A free fall cone penetrometer (FFCPT) probe was developed by ODIM Brooke Ocean Technology Ltd for use in offshore geotechnical foundation condition assessment and seabed characterisation. The probe measures acceleration and pore pressure as a function of depth of penetration into the seafloor. This combination of sensors provides two independent means of calculating undrained shear strength, as well as engineering variables that are used to identify sediment grain size characteristics. The Harpoon FFCPT couples to a large piston coring system, penetrating normally consolidated sediments 10 to 15m below the seafloor. Results show strong correlation of undrained strength profiles derived from the dynamic penetration response of the tool with predicted (normal consolidation) and measured (mini-vane and triaxial vane) strength profiles from companion cores, spanning a variety of sediment types and water depths. The FFCPT tool provides the potential for acquiring in situ engineering data rapidly and without the need for extensive sample collection and laboratory analysis.
Marine sediment physical property data are required for a number of applied purposes in addition to scientific understanding. Engineering applications include offshore dredging, aggregate mining, hydrocarbon exploratory drilling and production development, and pipeline trenching (as a few examples). Naval defence require seafloor property information for mine burial assessment and understanding seafloor acoustic propagation characteristics. Geoscientists require equivalent information for assessing conditions of slope stability, sediment transport and linking geological to geophysical data. Biologists require seafloor classification information for habitat assessment.
The principal means for acquiring such data of near-surface seafloor sediment engineering data has been either acquisition of sediment cores combined with laboratory measurement of recovered sediment, or in situ testing with conventional cone penetrometer testing (CPT) using drill rigs or similar technologies1. Equipment for such in situ data in deep water are largely restricted to the Penfeld CPT2 and a recent development of a free fall cone penetration tool by a group at the University of Bremen3. ODIM Brooke Ocean Technology (ODIM BOT), with funding support from Natural Resources Canada and Petroleum Research-Atlantic Canada, developed two models of a free fall cone penetrometer in response to this recognised need for in situ offshore engineering data that could be provided rapidly and yet ensuring safe and economical development of offshore resources.
Shear strength of sediments is the internal resistance per unit area that the sediment possesses to resist failure4. It is an important parameter for foundation of seafloor structures and for understanding the stability of the slope. Shear strength is determined in several ways. In situ methods, such as the field vane shear test and CPT, determine the shear strength indirectly but avoid some of the problems of disturbance associated with the extraction of soil samples from the ground5. Coring and laboratory methods, on the other hand, yield the shear strength directly and give valuable information about the stress-strain behaviour and the deployed from either a moving vessel profiler (MVP) winch or standard winch. From the latter, the vessel must be stationary and cable is flaked on the deck providing slack for the free fall operations.
