zeolite to uniformly disseminate gas bubbles inside the clay matrix. Results from controlled rate-of-strain (CRS) tests indicated that as the gas content increases, there is a reduction in the interpreted preconsolidation pressure, although the rigidity of the clay with more gas increased throughout the test. Minivane test results indicated that the undisturbed shear strength decreases as the amount of methane gas increases, while the residual and remolded strengths remain practically unchanged, i.e., are independent of the gas content. Similarly results from triaxial tests indicated that the undisturbed shear strength is reduced as the gas content increases, but there was no change in the failure mode. Interestingly, the normalized shear strength increased for the clay with gas, when the samples were tested at 100 percent of deformation per hour. It is theorized that the methane gas bubbles interact with both the clay platelets and the pore water, and, to certain point, bear part of the load, thus modifying the distribution of the load in the soil structure; that is to say, there is a partial load transfer from the gas bubbles to the soil structure, as the clay particles confine the methane gas.
Relative changes in Waste shear strength parameters as a function of strain level and stress path are investigated based on the results of 16 direct simple shear (DSS) tests, one direct shear (DS) test with four stages, and three triaxial tests. The magnitudes of shear strength parameters obtained from drained DSS tests and undrained DSS tests with pore water pressure measurement were comparable. This was the case even though the effective stress path in both approaches was different. Data indicated the dependency of the mobilized strength parameters on strain, or deformation level. Generally, stress-deformation response increased monotonically with no well defined peak or ultimate stress levels. The results of the DSS and DS tests show no dependency of the strength parameters on the stress level. Results from DSS and DS indicated a range of effective strength parameters of 9 to 14 kPa for cohesion and 23°–29° for friction angle. Data from the triaxial testing showed dependency of the shear strength parameters on the initial compression stress level. Given the number of potentially confounding issues associated with the measurement of shear strength, it is rather important to also report information on sample collection methods, sample age and chemical composition, sample processing, sample composition, the size of testing equipment and level of strain (instead of ultimate or peak) at which the strength parameters are evaluated.
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