Offshore Geotechnical Survey
Taka Hydrocore owns and operates geotechnical drilling rigs which complete with heave compensated systems which can be set up onto vessels of opportunity anywhere. The drill rigs use standard 5.5″ API drill pipe and have a capability to drill to 250m or other type of drill pipes. Wireline tools are available for deployment inside the drill pipe to perform Cone Penetration Tests (CPT) and to recover undisturbed samples. The drilling system also incorporates a coring facility enabling core recovery of soil and rock. The wireline system can also be used to raise and lower downhole geophysical logging tools.
CPT Piezocone Cone Penetration Test (PCPT or CPTu or CPT) with pore pressure measurement are performed in accordance with ASTM D-5778. The system used are meet the standards set by International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE), British Standard (BS) and ASTM standard. The 10 ton capacity wireline-operated PCPT system that uses an hydraulic Seabed Frame of 11 ton equipped with 1.5m external jack to ensure 2 ± 0.5 cm/sec penetration rate in accordance with ASTM Standard. This Seabed Frame acts as a seabed stabilizing/base for reaction and a stable feed system for 2 ± 0.5 cm/sec CPTu penetration for uninterrupted / continuous. Bottom Hole Assembly (BHA) having special locking device will be lowered into first API drill pipe/collar and the API Pipe will be clamped by the external jack system. The vertical thrust capacity of this equipment is 5ton (50kN) with penetrating stroke of 1.5m (optional to 3m). The 10cm2 AP van Den Berg electric piezocone with AP van Den Berg GOLOG data acquisition system (secondary equipment is Geomil PCPT system) are utilized. The system includes the umbilical and downhole tools which attached to the drill pipe. Real time data acquisition using a laptop with display on the screen during testing will enable the CPT Engineer/operator monitoring the testing. All AP van den Berg cones have a valid calibration certificate. The calibration work is also be performed before the first penetration of the equipment every work-day in the field. To maintain the accuracy of measurement, the cone tip is regularly cleaned to remove clays or colloids that trapped in piezocone tip. Data processing of PCPT log were performed using GO4 software which are present Qc (tip resistance) vs depth, fs (sleeve friction) vs depth, u (pore water pressure) vs depth, i (inclination) vs depth. This software has capability to present the estimated soil properties including corrected estimated Qc, corrected fs, relative density, undrained shear strength, estimated soil layer, etc.
The offshore sampling is use a downhole BHA (Bottom Hole Assembly) with constant WOB (Weight on Bit) controlled by heave compensator system. This BHA will be automatically attached to the API drill pipe. The following offshore proven methods which in accordance with offshore international standards will be employed in undisturbed sampling.
Undisturbed sample in very soft to soft (or medium stiff) clay layer
Wireline Hydraulic piston sampler with 76mm OD thin walled tube stainless steel will be used to obtain high quality undisturbed soil samples in very soft and soft cohesive materials, in accordance to ASTM D6519 Standard.
Undisturbed sample in medium stiff to very stiff clay layer
Wireline Push sampler, a Thin walled Shelby tube 70mm ID with Bottom Hole Assembly (BHA) will be used for undisturbed sampling in accordance with ASTM. The stainless steel tube of 76mm OD is used for sampler. The Shelby tube push sampler with its BHA will penetrate at constant pressure.
Sampling in sand & hard Clay
Wireline Thick walled Shelby tube 3” will be driven by wire line hammer or a wireline Mazier sampler will be used for sampling in sand and hard clay. Where the thick walled hammer sampler can retrieved the sand sample, the specific sand sampler with attached catcher will be utilized. The thick wall split spoon sample will also available for this sand sampling work. Wireline triple core barrel HQ3 with diamond bit will be utilized to obtain good core samples. The undisturbed soil samples (UDS) were extruded in onboard/offshore laboratory. The extruded samples are divided into 2-3 parts. The 1st and/or 2nd parts are tested in offshore lab, the reamining parts are covered with aluminium foil, placed in PVC and the space between PVC and aluminium foil will be sealed with wax. All undisturbed samples will be clearly marked to show the borehole identification, the depth from which the sample was taken, top and bottom of the sample and many other information which may be helpful in determining sub-surface condition. All undisturbed samples will be protected in every possible way to avoid disturbance of the samples during transportation. All samples will be protected from excessive heat. All samples will be carefully packed to prevent damage during storage or transportation. The samples will be sent to osnhore laboratory as soon as possible after teh vessel demobilization. The extruded samples will be photographed using at least 10 Mega Pixel camera resolution. A photographic colour chart will be included in each chart in order to have a colour reference. The time between commencement of preparation and the examination of the prepared and photographical smples will be minimized to prevent loss of moisture from the core samples. All soil and rock description will be in accordance with ASTM D2487 and D2488.