Harpoon Free Fall Cone Penetrometer
Application to Deepwater Seafloor Site Investigation
The Harpoon FFCPT adds an in situ geotechnical test capability to any conventional piston or gravity coring system. 
The FFCPT mounts in place of the soil sampling cutting shoe and can operate autonomously in any water depth up to 3,500m. The FFCPT tool is driven to refusal by the kinetic energy of the piston coring equipment. A high speed data acquisition system continuously monitors the sediment shearing response, by means of a suite of robust sensors mounted inside the FFCPT tool.
Onboard sensors include a 2g, a 5g and a 50g accelerometer (for sensing the sediment dynamic penetration resistance), a 100 psid pressure transducer (for sensing dynamic porewater pressure response) and an optical backscatter sensor (for mudline detection).Harpoon FFCPT equipment is easily mounted and deployed using existing piston coring systems, making the technology transferrable to any vessel capable of coring operations.
The continuous profiling of the sediment column and generation of high-quality CPT-type data means that reconnaissance geotechnical site investigations can now be routinely carried out at deepwater locations.
To the left is a photo of the Harpoon FFCPT tool, after recovery to the ship’s rail. Field test results shown below were obtained with the assistance of the Geological Survey of Canada (Atlantic), during a 2004 piston coring project on the Scotian Slope. The GSC Atlantic piston coring system weighed approximately 1,500kg and the FFCPT probe diameter was 16.8cm, giving an end area of 222cm2.Analysis of high quality piston core samples showed the sediment column to comprise 1.6m of biologically-cemented silty clay, overlying soft low plasticity silty clay which extended to the depth of investigation (11.7m). Peak miniature vane shear strengths (green dots) are plotted alongside FFCPT derived Su estimates, obtained from classical CPT penetration resistance and excess pore pressure methods.
The Harpoon Free Fall Cone Penetrometer Test (FFCPT) Operational Procedure
Harpoon FFCPT on workbench being prepared for deployment during July 2004 Geological Survey of Canada (Atlantic) research cruise. The FFCPT tool was designed for operation on the end of piston coring systems.
GSC Atlantic Large Diameter Piston Corer at rail onboard CCG Hudson. The photo shows the corer being disassembled after collecting a sediment core sample. The Harpoon FFCPT mounts on the core barrel in place of the cutting shoe.
Harpoon FFCPT shown supported by the vessel’s piston core barrel monorail system. When connected to the GSC Atlantic piston corer, the overall system weight is 1650kg. Different barrel adapter couplings allow the FFCPT to be used with other corers.
Harpoon FFCPT after being fastened to the core barrel. The porous filter ring is mounted above the shoulder of the conical point (hidden inside a sealing clamp). The filter connects external water pressure to a Tip Pore Pressure Transducer. A hand clamp maintains saturation in the porous filter prior to deployment.
View of the piston core barrel and Harpoon FFCPT, looking toward the corehead. The corer is laid horizontally in a rack at the rail for system setup and disassembly. At this stage the penetrometer is in low speed active acquisition mode. Just before impact with the seabed, it will be triggered into high speed data acquisition mode.
Close-up photo of the FFCPT. The tip has a replacement wear point and a bronze ring that allows the porous filter to be easily changed. The interior of the penetrometer houses the battery pack, sensor package and datalogger. Sensors include an Optical Mudline Detector, a Tip Pressure Transducer and 3 Vertical Accelerometers.
Photo showing the Harpoon FFCPT system ready for deployment. The chain falls and other arresting devices have been removed from the core barrels. Each core barrel section is 3m in length. Harpoon FFCPT tests have been successfully undertaken with as many as 5 core barrels mounted.
Harpoon FFCPT being deployed. The corer free trips and free falls into the seabed once the weight hanging from the horizontal trip arm contacts the mudline. The rigging, deployment and recovery of the Harpoon FFCPT piston corer takes less time than required for conventional piston coring.
Photo showing recovery of the Harpoon FFCPT system. The mud smear on the core barrel couplings shows the apparent depth of penetration. Data analysis provides a very accurate measure of the actual penetration distance. The core barrels bend as they are raised to the horizontal position where they are secured at the rail.
Harpoon FFCPT after a successful deployment. Note the sediment smear on the probe. The probe is attached to the core barrel by means of a split coupling, which also acts as a friction reducer. This adapter allows the penetrometer to penetrate deeper into the sediment.
Close-up photo of the FFCPT coupling, showing how sediment smeared the probe but not the core barrels. This friction reducing design significantly increases the overall depth of penetration by eliminating the build-up of skin friction on the core barrels. Penetrations of up to 30m are achievable with this arrangement.
The Harpoon FFCPT represents a breakthrough in marine geotechnical site investigation. In situ testing and collection of data for engineering analysis can now be undertaken at far less cost than ever before. The equipment can operate in 3,500m water depths and can be used with any coring system.For more information on the Operational Procedure for the Harpoon Free Fall Cone Penetrometer, contact ODIM Brooke Ocean.
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The Free Fall Cone Penetrometer (FFCPT) was developed jointly with Defence Research and Development Canada Atlantic.
Contents Copyright ©2008 ODIM Brooke Ocean
URL: http://www.brooke-ocean.com/ffcpt-harpoon-01.html (Last Updated: Apr 24, 2008)