Advanced deep-ocean rock-coring systems

Advanced deep-ocean rock-coring systems

McGinnis, Tim

New Class of Seafloor Coring System Provides Cost-Effective Extraction of Rock, Consolidated Sediment Core Samples

For a variety of geological, geotechnical, geochemical, and other studies and surveys of the seafloor, high-quality samples of the seafloor materials are highly desirable if not critical. A number of techniques and tools have been developed to collect these samples with advantages and disadvantages associated with each.

For rock sampling, the techniques include dredging for surface rocks, grabbing or cutting by ROV or manned submersible, and coring with a drill ship. Rock dredging is inexpensive but does not allow much control over where the samples are taken and is limited to material that is loose or can be broken off the surface. Drill ships allow position control and penetration into the seafloor material but this comes with high cost and limited availability.

The geologists and engineers at Williamson & Associates operate several deep-towed side-scan/multibeam/sub-bottom profiling sonar survey systems and frequently are tasked with obtaining seafloor samples for ground-truthing the sonar data or other scientific or engineering purposes. This interest in obtaining high quality seafloor samples has led Williamson & Associates to develop a series of remotely controlled seafloor rock-coring drills that offer a portable and costeffective alternative for obtaining vertical core samples from seafloor rock formations.

These drills offer improvements over previous systems by providing interactive drilling through the use of an extensive array of sensors and controls which give the operator accurate, real-time feedback on the drilling performance and allow fine control over the drilling functions to react and adjust to changing conditions.

UW 3-meter Drill

A seafloor rock-coring drill capable of obtaining a 3-meter long core sample was designed and built by Williamson & Associates for the University of Washington (UW) in 1991. The National Science Foundation, Office of Naval Research, and Washington Sea Grant provided funding.

The UW drill design has a frame with a triangular base 3 meters on a side and 5 meters high, weighs 1,800 kilograms, and can be disassembled to fit in a 20-foot shipping container. The drill uses a standard 0.68-armored coaxial cable and can be operated in water depths to 5,000 meters deep.

The rock-coring process uses standard diamond impregnated land coring tools that acquire a 33-millimeter diameter core sample. The operator has control over and feedback from bit weight, rotary speed, and flushing– water pressure. Due to the type and length of the cable, the system utilizes slow-scan video telemetry. Auxiliary controls include frame leveling and video pan & tilt.

The UW 3-meter drill has been successfully used to obtain core samples to water depths of 2,400 meters at several sites in the Pacific Ocean, including pillow basalts and volcanic sheet flows at a mid-ocean spreading center on the Juan de Fuca Ridge. Unfortunately, the drill was lost on a drilling program off the coast of Costa Rica when the umbilical cable became tangled in the drill base from a previous JOIDES Resolution Ocean– Drilling Program site.

Benthic Multicoring System

The next generation of drilling system, with the extended capability of obtaining a 20-meter-long core samples, was designed and built by Williamson & Associates for the Metal Mining Agency of Japan (MMAJ) in 1996. The Benthic Multicoring System (BMS) has an operating depth of 6,000 meters and the capability of recovering continuous core samples in hard rock to a depth of 20 meters. The BMS is currently installed on MMAJ’s research vessel Hakurei Maru No. 2.

The BMS uses standard coring tools to collect core samples with a diameter of 44 millimeters to a depth of 20 meters. A rotary magazine holds up to 30 coring tools consisting of core barrels, drill rod, and casing. A variety of diamond and carbide bits can be used to enable coring in a variety of materials. Borehole casing can be set to prevent hole collapse when coring through soft or unstable formations. The BMS is equipped with an extensive suite of sensors, five video cameras and has 60 hydraulic functions, including joystick-controlled thrusters for positioning prior to landing and telescoping legs for leveling on slopes. Data displayed to the operator from the onboard sensors provides real-time feedback from the coring process. Coring function controls are provided that include proportional control of bit weight, bit torque, bit-rotation speed and flushing-water flow. Several coring operations that involve a number of repetitive steps, such as making up, breaking down, and storing the drill tools are automated and controlled by the vehicle-computer.

The BMS has a liter optic telemetry system with four video and eight serial-data channels that transmit video signals and sensor data and control information between the drill and the operator’s console on the ship. The BMS operates over a 12,000-meter long electro-optical-mechanical umbilical cable.

The drill’s functions are driven from two 15-horsepower hydraulic power units. The positions of all the actuators involved in the handling of the drilling tools are monitored with proximity and/or linear displacement sensors. Pressure transducers are used to monitor the coring torque, bit weight, leg loads and flushing water pressure.

From the operator’s control screen, the position status of all tool-handling actuators and coring-system functions can be monitored as well as the type and location of tools stored in the magazine. Elevator-position, casing– depth, bit-depth and hole-depths are displayed to the operator.

The drill’s computer can do drillstring make-up manually or automatically. In manual mode, the operator uses a mouse to click on the appropriate icon for an actuator displayed on the control screen. The control screen updates when the appropriate signal has been received from the drill that the actuator has moved. By selecting the proper sequence of actuators, an operator can make up and break down a drill string. Manual operation is tedious, slow, and prone to human error so an automatic mode was developed. In automatic mode, the operator selects the desired operation from a menu. These operations include run/recover core barrel, run/recover drill rods, run/recover casing, etc.

During the auto routines, the computer advances through the steps in the routine, updates the screen icons, and sends messages to the operator. If an actuator fails to respond or is out of position, the routine will pause and prompt the operator to fix the problem, ignore the warning, or exit from the routine. The actual coring procedure is done manually with the operator controlling the drill rotation, bit weight and flushing water by sliding the control icons on the screen.

The casing can be used to keep cuttings from re-entering the hole and to keep the hole from collapsing when coring in unconsolidated material.

The BMS has been used on several deep-ocean drilling programs throughout the Southeast Pacific area to recover hard-rock core samples in water depths to 2,300 meters. In July 1998, the BMS was successfully operated at three sites in a volcanic area near the Western Margin of the Pacific Ocean about 700 miles southeast of Okinawa. These sites were located on the crest of a ridge, on a flat area near the base of the ridge, and on a small knoll on the slope of the ridge and ranged in water depth from 1,180 to 1,960 meters. Coring depths from 4.2 to 8.0 meters were achieved. The core samples contained volcanic sediments (fine-grained sand), sandstone, basaltic lava, and hydrogenous ferromanganese oxide crust. Two of the sites had unconsolidated sediment cover and required the use of the borehole casing.

Future Systems

Williamson & Associates participated in the design and construction of the portable remotely operated drill (PROD), which is currently undergoing sea trials. PROD is the next step in the evolution of remotely operated seafloor coring drills and will have a water-depth rating of 2,500 meters and a drilling depth of 100 meters. PROD is owned by Benthic Geotech which is a consortium that includes the University of Sydney (Australia), Nichyu Giken Kogyo (Japan), C. H. Warman (Australia), Protech Development (Australia), Williamson & Associates (USA) and others. Williamson & Associates and BGT will offer PROD for use on commercial, research, and other seafloor investigation projects.

Planned future enhancements include the ability to make in-situ measurements of porosity, resistivity, and other properties. Development of methods for hole re-entry and the sealing of samples are currently being done.

Conclusion

These portable seafloor-coring systems have proven to be reliable and functional and provide a seafloor sampling capability that is not available with any other existing equipment. The use of proven ROV and land-coring technology has minimized the amount of new design required resulting in reduced cost, risk, and time to design and build. /st/

By Tim McGinnis

General Manager

Williamson & Associates, Inc.

Seattle, Washington, USA

Tim McGinnis received his bachelor’s degree in ocean engineering from the University of Washington. He has 20 years of experience with the design

and operation of a wide variety of deepocean sonar, sampling, and data-acquisition equipment.

Copyright Compass Publications, Inc. May 2000

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