Comparison of Geotechnical Properties from Large-Diameter Long Cores and Borings in Deep Water Gulf of Mexico

Abstract

Large-diameter long piston cores (Jumbo Piston Corer, JPC) and Large-diameter Gravity Cores (LGC) were taken immediately adjacent to previously drilled geotechnical borings at three floating platform sites: Auger, Jolliet, and Marlin. This task was included as part of a more comprehensive NSF program on seabed processes in the deep water Gulf of Mexico. Sediment properties measured included bulk density, magnetic susceptibility, compression wave velocity, vane shear strength, and unconsolidated-undrained triaxial strength. A comprehensive geotechnical-testing program confirms the samples are high quality and shear strengths within the 63-ft core depth were comparable to the results of tests on the geotechnical borings. The exception occurred when gassy deposits were encountered. The use of the LGC and Multi-Sensor Core Logger (MSCL) in conjunction with the JPC proved to be valuable in assessing the quality and continuity of the piston cores. At the Auger and Marlin sites, there was good agreement between the sediment properties obtained from the borings and cores over the cored depth of 63 ft. At the Jolliet site, the values of strength obtained from the core in the upper 10 to 20-ft. were considerably higher than those obtained from the nearby boring.With modifications, the long coring system can be extended to take 100-ft samples. The use of large-diameter piston and gravity cores can provide an economical alternative to traditional borings for the design of shallow foundations for subsea completions, pipelines, suction caissons, and identification of geohazards. Project Description. This project was part of an integrated, interdisciplinary effort that includes substantial fieldwork, extensive laboratory analyses, and modeling of seabed processes (12). The fieldwork, including three deep-tow cruises and the long coring cruise, has been completed. The laboratory program focused on determining geotechnical and geological information and parameters necessary for the numerical modeling of seabed processes.

Case Study of Geotechnical Site Investigation using a Seafloor Drilling Unit, Large-diameter Cores, and Coring-vessel-deployed Cone Penetration Tests in the Gulf of Mexico

“…22 The large-diameter corer is lowered via a high-speed winch connected to the trigger assembly. 22 The large-diameter corer is lowered via a high-speed winch connected to the trigger assembly.…”

“…22 The large-diameter corer is lowered via a high-speed winch connected to the trigger assembly. 20,22 The 2012 geotechnical site investigation collected three large-diameter cores, one at each of the boring locations, which were taken at the center anchor of each mooring cluster. 6,22 These continuous soil samples can be logged with a Multi-Sensor Core Logging System (MSCL) and can provide visual confirmation of site variability.…”

Pipeline Embedment in Deep Water: Processes and Quantitative Assessment

“…Modern techniques for obtaining large diameter piston cores have led to improvement in sample quality over most of the sample length [3,4], but any slight delay in triggering the piston of gravity corers will lead to gross disturbance of the very shallow soils, and also to uncertainty in the depth datum [5]. The development of a static sampler activated from a fixed frame [6] should lead to further improvement, provided the shallow material has sufficient strength to allow samples to be prepared for laboratory testing.…”

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The paper outlines the processes that determine pipeline embedment in soft sediments typical of deep water, and describes quantitative methods for assessing the contribution to pipeline embedment from each process. All such measurements become more challenging in very soft sediments, either because of disturbance to the recovered samples or, for the in situ testing, due to minor errors in the depth datum or load cell zero readings, both of which become much more significant at shallow depths and low strength material.Modern techniques for obtaining large diameter piston cores have led to improvement in sample quality over most of the sample length [3,4], but any slight delay in triggering the piston of gravity corers will lead to gross disturbance of the very shallow soils, and also to uncertainty in the depth datum [5]. For each process, non-dimensionalized solutions are provided to evaluate embedment for given pipeline and soil properties, and different conditions operative during pipe-laying.

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Physical and Mechanical Characteristics of Shallow Surface Sediments in the Shenhu Area, South China Sea

“…Some marine engineering activities of human, such as the test mining of hydrate (Li et al, 2018), are likely to damage the stability of seabed sediments and lead to the occurrence of geological disasters . Young et al (2000) studied the characteristics of the Gulf of Mexico seabed sediments by using the large-diameter core, including volume density, cross-plate shear strength, and unconsolidatedundrained triaxial strength. Francisca et al (2005) studied the shallow surface sediments in the Gulf of Mexico and obtained that the near-surface sediments in this region were mainly composed of clay and powder grains.…”