The DISC Drill is a tilting-tower electromechanical drill designed to take 122 mm (4.8 inches) diameter ice cores to depths of 4000 meters with variable core lengths up to a design limit of 4 meters. (The drill is currently able to recover cores up to 3.5 meters long.) Because of the great depths, the drill is designed to operate in a drilling fluid, which provides the hydrostatic compensation necessary to prevent closure of the borehole due to the "glaciostatic"pressure of the surrounding ice.
The rotating drill head contains 4 razor sharp cutters that shave out an annulus of ice, which the 14 m long drill slides down into. As the drill slides down into the annulus it slides over the core, which is 12.2 cm in diameter. When a cable pulls up the drill, four cams grab the core and fracture it. After the drill is pulled back to the surface, it is lowered from a vertical to a horizontal orientation. An arrow is marked on the core to indicate which side of the core was facing north when the core was in the ice sheet.
The DISC Drill consists of several major subsystems.
The sonde is the down-hole portion of the drill system that cuts the ice. The sonde consists of a cutter head, a core barrel in which the core is collected, screens to remove chips from the drill fluid, a motor and transmission to drive the cutter head, a pump to circulate ice cuttings in the drill fluid through the sonde, an instrumentation/ control section, and an anti-torque section to stabilize the drill in the borehole and the mechanical, electrical power, and optic fiber terminations of the cable which include rotary joints that allow the drill sonde to rotate relative to the cable.
The tower consists of a mast (constructed of modular truss sections) at the top of the borehole with a number of pulleys to allow for the raising and lowering of the sonde and positioning the sonde for removal of the core. The DISC Drill tower tilts from vertical to horizontal to allow easy core removal and drill sonde servicing.
The winch provides the means of hoisting the drill from the borehole. The components of the winch system include the winch drum, which can hold 4,400 meters of cable, a level-wind system that allows for the payout of cable from the winch drum with a single sheave, and a drive system. The winch is electrically driven and unusual in that the drum axis is parallel to the drill cable as the cable runs between the winch and tower with the level wind traversing back and forth across the width of the drum in line with the cable. This configuration allows the winch to be connected to the tower base resulting in a smaller footprint.
Control for the entire drill system is PC based with the user interface being developed using LabView. Data from the drill sonde, the tower, and the winch are displayed and drill control parameters set using the computers. The computers are installed in a 1.78 meter x 3.53.meter x 2.00 meter insulated and temperature controlled room which is positioned to allow the operators to observe the drilling equipment and operations.
Drill Fluid and Fluid Handling
The borehole fluid provides hydrostatic compensation, necessary to prevent closure of the borehole due to glaciostatic pressure of the surrounding ice. The first project utilizing the DISC Drill (WAIS Divide) uses a two-part fluid similar to that used by the European deep coring projects. The heart of the fluid handling system is a tank in which the two drill fluid components are mixed. All associated measurement devices, valves and pumps are integrated into the tank assembly and the mixing is controlled by a microprocessor. The remainder of the system consists of piping to transport the fluids to the tank for mixing and piping to inject the mixture into the borehole. Fluid that drips from equipment and that is recovered from the ice chips is returned to the borehole. A centrifuge is used to remove fluid entrained in the recovered ice chips; the fluid is returned to the fluid system.
Screen cleaning equipment includes a station where the screens are removed from the housing and the individual screen sections cleared of ice chips. The screen sections are reinserted into the housing at the screen cleaning station. The screen cleaning station is semi-automated. The chips are collected in a bucket that is inserted into the centrifuge for the removal of fluid from the chips.
Equipment not directly involved with the drilling operation, but integral to the system, includes a shop facility and a gantry crane. The shop, designed to facilitate repairs to the drill in the field, is contained in an expandable container shelter housing a machine shop, a welding shop, and an electronics shop. The A-frame gantry crane runs on rails that are used by the smaller core handling crane and is used to move equipment, particularly during drill system installation and “tear-down.”