The Rapid Access Ice Drill (RAID) is designed for subsurface scientific investigations in Antarctica. Its objectives are to drill rapidly through ice, to core samples of the transition zone and bedrock, and to leave behind a borehole observatory. These objectives required the engineering and fabrication of an entirely new drilling system that included a modified mining-style coring rig, a unique fluid circulation system, a rod skid, a power unit, and a workshop with areas for the storage of supplies and consumables. An important milestone in fabrication of the RAID was the construction of a North American Test (NAT) facility where we were able to test drilling and fluid processing functions in an environment that is as close as possible to that expected in Antarctica. Our criteria for site selection was that the area should be cold during the winter months, be located in an area of low heat flow, and be at relatively high elevation. We selected a site for the facility near Bear Lake, Utah, USA.
The general design of the NAT well (NAT-1) started with a 27.3 cm (10.75 in.)
outer casing cemented in a 152 m deep hole. Within that casing, we
hung a 14 cm (5.5 in.) casing string, and, within that casing, a column of
ice was formed. The annulus between the 14 and 27.3 cm casings provided the
path for circulation of a refrigerant. After in-depth study, we chose to use
liquid CO
Characterizing and mapping continental bedrock beneath the Antarctic ice sheets is a fundamental objective of the US National Science Foundation's Antarctic Program (Goodge and Severinghaus, 2016). However, drilling through thick ice using existing ice-coring equipment is a slow process that has required multiple working seasons, and these systems were not designed for recovery of sub-glacial bedrock samples. The development of a mechanical drilling system that can rapidly drill through ice and core the base of the ice sheet and underlying bedrock is a high priority for the scientific community (Clow and Koci, 2002; Gerasimoff, 2012). Other priorities include finding the oldest ice in Antarctica in order to enhance our understanding of past global climate changes by linking changes recorded in the polar ice sheets with complementary marine sediment records.
The goal of the Rapid Access Ice Drill (RAID) is to rapidly drill a thick
section (up to 3300 m) of Antarctic ice and to collect core samples of the
transition zone and underlying bedrock. We expect to drill five or six holes
in a 2-month drilling season. To accomplish this, we need to achieve
penetration rates through the ice of 3 m min
To mitigate the risks associated with the performance of this new drilling system, a series of bench and field tests were designed to evaluate individual components and system integration before deployment to Antarctica. The North American Test (NAT) was the first integrated RAID system test where drilling and fluid circulation functions were tested under conditions that were as close as possible to those expected in Antarctica. The RAID is designed to use ESTISOL-140 (ESTISOL) as a drilling fluid because it has a density similar to that of ice, it has a low freezing point, and it has good transparency, allowing the hole to be logged optically following drilling (Sheldon et al., 2014). The FRS processes the returned ESTISOL and included ice chips, separates the ice, and re-circulates the ESTISOL down the hole. RAID utilizes pressurized reverse circulation to drill through the ice; i.e., ESTISOL is pumped down the annulus between the rods and ice, and the returned fluid and ice cuttings flow up the inside of the drill rods. This is done to mitigate the risk of cuttings freezing in the small annulus between the ice and the drill rod. The FRS is a key component of the fluid circulation system, and is a unique development under this project. As with all prototype equipment, it requires comprehensive testing under expected operating conditions.
In order to accomplish the RAID system test, we designed and built a facility where we could drill ice and test the rig's drilling and coring capabilities as well as the functioning of the FRS. In brief, the NAT facility consisted of a hole where we froze a column of ice and conducted a series of system tests. Other test facilities for testing ice drilling include a 60 m deep hole at CRREL (National Research Council, 1986) and a 13 m deep facility at the University of Alaska (Das and Jois, 1994). A 76 m deep well at the University of Wisconsin was established as part of the IceCube development and was used in early 2016 to test the ASIG drill (K. Slawney, personal communication, 2017).
The NAT facility was established on private land near Bear Lake, Utah, and
was permitted through the Utah Division of Oil Gas and Mining. This site met
a number of criteria. First, this region is often one of the coldest areas
in the lower continental United States. We wanted to replicate temperatures
that the equipment and personnel would experience in Antarctica. Second,
operations in Antarctica are expected to take place at elevations of 3000
to 4000 m. These elevations will impact the performance of both machinery
and personnel; the elevation at the NAT site is 1824 m above sea level. Lastly,
since we wished to freeze a column of ice to a depth of 137 m, we
were working against natural heat flow. Therefore, we chose a region in northeastern
Utah that has relatively low heat flow (60–80 mWm
Drilling was initiated in January, 2015, by augering a 61 cm (24 in.) hole
to a depth of 12 m (40 ft). A 40.6 cm (16 in.) diameter conductor
casing was cemented in place. A 37.5 cm (14.75 in.) hole was then drilled
to total depth of 155 m by an Atlas Copco RD-20 rotary drill, utilizing air
and foam. Then, a 27.3 cm (10.75 in., L-80, 45.5 lb ft
A 14 cm (5.5 in., L-80, 17 lb ft
Schematic diagram of the North American Test facility.
Cooling process diagram for the NAT facility.
A thermistor string was critical for monitoring the temperature of the well during the testing activities. A Geokon thermistor string with digital read-out at 15 m intervals was installed in the annular space between the 27.3 and 14 cm casing strings.
Liquid CO
As LCO
Figure 3 shows the cooling history of NAT-1 starting on 5 February 2015 and
following completion of the test facility. On 9 February, CO
Temperature with depth showing the cooling history of the North American Test facility.
On several occasions we ran out of LCO
Freezing water produces roughly 207 MPa (30 000 lbs in.
To calibrate the process, we added 3.2 kg of cubed ice to a piece of 14 cm casing and then added water. Together, 3.2 kg of ice and 2.5 L of water filled approximately 50 cm of the 14 cm casing. In the borehole, ice cubes were allowed to free fall to the bottom. The water, however, was a more difficult problem since it needed to be delivered to the bottom of the hole without freezing to the casing. A simple fluid dispenser was devised based on a reverse-bailer concept that would introduce approximately 53 L of water to the bottom of the hole using the drill's wireline. Impact of the fluid dispenser with the top of the ice released a rubber plug, and the water flowed into the ice. The freezing process then took place in 3.3 m increments, and the duration of a complete freezing cycle was 3 to 4 h. A video log was run at the end of each ice–water loading cycle to verify placement and the correct depth of the ice.
Details of the testing procedure will be covered separately; however, the principal objectives were to evaluate bit design, to exchange a face-centered bit for coring assembly, to drill at different rates, and to confirm the functioning of the FRS. The RAID is designed to be capable of drilling rapidly through the ice in order to maximize the number of holes that can be drilled in an Antarctic summer field season. To achieve this objective, the rate of penetration is of paramount importance. The range of components that affect the penetration rate introduces too many variables to accurately determine the rate mathematically and therefore must be tested to see how hard the systems can be pushed to maximize ice drilling speed while ensuring a high degree of safety. The drilling process used an 88.9 mm bit and NRQ V-wall core rod (69.85 mm; 2.75 in. OD). The main parameters that have a direct effect on penetration rate are rotation speed, flow rate of the fluid circulation system, capacity of the FRS system to process debris and ice cuttings and return a clean drilling fluid, pressure exerted on the borehole to maintain the fluid flow required, and the cutting capacity of the drill bit.
Achieving progressively higher penetration rates was the chief factor
guiding our testing sequence. The underlying rationale for this testing
approach was that if the drill can operate successfully at its maximum
penetration rate (3.0 m min
Video logs were run in the hole following each test. The logs revealed an unanticipated problem with the ice-forming process. In the process of adding ice cubes and then saturating the ice with water, we observed that the top 50 % of the 3.3 m segment formed compact ice, and the bottom 50 % of the volume was loosely bonded ice cubes. Attempting to freeze a 3.3 m section was too aggressive. Water did not flow completely through the cubed ice, resulting in a section of completely frozen ice that drilled smoothly and an underlying section where the ice would break off in pre-formed cubes during drilling.
After completion of the half-speed testing segment, we re-froze the hole using only water so that we could attempt to drill the full-speed test in as pristine ice as possible. We did this in increments of about 1.5 m, and the time to freeze these segments was about 4–5 h.
Frequently, the drill bit came into contact with the casing wall, resulting in damage to the cutters on the bit. Drilling with an 88.9 mm outer-diameter bit in a 124.2 cm inner-diameter casing provided too little space for the bit to wander, and any future work should consider using a larger inner-casing string.
The coring tools were tested and we were able to retrieve core, though not of the length we desired. The concrete placed at the bottom of the 14 cm casing was powdered by the extreme cold and was not an appropriate analog for hard rock.
Designing NAT-1 devoted a considerable amount of study to determine the best
mechanism for cooling the hole. We considered the option of using a
mechanically pumped coolant, such as Dynalene, in place of LCO
The authors declare that they have no conflict of interest.
This project was funded by the US National Science Foundation through grants 1242027 and 1419935 to the University of Minnesota Duluth. We would like to express our appreciation to Dan McClellan, Philippe Wyffels, Blaise Stephanus, and John Eckels for their contributions to the project. Edited by: T. Wiersberg Reviewed by: A. Pyne and one anonymous referee