4.1. General outline of components

As shown in Figure 2, the RAID drilling system is designed to penetrate three sections of the ice sheet – porous firn, non-porous glacial ice and a basal section consisting of deep ice, glacial bed materials (e.g., till) and bedrock below. The corresponding drilling sections are divided into stages for installation of firn casing, cutting an ice borehole and coring of all solid materials near the base of the ice sheet. In order to drill each of these three sections in sequence, the principal design elements of the RAID drilling system include: (1) a diamond drilling rig, modified to fit inside a traversable module; (2) a pipe rack, for storage and delivery of auger flights, casing pipe and drill rods to the rig; (3) diesel power generators; (4) a mechanical shop and inventory module; (5) a custom-designed fluid recirculation system; (6) metal-pipe casing and an inflatable packer unit to make a fluid-tight connection through porous firn between the drill rig and impermeable glacial ice below; and (7) a wireline-deployable bottom-hole diamond drilling and coring assembly capable of retrieving ice, sediment and rock cores. These functions are housed in five modularized and sled-equipped ISO-shipping container-type units.

Fig. 2. Schematic diagram showing the main elements of the RAID design using a conventional rotary drilling rig operating in flooded reverse circulation mode in jointed hollow drill pipe. The RAID system involves three stages of drilling and coring, divided here into casing, borehole and coring sections (a–c). Firn casing is capped at the surface and sealed in non-porous ice by an inflatable packer in order to maintain borehole fluid pressure. The ice borehole will be made with hardened ice-cutting bits in a reverse fluid circulation mode in order to evacuate cuttings. Coring will be done with a wire line BHA using thin-kerf diamond drill bits in a normal fluid circulation format.

The main drilling components at the surface consist of a modified rotary drilling rig operating in flooded reverse circulation for removal of cuttings and a fluid recirculation system custom-designed to separate ice-cuttings from the drilling fluid and recycle the fluid into the borehole in order to use it efficiently and minimize supply. The upper firn section will be cut by the drilling rig using 7.5 inch (19 cm)-diameter ice augers and then cased with 4.5 inch (11.4 cm) outer diameter (O.D.) steel or composite casing pipe. Casing pipe will be sealed at the bottom against non-porous glacial ice by use of an inflatable rubber packer in the shape of a torus, through which drill pipe can be inserted. A packer enables fluid overpressure in the borehole to be maintained without leaks into the porous firn, and resists both upward and downward forces imparted to the casing by fluid pressure or movement of the drill string. This fluid overpressure in the annulus is crucial during deep drilling to compensate for viscous pressure losses during fluid circulation, in order to prevent warm ice from closing in on the drill pipe in the deepest part of the hole. The RAID packer is an industry-standard 7 inch (17.8 cm) O.D. model rated to 600 psi (4.1 × 10⁶ Pa) of inflation pressure and can be inflated by any fluid. Tests in ice indicate that air is the best medium for inflating the packer to a desired pressure of 400 psi (2.8 × 10⁶ Pa) in order to prevent leakage and slippage. A deep 3.5 inch (8.9 cm)-diameter borehole will then be cut in the ice by rotating NRQ size drill rods (2.75 inches, or 7.0 cm, O.D., in 10 m rod lengths) equipped with tungsten-carbide or hardened-steel bits. A diverter bolted to the top of the casing with a gasket flange has an inlet that allows the drilling fluid to be pumped downward within the annulus between the casing and the drill rods (reverse circulation), then down the annulus between the ice borehole wall and drill rods, and finally to pass across the bit face in order to flush ice cuttings up through the center of the drill rods. Drilling fluid will be circulated by pumps on the drilling rig and pass through a series of mechanical, gravity and melting-coalescing stages to separate ice from drilling fluid before it is recycled into the borehole. RAID will use ESTISOL 140 as the drilling fluid (see below). Ice, bed and bedrock cores (1.5–1.9 inch, or 3.8–4.8 cm, diameter) will be retrieved into core barrels using a wireline-deployable bottom-hole assembly (BHA) equipped with thin-kerf diamond rotary bits. This BHA fits within the drill rods and so can be deployed without pulling the drill string prior to coring. The entire operation of cutting a borehole and retrieving core will be completed with only one trip of the main drill string to depths of 2500–3300 m, significantly minimizing total time at each drill site. To the extent possible, the RAID drilling system uses off-the-shelf, industry-standard components; in many cases, however, individual components were custom designed or sized for appropriate application in the RAID system. Key components of the RAID system are detailed below.

Field deployment of the RAID drilling system in Antarctica is shown schematically in Figure 3. The system consists of five main drilling modules, all containerized and towable over the ice cap, plus supporting equipment and berthing for a minimum complement of drilling staff and all the consumables required for traversing and drilling up to six sites per field season. RAID is designed to be logistically autonomous.

Fig. 3. Field layout for RAID (schematic and approximately to scale), including five main drilling modules (brown). Tower shows location of drill rig platform. Not shown are hoses and wiring to connect drilling components, fuel bladders, camp facilities, tractors and other equipment. Drill pipe rack can hold up to 3500 m of drill rod, depending on rod diameter. Drilling fluid and fuel will be supplied in flexible bladders.

4.2. Drilling rig and drill-rod modules

The centerpiece of the RAID design is the drilling rig itself. After consideration of commercially available rigs, we identified the Boart Longyear model LF230 as the best choice for the RAID system. Manufacturer specifications for this coring drill are available at http://www.boartlongyear.com/drilling-equipment/surface-coring/lf230/. This rig has multiple advantages: (1) appropriate power, torque and lift capacity (22.7 ton pullback, or 20 593 kg); (2) adaptable for normal and reverse circulation drilling; (3) up to 9 m rod pull; (4) simple, hydraulic chuck drive; (5) uses industry-standard drill pipe, bits, and casing; (6) suitable for wireline coring; (7) compact design and skid-mountable; (8) manual leveling capability; (9) convertible to an electric-over-hydraulic prime mover; (10) operable by a 3-person drilling crew; (11) factory engineered to operate at −40°C; and (12) readily available on the commercial market. The RAID drilling rig has been modified with a 300 HP electric motor running hydraulics in place of a diesel engine, along with soft-start capability provided by an AC solid-state motor control, which decreases the weight of the system, provides greater energy efficiency, and provides for greater ease of use in the Antarctic environment.

The drilling rig and drill-rod rack modules were custom-built from cold-rated C-channel steel, bolted to form two 45 ft (13.7 m) and 40 ft (12.2 m), respectively, ‘flat racks’ that are mountable on sleds and equipped with retractable vinyl-coated woven fabric side and top curtains for access during drilling operations. The drill-rod rack is equipped with an auger trolley that allows the drill crew to move hollow-stem auger sections to the front of the pipe rack skid with little effort. The drill rig and drill-rod modules are outfitted with hydraulic leveling legs and will be supported by rig mats in the field to reduce ground pressure. When deployed, the drilling rig and drill-rod modules will be joined by a fixed platform and the drilling crew work area will be covered by a woven fabric canopy for environmental protection (Fig. 4). The canopy is light enough that the hydraulic rams of the drill mast can lift it into place, but durable enough to resist snagging and wind; it is designed to withstand wind loads of 80 mph (36 m s⁻¹).

Fig. 4. Field deployment of drill and rod modules on sleds. Drill rig (background) and drill-rod racks are joined by a work platform and covered by a canopy of reinforced fabric.

4.3. Fluid recirculation system

A custom-designed fluid recirculation system was designed for RAID to move fluid and ice cuttings between the surface fluid supply reservoirs, the wellhead and the borehole (Fig. 5). It is a closed-loop system capable of both normal and reverse circulation. The fluid recirculation system itself houses all of the equipment needed to prime the borehole with fresh drilling fluid, separate ice cuttings from the drilling fluid and filter the fluid for recycling into the borehole (Fig. 6). We modified a standard 40 ft (12.2 m) Hi-Cube ISO container by dividing it into cold and warm rooms to accommodate a two-stage separation process. The cold room, designed to remain at ambient conditions, contains fluid storage and supply tanks, pumps, filters and a screen-shaker unit used to separate ice chips and recover drilling fluid. The larger warm room contains tanks for melting ice cuttings, coalescing drilling fluid, collecting waste water and returning filtered fluid to the cold-room supply. The warm room also houses pumps and filters; a large-capacity compressor used for maintenance as well as delivering air to the borehole during air lift operations; a boiler unit for warming the melting tank; a programmable logic control interface to monitor temperatures, fluid levels and pressures; an electrical cabinet and motor control center and space heater. During drilling operations, fluid is provided to the supply tanks in the cold room from an external supply in flexible bladders and then pumped into the borehole (blue fluid paths in Fig. 6). Fluid returning from the borehole contains ice cuttings and is introduced across the screen-shaker in the cold room, which mechanically separates fluid from ice on two sloping screens of coarse and fine mesh size, vibrating by an eccentric cam. Shop and field tests of the shaker indicate that we can achieve ~90% recovery of pure fluid from this step alone. Fluid passes through the screens and into a return tank below. Mostly-dry ice cuttings then fall off the shaker screens, set across a pass-through into the warm room and into the melting tank.

Fig. 5. Generalized schematic diagram illustrating the closed-loop fluid recirculation system that connects the surface fluid reservoirs with the wellhead and borehole. Details of the fluid-processing module are shown in Figure 6.

Fig. 6. Simplified schematic diagram of fluid recirculation system (FRS) shown to scale as built out in a 40 × 9.5 ft (12.2 × 2.9 m) Hi-Cube ISO container. FRS is divided by a partition wall into unheated and heated spaces (cold and warm rooms, respectively). Blue lines show general path of drilling fluid; magenta lines show warm fluid circulation loop. Not shown: electrical panels, power distribution, fire suppression, lighting, gauges and readouts and other minor components.

A closed-loop warm fluid line from the boiler, coupled with waste heat captured from the power generators, provides energy to melt the remaining ice (red fluid paths in Fig. 6). Because ESTISOL is an oil-based, water-insoluble fluid that floats on liquid water, the two-phase fluid is pumped to a coalescer tank where ESTISOL is skimmed off the water and captured in a fluid return tank below. Waste water from melting of ice is virtually free of drilling fluid and periodically purged to the outside. The two-stage process of separating ice and drilling fluid, combining mechanical and melting-coalescing steps, is highly effective and capable of keeping up with the maximum drilling rates achieved by the LF230 drill.

4.4. Other modules

Other modules include the drill-rod rack skid, built in the same configuration as the drilling rig skid, and power and shop modules each fitted-out in 40 ft (12.2 m) ISO containers. The drill-rod skid includes space for all the necessary drill rods, augers and casing pipe to drill through 150 m of firn and 3300 m of ice. It includes a hoist and trolley system for moving and lifting drilling components. Power is provided to the RAID equipment from a central generator module containing twin Caterpillar C- 18 500 kW diesel generators. The generators are redundant and typically only one is required for all drilling and support operations. Power is distributed to the other modules through an elevated cable system that is assembled at each drilling site. A ventilation system allows for control of temperature within the generator module in both automatic and manual modes. The shop module is configured with ample storage for drill bits, bottom-hole tooling, spare parts, and consumables and a basic workbench with machine tools and welding equipment for field repairs. All five RAID modules are outfitted for fire-suppression, consisting of an ABC dry chemical system operable in both automatic and manual modes. Fire detection is possible by thermal sensors, linear detection wire, infrared, push-button and remote controls, and the system is connected to a main fire control panel at the drill station operation interface.

4.5. Tooling

The combined drilling and coring requirements of the RAID system while operating at depths up to 3300 m and cutting through different materials mandated a unique solution. RAID uses a BHA consisting of an outer ice-cutting bit and tubes that remain in the borehole at all times with the drill rods (Fig. 7a). The BHA attaches to the outer bit and is in line with the drill rods; it is the chambering device for both the non-coring assembly used in ice cutting and the inner coring assembly. Either a face-centered ice-cutting bit or coring tools are inserted in and removed from the BHA by wireline. This allows us to change from drilling to coring without pulling the entire drill string. From bottom to top, the BHA consists of the outer bit, landing ring, outer core barrel, rotational driver coupling, vertical locking coupling, reaming shell and adapter coupling. The locking coupling hosts the latch head that attaches to the inner core barrel. Separate reaming shells can be used to help centralize the drill string and condition the borehole.

Fig. 7. Schematic diagrams showing bottom-hole tools, including: (a) non-coring bottom-hole assembly (BHA), with reaming shell and outer ice-cutting bit; (b) face-centered bit for cutting ice, latched into position within the BHA; (c) diamond coring bit, substituted by removing the inner ice-cutting bit and latching in on wire line; and (d) a sequence of coring steps showing progressive advance of the coring bit and core barrel with extensions as needed. Core barrels are available in 1.5 and 3 m lengths. Not to actual scale. Not shown are inner landing ring, latching mechanisms for vertical and rotational locking and pulling tools.

The RAID system has three primary drill bits used in the ice drilling and coring sections of the hole. The outer bit, which is the primary cutting mechanism through the ice, has a 3.5 inch (8.9 cm) O.D. If required, the bit diameter can be increased and will not adversely affect drilling of the hole; however, doing so will increase the drilling fluid requirements. The outer bit is ~20 cm long and houses the landing ring, a hardened piece of steel designed to stop downward momentum of the inner tools when the landing shoulder contacts the landing ring. Ice-cutting outer bits are available in both hardened steel and tungsten carbide formats. We have also designed bits with cutters of different materials such as high-speed steel, carbide and diamond that will allow us to drill through ice and expose diamond cutters if contact is made with hard materials such as an entrained glacial boulder or compacted till. The second-stage non-coring assembly is a wire-line retrievable inner barrel with a second face-centered bit (Fig. 7b). It consists of the inner ice-cutting bit, landing shoulder, spacer tube and latch head. This tool is used for drilling operations without taking core samples. It is the crux of the RAID concept and is designed to penetrate the ice sheet as quickly as possible, allowing cuttings and drill fluid to pass through its inner diameter. This tool, along with the design of the outer barrel, allows a drill crew to drill ahead to a designated depth and then, with the wire line, retrieve the non-coring assembly in order to sample ice, transition of the ice/bedrock, bedrock and afterwards redeploy the non-coring assembly if needed. A third bit is used to core ice, glacial bed materials and rock (Fig. 7c). It is patterned after a standard mining industry hard-rock bit, with different configurations to handle ice, till, cobbles and rock. This bit has as thin a kerf as possible in order to allow passage through the BHA bit and retrieve the largest possible diameter core (~1.5 inch, or 3.8 cm, diameter).

RAID uses custom-designed ‘running’ and ‘pulling’ tools to deliver and retrieve the inner barrel from the BHA. Designs of the latch head and the running and pulling tools are based on designs used in the oil industry and for marine scientific drilling; they are positive locking devices that prevent the inner barrel from disconnecting. Because the drilling fluid is less viscous than typical water-based muds, the inner barrel cannot be dropped inside the rods and allowed to free fall; these tools are deployed in and out of the drill string via a 3/16 inch (0.5 cm) swaged wire rope that is spooled by the wire line winch of the LF230 drill rig.

A custom-designed coring assembly will be used to take spot ice cores, drill through the boundary zone and glacial bed, and into bedrock. The coring assembly protrudes from the BHA in 1.5 and 3 m increments, or multiples thereof. To recover 25 m of core, the coring assembly first latches into the BHA, advances by rotation of the drill string, collects up to 3 m of core and then is retrieved by the pulling tool (Fig. 7d); after core is removed from the barrel, additional increments of core will be taken by adding extensions to the core barrel and the tool is run to the bottom to begin another section of coring. Coring rods are designed in 3 m lengths of B size (2.2 inch, or 5.6 cm, O.D.). Coring continues in this incremental fashion, with the BHA moving up and down within the previously drilled borehole enough to advance the coring assembly, until the desired amount of core is recovered with successive trips of the wire line.

4.6. Key design innovations

Since completion of an initial feasibility study undertaken in collaboration with IDDO (Gerasimoff, Reference Gerasimoff2012), which identified a jointed-pipe rotary drill as the most appropriate technology for this application, subsequent design work completed by DES and the RAID scientific team has resulted in the following key design features and innovations:

1. (1) Flooded reverse fluid circulation. A key element in the RAID design is use of a flooded reverse-circulation mode of drilling. In reverse circulation, the drilling fluid is pumped downward along the annulus on the outside of the drill pipe and returned along with cuttings upward through the middle of the drilling pipe (Fig. 8). This method was adopted in order to maximize the diameter of ice and rock cores (requiring a larger inner diameter (I.D.) drill rod), while at the same time minimizing the risk of clogging by ice chips in the narrow, outer annulus. The RAID reverse circulation system has been designed to maximize flow to achieve rapid borehole cutting. In field tests conducted in North America, we achieved ice-cutting rates of 2.5–3.0 m min⁻¹. During rock coring, however, the drilling system will be returned to normal mode, in which fine rock cuttings are lifted up the outer annulus and a high flow rate provides adequate cooling and lubrication at the cutting surface.

2. (2) Custom mechanical/coalescing fluid recirculation system. Perhaps the most challenging and innovative part of the RAID design is the fluid recirculation system. Because of the need to carry all of the drilling fluid required for multiple holes while traversing and to minimize the total amount of fluid used in keeping with environmental stewardship, RAID must continuously recirculate drilling fluid, remove ice cuttings and recycle fluid without substantially warming ice in the borehole. We designed a containerized fluid recirculation system consisting of a series of tanks and pumps that is capable of moving fluid through the borehole and separating the fluid-ice mixture in a two-stage process involving both mechanical screening and melting/coalescing stages, as described above (Fig. 6).

3. (3) Narrow-wall drill rods. To save on weight of the drill string at depths between 2000 and 3300 m, we will employ custom steel drill rods consisting of a narrow-wall mid-body section fitted with standard Boart Longyear-type threaded box-and-pin end joints. We considered composite drill rods consisting of an aluminum mid-body fitted to steel end joints because aluminum provides advantages of lighter weight yet greater tensile strength at cold temperatures. However, contrasting thermal expansion of steel and aluminum raised concerns about the viability of the metal joints at low temperatures in Antarctica.

4. (4) Custom rod rack for inventory and driller operation. A custom-designed rod rack is included with the RAID design. It is built on a bolted steel rack like that used for the drilling rig, and it includes a sliding trolley for auger flights, racks for casing pipe and drill rods, retractable vinyl side curtains and top cover, and a modular catwalk. It is capable of carrying up to 3500 m of drill rods. This unit, when mounted on skis, will be connected to the drilling rig platform by a modular deck to create a workspace for ease of rod handling to and from the drilling rig (Fig. 4).

5. (5) Airlift. To enhance the ability of the fluid circulation system to maintain flow and appropriate pressure compensation during making of the deepest parts of the borehole, an air-lift system will be employed. In this standard oil-field and rock drilling practice, compressed air is injected into the inside of the upper ~60–150 m of the drill pipe to reduce the overall density of the upward-flowing fluid-ice mixture. The resulting increase in its buoyancy augments the driving force for fluid circulation, overcoming frictional pressure losses and thus ensuring a rapid flow rate of the drilling fluid. Use of an air-lift system will also help maintain the desired fluid pressure compensation in the deeper, warmer parts of the borehole, thereby reducing the likelihood of warm, ductile ice ‘ballooning’ into a cavity in response to fluid overpressure. The air-lift process and fluid pressure compensation are discussed separately below.

6. (6) Augering of firn. It is necessary to case the upper ~130 m of the borehole passing through the porous and permeable firn in order to prevent loss of pressurized fluid during drilling. The borehole in firn will be larger diameter (7.5 in, or 19.1 cm) and cased with steel pipe (4.5 in, or 11.4 cm). We have elected to make this larger-diameter hole for the casing with conventional ice augers deployed by the LF230 drill rig because augers are fast, energy efficient, compact, proven in similar settings (e.g., South Pole), and exploit the rig's high torque capability. Other options were considered, such as a hot water drill, impact driver, or a rotary air (RAM) drill, but these were discarded as too costly in terms of fuel consumption, larger compressor requirements, greater pumping/lifting requirements and possible damage to casing sections. The RAID system will use a light soil auger borrowed from the geotechnical industry, with a 7.5″ (19.1 cm) O.D. bit appropriate for cutting ice. After augering a firn borehole to accommodate casing pipe, and once casing is set and a seal is created in non-porous ice with a packer, the ice-cutting process will begin using the RAID rig with the conventional drill rods.

7. (7) Inflatable packer. To seal the base of the casing against non-porous ice (at ~130 m depth), we will use an inflatable casing ‘packer’ with a flexible rubber membrane that is inflated to seal against solid ice. This industry-standard packer has an outside diameter of 7 inches (17.8 cm) and can be fitted with spikes or studs to ensure good coupling against back-pressure from the borehole fluid. The packer performed as expected in a recent test at the firn-ice transition near McMurdo Station, making a leak-tight seal when inflated with air to a pressure of 350 psig (2.4 MPa).

8. (8) Wire line bottom-hole assembly. To reach deep levels in the ice and retrieve ice and rock cores on a rapid timeframe, the RAID design consists of a BHA into which ice-cutting and coring tools can be deployed on wire line. After reaching the base of the ice sheet, removal of a detachable inner ice-cutting bit will allow us to insert a wire line coring system that will latch into the outer drill pipe and advance behind a diamond coring bit. Such a wire line system will allow us to take the required cores without tripping drill pipe, saving considerable time while on site. For coring, sections of core barrel and an attached coring bit are lowered into the drill string via a dedicated wire-line winch located in the drill module and mechanically latched to the BHA upon landing (Fig. 7). Positive latching provides both vertical and rotational coupling between the coring tools and the bottom-hole assembly. Once coupled to the drill string, the inner core barrel and coring bit move in synchronous rotation with the drill string, which is rotated by the drilling rig. The coring bit advances by weight-on-bit, while the outer ice-coring bit remains at the same level in the ice borehole. Once a coring section is completed, the inner core barrel is lifted on wire line to the surface, core removed, a core barrel extension is added and the process is repeated.

9. (9) Environmental canopy. We designed a removable fabric canopy that can be hoisted up the drill mast and guyed out to anchors, making a taut tent-like cover to protect the rig and drilling crew from the Antarctic elements. It is designed for quick rig-up and rig-down (<1 hour), and also to minimize wind loading.

10. (10) Heat-efficient design. To the extent possible, we have designed the RAID system with heat-exchange capability in order to recover as much waste heat as possible for use elsewhere, and, likewise, for efficient cooling using ambient air temperatures. For example, excess heat from power generation will be used to melt the ice chips during fluid recovery, whereas a heat-pump concept will be applied to chill this fluid before it is re-injected into the borehole.

11. (11) Containerized subsystems. Each of the main components are designed to fit within standard-dimension ISO containers and/or flat racks of similar dimension. These containers are designed to fit on either skis or inflatable pontoons for over-snow traversing. They will be fitted with leveling, hydraulic, control and heating systems, fluid pumps, catwalks, insulation and retractable side and top curtains. The containerized systems are designed to be strong and weatherproof, yet provide ready access to the drilling crews.

12. (12) Arctic ratings for components. All critical components will have industry-grade Arctic ratings to reduce the risk of failure in Antarctic conditions. Arctic-rated equipment will include power equipment, motors, fluids, seals, hoses, steel, fire safety equipment and electric wire jacketing.

Fig. 8. Schematic diagram showing the upper portion of the drill string from the surface down to non-porous ice. Fluid circulates from the borehole to the fluid recirculation module via the drill-pipe swivel and diverter at the surface. Firn casing is sealed below within non-porous ice by use of an inflatable packer, through which the drill string penetrates. Fast ice cutting is made possible by use of flooded reverse circulation, in which drilling fluid flows downward within the annulus surrounding the drill rods, passes over the cutting face and then carries ice cuttings away up the inside of the drill rods (inset).

4.7. Drilling fluid

The choice of drilling fluid affects the design in numerous ways, such as compatibility with elastomer seals, the fluid viscosity's impact on the permissible width of the annulus and the fluid density's impact on the required fluid level for long-term borehole stabilization. For this reason, we decided to choose a drilling fluid early in the design process. A wide range of fluids was considered, including n-butyl acetate, silicone oil, and refined derivatives of kerosene such as ISOPAR-K. Our search was heavily influenced by the experience of the ice-coring community in identifying a replacement for the halocarbon HCFC-141b, which has been phased out by the Montreal Protocol to protect atmospheric ozone. Borrowing on pioneering work by Danish scientists in successful ice-coring operations in Greenland and Antarctica, we decided at an early stage to adopt ESTISOL 140 as the drilling fluid for RAID (Sheldon and others, Reference Sheldon, Popp, Hansen and Steffensen2014). This fluid has all of the desired properties listed in the technical requirements above, satisfying the needs for low viscosity at low temperature, drill lubrication, ice-chip flotation, borehole pressure stabilization, optical quality, safety, modest cost and environmental suitability.

4.8. Air-lift process

For the air-lift process, 60 m of 3/4 in (1.9 cm) diameter aluminum rods are inserted into the drill rods and secured to a swivel designed for air lift using positive-locking quick-disconnects. The air-lift rods are run into the drill string one section at a time and a check valve is used to create neutral buoyancy. During drilling, compressed air is injected into the aluminum rods and adjusted as needed. A check valve at the bottom prevents back-flow of drilling fluid. Maximum airlift capability is achieved at a depth of ~400 fbs (121 mbs). To add more drill rod, the air-lift rods are detached and then reconnected to resume drilling using the following procedure.

At the end of a drill run (9 m), flow is stopped and the drill rods are secured in the foot clamp and chuck. The swivel is unthreaded from the drill rods and pulled up to expose the air-lift tube connection, and the neutrally-buoyant air-lift rods are disconnected. The swivel is attached to a new rod from the rod rack and pulled up over the drill head. The air-lift rod is then manually inserted upward into the suspended drill rod until it connects with a coupler at the base of the swivel. Once a positive connection is made, the drill rods above are threaded to the rods in the chuck. Flow of ESTISOL and air is resumed and drilling proceeds.

4.9. Borehole pressure considerations

The central concept of RAID is to keep the logistical footprint small and nimble, which translates directly to keeping the borehole diameter as small as possible due to the fact that the volume of drilling fluid required increases as the square of the borehole diameter. For example, a 3.5 inch (8.9 cm) diameter borehole to a depth of 3300 m will require 5400 gallons (20 440 l) of fluid, whereas a 7 inch (17.8 cm) diameter borehole requires >21 000 gallons (79 500 l) of fluid. However, as the borehole diameter becomes smaller, frictional pressure losses in the moving fluid increase. This pressure loss makes it increasingly difficult to maintain adequate borehole pressure during drilling, which is necessary to prevent the ice from closing in on the drill pipe and stopping fluid circulation. For this reason, we engaged in a lengthy optimization process, iterating between engineers and scientists on the design team, informed by a fluid-mechanical model of borehole pressure and frictional pressure loss (based on the Darcy–Weisbach equation) that we constructed for this purpose.

Briefly, the model is steady state and starts with ice density, ice velocity and temperature profiles computed from a modified Herron and Langway (Reference Herron and Langway1980) firn densification model, a Dansgaard and Johnsen (Reference Dansgaard and Johnsen1969) 1-D ice flow model, and a 1-D advection-diffusion heat flow model (Fig. 9). The model takes as inputs surface temperature, snow accumulation rate, geothermal heat flux at the ice/rock interface and ice thickness. Temperature-dependent density and viscosity of ESTISOL were computed and used as inputs to the Darcy–Weisbach equation to predict fluid pressure as a function of depth (Fig. 10).

Fig. 9. Modeled steady-state temperature and velocity profiles at an East Antarctic plateau RAID site. Ice thickness is assumed to be 3000 m, geothermal heat flux of 50 mW m⁻², mean annual surface temperature of −55°C and snow accumulation rate of 3 cm a⁻¹ of water equivalent.

Fig. 10. Modeled dynamic pressure losses for a typical RAID hole, expressed as the difference between fluid pressure and ice hydrostatic pressure. Thus, a negative value indicates that the borehole will tend to close. During operations, the surface injection pump (Bean pump) and/or the airlift will be used to compensate for the differences shown here. Static curve shows pressure differences when fluid is not flowing. Assumptions: 30 gal min⁻¹ (113.6 l/min⁻¹) fluid flow; 3.5 in (8.9 cm) hole diameter; 2.75 in (7.0 cm) pipe O.D.; 0.0057 m wall roughness; Darcy–Weisbach formulation; Re >2300 (turbulent); 50 mW m⁻² geothermal flux; −55°C mean annual surface temperature; 2 cm/a w.e. snow accumulation; ESTISOL 140 drilling fluid.

Note that a major source of uncertainty in the calculation stems from the position of the drill pipe in the borehole – if it is centered, pressure losses are approximately twice as severe as if the pipe lays on the wall (Piercy and others, Reference Piercy, Hooper and Winny1933). Experienced drillers report that pipe generally lays on the wall (C. Delahunty, personal communication, 2013). To some extent the risk of hole closure can be mitigated by overpressuring the fluid at the surface injection point, but this pressure must not exceed the hydrofracture strength of polar glacial ice, which we found to be 190 psig (1.3 MPa) in −17°C ice during a recent test near McMurdo Station at the firn-ice transition. It is expected that the fracture strength of ice will be greater at the colder temperatures of the Antarctic Plateau (−50°C), but no tests have been done at lower temperature to our knowledge.

The opposite risk, of ‘ballooning’ the borehole in warm soft ice, occurs when fluid circulation is stopped and the fluid pressure returns to its static value (Fig. 10). Due to the fact that the firn is less dense than ESTISOL 140, the static pressure is always overpressured with respect to the ice while the fluid column extends all the way to the surface. After drilling, the fluid level will be lowered to closely balance the borehole pressure with the ice hydrostatic pressure in the warm soft ice, to prevent borehole deformation. This will be accomplished by evacuating ESTISOL in the borehole to a level below the packer by pumping air to the diverter, thereby forcing drilling fluid upward within drill rods to the fluid recirculation system for recovery. After removing the drill rods, the packer will be deflated and removed along with the primary casing. To prepare the boreholes for future down-hole observation, new plastic casing will be installed to the packer depth, capped and back-filled with snow to seal the annular space. Calculations show that the final fluid level needed for long-term borehole stabilization will be near the firn-ice transition.

6.1. North American test

In early 2015, we completed a system-level field test of the RAID drilling system in Utah (North American Test, NAT), which demonstrated the ability of RAID to drill through ice and take core with its principal components consisting of the drilling rig, fluid recirculation system and bottom-hole assembly. A site in the mountains of Utah was chosen because of historically cold temperatures, high elevation, low terrestrial heat flow and proximity to the DES facilities in Salt Lake City. All components of the RAID system functioned successfully as designed, and we demonstrated the capability of the drilling rig and fluid recirculation system to drill ice and take core in cold-weather, high-altitude field conditions. In support of the field tests completed in Utah, we also undertook a comprehensive set of bench tests and computational fluid dynamics modeling studies that validate key components of the RAID system and the drilling process.

In order to test the drilling and fluid-processing functions of RAID in an environment that is as close as practical to that expected in Antarctica, we first designed and implemented a unique cryogenically-cooled drilling test facility to make a column of ice by freezing water with liquid CO₂. This method was implemented at a test facility at an elevation of 1824 m near Bear Lake, Utah, in early 2015. As summarized by Nielson and others (Reference Nielson, Delahunty, Goodge and Severinghaus2015, in review), the general design of the NAT well consisted of a 10.75 inch (27.3 cm) outer casing cemented in a hole to a depth of 152 m. Within that casing, we hung a 5.5 inch (14.0 cm) casing string within which a column of ice was formed by circulating a refrigerant in the annulus between the 5.5 and 10.75 inch casings. A thermistor string was installed in the annular space between the two casing strings to monitor temperatures at 15 m intervals. We used liquid CO₂ as a refrigerant because it exists as a multi-phase material to low temperatures (triple point of −57°C at 5.1 atm, or 5.2 × 10⁵ Pa) and it is easily circulated, inexpensive and readily available. In order to minimize the likelihood of the casing splitting due to the volume increase associated with freezing water, the hole was cooled first and then ice was formed in increments from the bottom upward. Refrigeration of the ice column yielded down-hole temperatures of −20 to −30°C to a depth of 137 m.

During the NAT, we successfully demonstrated most of the technical requirements for RAID, including: (a) borehole penetration rates of up to 2.3 m min⁻¹ (design limit of the LF230 rig), which corresponds to <24 hours to cut 3300 m not including drill rod cycling; (b) cutting a continuous 3.5 inch (8.9 cm) borehole; (c) successful operation of the drilling fluid recirculation system for ice chip removal and chip disposal at borehole temperatures similar to those we will encounter in Antarctica; (d) taking cores of ice and ‘rock’ (represented by cement) using the bottom-hole coring assembly; (e) mobility of the containerized modules; (f) operation of the drilling system with a crew of three; and (g) effective use of ESTISOL 140 as a drilling fluid at high elevation and cold air temperature. We were able to meet these requirements because of the combined capabilities of the drilling rig and effectiveness of the fluid recirculation system to keep pace with the production of ice cuttings. The main parameters that have a direct effect on penetration rate are: rotation speed (rpm) of the drill head; flow rate (gpm) of the fluid circulation system; the capacity of the fluid recirculation 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 cutting capacity of the drill bit. All drill bit configurations worked as expected, and we experienced very even cutting and uniform fluid pressures in zones of clear, well-formed ice; similar cutting performance is expected in homogeneous Antarctic ice. Rotation rates during drilling were varied from 200 to 600 rpm, but we found that an ideal performance was achieved at 400–500 rpm and pumping ESTISOL at 20–25 gal min⁻¹ (75–95 l min⁻¹, about half the maximum rate anticipated), thereby significantly reducing flow rate through the fluid recirculation system. The BHA worked as expected and rock-coring system yielded recovery of short cement cores at the base of the casing. The main functions yet to be tested include boosting our maximum cutting rates (up to 3 m min⁻¹) and use of the air-lift system to manage fluid circulation in deep boreholes, both of which require deployment of the system to Antarctica. At deeper levels planned for Antarctica, greater weight on bit should support optimal cutting rates. Once a drilling routine was established at the NAT, we achieved a cycle time for 9 m long drill rods of ≤5 min, which supports drilling to a depth of 3300 m in <48 hours.

6.2. Antarctic field trials (AFT)

We are currently planning a set of AFT for the 2016/17 austral summer in order to validate our operational plan, evaluate full integration of the drilling system in a deep-ice setting and learn from drilling operations in order to fully prepare for science drilling as an autonomous, traverse-capable system. Antarctica is the only place to effectively integrate the entire drilling system and evaluate the capability of those components designed for operation in thick firn and ice. In order to prove out the operational capacity of the RAID system, the goals for our proposed field trial include traversing over snow, site set-up, augering of firn and installation of casing, drilling through thick ice, retrieving ice and rock cores, rigging down, stabilizing the borehole and redeploying to a base station. The AFT will be conducted in the McMurdo Station operational area at a site of thick, grounded ice in order to take advantage of the established US Antarctic Program transport and logistics system, thereby reducing risk, cost and footprint in Antarctica. This approach represents the most logical, logistically light and cost-effective way to evaluate the full operational capability of the RAID system.