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

Abstract

Abstract This paper compares results of a deepwater geotechnical investigation using a seafloor drilling system, large-diameter core samples, and cone penetration tests performed from a coring vessel with results of a geotechnical investigation performed from a conventional drillship. The paper reports the first use of a seafloor drilling unit in the Gulf of Mexico to conduct a geotechnical site investigation for the mooring design of a floating platform. Common practice in the Gulf of Mexico for geotechnical investigations of anchor spreads for moored production facilities has been to perform seafloor cone penetration tests and to drill and sample one or more soil borings. "Soil borings" are prescribed but not defined by 30 CFR 250.19 API RP 2SK1 recognizes currently available site investigation tools, such as large-diameter core samples, that could be used in lieu of rotary soil borings; however, some certified verification authorities/agents (CVAs) have been reluctant to accept the use of API-accepted methods of site investigations in the Gulf of Mexico. Seafloor drilling units have been used successfully in various parts of the world, and cone penetration tests from a coring vessel have been performed and studied at numerous sites, both in the Gulf of Mexico and worldwide. This paper demonstrates that a seafloor drilling unit, large-diameter cores, and coring-vessel-deployed cone penetration tests integrated with AUV subbottom profile data can be used in the Gulf of Mexico without reliance on samples obtained from a drillship. Cone penetration resistance is compared using different cone systems and deployment methods at nearby locations. Results are essentially identical. Representative soil sample data are compared, and geotechnical laboratory test results are very similar. The paper confirms that when ASTM standards for cone testing and thin-walled-tube sampling are used, consistent results can be obtained from site investigations performed from seafloor drilling units and coring vessels or purpose-built geotechnical drillships. Integrated site investigation programs using technologies such as seafloor drilling units, large diameter coring, and cone penetration testing from a coring vessel should be acceptable for site investigations without special regulatory approvals beyond those that would apply to a site investigation program performed from a purpose-built geotechnical drillship.