RAID CONFIGURATION AND OPERATION
The RAID system consists of five ski-mounted modules – drill, rod, fluid recirculation, shop and power (described here). The first two were built in custom-designed and fabricated cold-tolerant steel frames and the latter three modules were fitted inside 40 ft standard ISO shipping containers. Drilling is done with a modified Boart Longyear LF-230 rotary exploration-type drill using custom augers for making boreholes in firn, and with NQ drill rods equipped with custom ice-cutting and coring tools for making boreholes in ice and rock. The system is driven by an electric motor powered by redundant diesel Caterpillar C-18 500 kW generators, operating alternately. The modules are mounted on ISO-3 ski systems with an articulated, walking-beam design for flexibility during movement over uneven surfaces without stressing the module structures. During field drilling operations, the RAID modules are aligned in two rows, in close enough proximity to allow power and hose connections. The modules were moved to Minna Bluff and between drill sites using Caterpillar Challenger and Case Quad-Trac tractors. Once in place, a fabric canopy is erected over the work deck connecting the drill and rod modules, supported by the drill mast. Liquid consumables required for operation are diesel fuel, glycol used in heat exchange, and ESTISOLTM 140 drilling fluid.
During drilling, the system is operated by a lead driller on the LF-230, with assistance from 1 to 2 drill hands to handle auger and pipe assembly and disassembly, and one person to oversee the automated fluid recirculation system (FRS). Ideally, regular operation of the RAID system can be managed by a 3–4 person crew.
FLUID RECIRCULATION SYSTEM
The FRS is a custom-designed module for supplying cold drilling fluid to the borehole and separating drilling fluid from ice cuttings as they are evacuated from the borehole. It is the heart of the RAID system. A schematic layout of the FRS is shown below, as are photographs of the main components inside the FRS. The FRS is divided into a cold side kept at ambient temperature and a warm side heated sufficiently to maintain liquid water above freezing.
Supply of fresh, cold drilling fluid is provided by tanks in the cold room prior to injection into the borehole.
Drilling fluid returned from the borehole with suspended ice cuttings enters the cold room and undergoes a first-stage mechanical separation from cuttings by means of vibrating shaker screens. Clean, cold fluid is captured below the screens and pumped through filters back to the supply tanks.
The remaining cuttings wetted with drilling fluid are passed through a room divider and dumped into a melting tank to create a two-phase mixture of water and drilling fluid, and then this mixture is pumped into a second-stage coalescing tank where the lighter oil-based drilling fluid is floated above water, skimmed by a tubular weir, and the two liquids are then drained into separate single-phase tanks. Processed fluid is then filtered before returning to the cold-room supply tanks where it is pre-chilled prior to pumping back into the borehole.
All of the tanks, pumps and filters are monitored by a programmable logic control system in order to maintain tank levels, exchange rates and temperature.
ESTISOLTM 140 was selected as the optimal drilling fluid for the RAID system because of its ideal density, low freezing temperature, hydrophobic property, low viscosity at cold temperatures and colorless, translucent property suitable for borehole optical logging. ESTISOLTM 140 is a synthetic, ester-based solvent with some known negative material effects and physical irritation. It is absorbed by, expands and degrades some common materials used in seals, pipe and tool housings (e.g. polyvinyl chloride, nitrile rubber, and many common plastics), so appropriate alternatives must be used. The fluid has a sweet odor that some people find unappealing, but it evaporates quickly after absorption in clothing and absorbent materials if left in a warm, dry setting. The fluid also can be a skin irritant, particularly causing severe drying due to removal of natural oils. Our drillers generally find the workability of ESTISOLTM 140 to be superior to other drilling and machine fluids, including diesel fuel, and with proper protective clothing and ventilation it is easy to work with. Despite its low vapor pressure compared to other drilling fluids, some physical side effects can be anticipated due to evaporation from clothing in heated and poorly ventilated spaces on the Polar Plateau.
RAID’s 8.9 cm (3.5 in) ice boreholes require 0.62 m3 (164 gal) of drilling fluid for every 100 m of borehole length, plus provision for filling supply tanks in the FRS, making hose connections, and possible losses due to hydrofracture. After scientific drilling, all RAID boreholes will be left filled with drilling fluid in order to maintain open boreholes for 5 years for follow-up wireline logging, so an adequate supply of drilling fluid must be provided at the beginning of a drilling season.
The RAID system can be divided into five major parts: the surface drill, the FRS, an upper firn layer requiring casing to prevent fluid escape, a middle ice borehole, and a lower coring section, as shown diagrammatically below.
In the borehole sections, a combination of ice cutters on NRQ drill rods and a bottom-hole coring assembly allow for: (a) fast ice borehole cutting up to 3300 m depth; and (b) efficient wireline coring up to 25 m length using custom latching tools. Individual tooling is shown below.
BOREHOLE AND CORING TOOLS
RAID uses a combination of stainless steel ice-cutting bits, formatted with both outer and face-centered cutters, the latter of which can be removed with wireline latching devices. Our bottom-hole tools are custom-built by manufacturers in the U.S.
RAID considered a variety of ways to quickly cut a wide hole in firn to accept an inflatable packer. Ultimately, we settled on use of modified soil augers. Our current iteration of these are custom-made steel augers fabricated in 5-ft sections, with helical flights welded to a hollow central stem to reduce weight. The bottom auger is faced with a 6″ OD steel cutting head optimized for most efficient cutting and chip transport. Individual or paired auger sections are joined by raising on the drill’s main line as they are introduced to the borehole. Our augers are custom-built by a manufacturer in the U.S.
RAID uses an industry-standard borehole packer to make a fluid-tight seal against the auger borehole wall before we start fluid circulation. A packer consists of a section of casing pipe wrapped with an inflatable rubber membrane for making a seal with the borehole. The packer is inserted on HWT dimension steel casing to a depth considered to be impermeable and inflated using pressurized nitrogen gas. Our packer is custom-built by a manufacturer in Australia.
In collaboration with engineers from the Ice Drilling Program, we designed and built a special chip ‘bailer’ for better efficiency of removing cuttings from the auger borehole and for creating a smoother wall on the borehole than the augers provide. The bailer is essentially a refined auger with an outer tube to catch all the cuttings. During our most recent field trials, this unit worked exceptionally well and was operated with the IDP 4-in Drill. In future, we plan to use the bailer with the RAID rig.
RAID uses off-the-shelf Boart Longyear steel drilling rods (NRQ size) that thread together in 3 m (10 ft) sections. Once a packer is in place and drilling starts, fluid is restricted to the space below the packer and around the drill rods, inside the hollow drill rods, and in the hoses connecting the drill to the FRS. All of these components are manufactured in the U.S.
A dust logger is a device designed to measure ice properties by shining a laser light into the ice surrounding a borehole and detecting the backscattered light. A dust logger is commonly used in Antarctica and Greenland to record layers of volcanic ash and continental mineral dust to rapidly date ice, much like using a bar code on a retail product. RAID’s dust logger was designed and built by Ryan Bay (UC-Berkeley) as a slim modification of earlier designs. Details of the dust logger and our results from AFT3 are reported here.
The RAID system is designed to be mobile, enabling us in principle to complete several boreholes per season. In essence, as an exploratory platform, RAID’s approach is akin to spot drilling rather than stratigraphic-type drilling that is completed with ice-coring equipment. The emphasis is on speed to get to the bottom, rather than recovering a complete ice core record.
Getting to and from drill sites is done by traversing the sled-mounted modules with heavy tractors. During traverses, the tractor train will pull the RAID modules, fuel and drilling fluid in bladders on cargo sheets, cargo decks, and a crew module with quarters, galley, comm’s, and other traverse kit. A smaller tracked vehicle will lead the way with ice-penetrating radar for scouting safe routes.
Moving day is a well-choreographed ballet. Mast lowered and stowed. Canopy bundled up. Power down. Hoses disconnected. Power cables disconnected. Stow loose gear. Dig out skis and jack stands. After loosening up frozen-in skis with a gentle tug from a tractor, we’re on our way.