Geophysical survey often provides the principal sub-bottom information for offshore developments, particularly in deepwater areas. Ground investigation - sampling and in-situ testing - serves as ground-truthing tool and provides parameter input for geotechnical design models. Measurement of shear wave velocity is feasible for practically any solid. This includes gassy, underconsolidated clays and heavily overconsolidated ground, not uncommon in offshore geotechnical practice. This paper addresses benefits and limitations of measurement of in-situ shear wave velocity in offshore environments. Factors include
on-bottom equipment time for deepwater operations in combination with cone penetration testing (CPT),
operational constraints, and
geotechnical interpretation and integration
Energy supply to deepwater tools is one of the key challenges for the offshore industry. This applies to the use of underwater vehicles, foundation installation tools, and also to operation of geotechnical investigation equipment. Addition of shear velocity measurement implies energy consumption for operation of a shear wave hammer. This will affect on-bottom time of battery-operated CPT equipment for deepwater investigation.
Important other offshore operational constraints include robustness and depth interval control Operational robustness is a must for any offshore activity, with growing importance as developments move to increased water depths. Shear wave velocity measurement is no new development. More than 10 years of experience in shallow water is available for use in deepwater exploration.
Depth interval control is particularly important for shear wave velocity measurement using wireline equipment that latches in a heave-compensated drill string operated from a vessel. Such control is limited, particularly where interpretation relies on measurements made in adjacent strokes of the wireline tool Verification with measurements made within a single stroke is important Geotechnical interpretation benefits from the use of a single tool for acquiring CPT information and shear wave velocity. CPT information provides detailed stratigraphic information and input into analysis of an ultimate limit state or failure of a facility. The additional shear wave velocity measurement can serve as input into seismic ground response analysis and/or static deformation response of a facility. Selected examples illustrate opportunities for data interpretation and use.