4.1. Passive replicate coring drill

Two variations of the passive drill system were considered: a whipstock positioned on sacrificial aluminum support tubes (type 1 ; Fig. 3a), which is the less expensive solution using a whipstock; and a whipstock positioned above a remotely actuated clamping mechanism (type 2; Fig. 3b), which requires the design and fabrication of the mechanism and would be, consequently, more expensive.

1. The advantages are:

2. The passive drill concept is less complicated and therefore less expensive

3. The replicate borehole could be available for logging

The disadvantages are :

1. Type 1 would not meet requirements (a), (c), (d), (g) and (h)

2. Type 2 would not meet requirements (h) and (n)

3. Risk of intersecting the original borehole

4. Risk of losing the drill stuck during the deviation

5. With the type 1 whipstock, the last 400–450 m of the main hole are ‘plugged’, precluding any possibility of logging without reaming, because the whipstock and the sacrificial support tubes cannot be easily removed from the parent borehole

6. Potential environmental impact of materials used to set a whipstock due to difficulty of removing these materials if they are stuck in the parent borehole

7. The type 1 whipstock system is not reusable

The actuator-based system is chosen in preference to the originally intended whipstock system (type 1 or 2). This choice was made based on two major characteristics.

1. 1. The RCS should be capable of performing multiple deviations at the same depth, leading to serious concerns that boreholes may accidentally intersect. This could be avoided by steering the drill using the actuators.

2. 2. There is a desire to apply the system to other deep drills. Other drills are less suited for a whipstock-based approach than the DISC drill (non-rotating barrel, low fluid flow and low power budget).

A consequence of these two aspects, and the resulting solution to overcome the penalties they represent, is an active replicate coring drill system presented below.

4.2. Active replicate coring drill (IDDO design; Fig. 4)

The advantages are :

1. Meets all of the science requirements

2. The system concept is modular in nature and can therefore be integrated into the base DISC drill or other drill systems

3. The electromechanical design of the active steerable replicate system will allow for monitoring of the existing three-dimensional orientation of the DISC drill to avoid intersection with the main borehole

4. Eliminates the potential of getting a whipstock stuck in the main borehole

The disadvantages are:

1. More expensive than the whipstock approach

2. Entering the deviation borehole after it has been established is precise and will require active navigation during operation

3. The actuator sections add length to the drill, but screen and core barrels will be shortened, resulting in a slightly longer drill.

Based on the analysis of these two major approaches and after consultations with the science community, IDDO decided to pursue the active steerable replicate coring drill. Two major advantages of the active system led to this decision, meeting all of the science requirements and the possibility of integrating the modular RCS into the DISC drill and other drill systems in the future.

5.1. Actuator sections

The actuator sections (Reference Gibson, Johnson, Shturmakov, Mortensen and GoetzGibson and others, 2014) added to the base DISC drill enable the drill to be tilted in the borehole and pushed against the borehole wall to make the desired deviation. The actuator subsystem consists of two three-lever/cam actuators to ensure that forces can be delivered in any radial direction (Figs 7 and 8). The three cams are located 120° apart. Power is applied using three brush-type electric d.c. motors. The motors and reducers are maintained in a hydraulic fluid with a pressure compensator similar to that used in the DISC drill’s existing motor section. The linkage assembly is exposed to drill fluid. Each lever can be individually extended to the 80° angle and is equipped with a mechanical fuse.

Fig. 7. Actuator module schematic.

Fig. 8. Actuator section with actuator arms extended.

The design is relatively simple and minimized the modifications needed to the base DISC drill.

5.2. Control system

The replicate coring control system serves two major functions: to be able to situate the drill in the required coordinate position and to be able to correct this position by steering itself and executing the operator’s command (Reference Mortensen, Johnson and ShturmakovMortensen and others, 2014a).

The replicate coring control system consists of a few major components and includes Glassman 1kV regulated d.c. power supply for the drill, and surface and onboard computers. The surface computers, used with the base DISC drill and LabVIEW software, provide the user interface.

The six levers (three per actuator section) are used to achieve the deviation capability of the drill. Each of the levers is motor-driven with a closed-loop motor controller for each motor. This requires controlling six independent axes. For deviation to occur in a desired direction, two of the three levers of each actuator module need to be moved correctly to direct the forces from the levers necessary for the deviation at the correct azimuth. The drill operator can input the azimuth as well as the amount of force applied by the levers.

The control system design makes the drill capable of easily providing the desired force against the borehole wall with minimum operator intervention. Figure 9 shows the system control room, and Figure 10 shows one of the control displays.

Fig. 9. Replicate coring control room.

Fig. 10. Replicate coring control display.

The downhole portion of the control system includes a 15.2 mm diameter fiber-optic communication cable (Reference Shturmakov and SendelbachShturmakov and Sendelbach, 2007), custom-made by Rochester Wire and Cable; eight servomotor drives; six actuator motors; a cutter motor and a pump motor; a sensor control system, along with temperature, pressure, navigation and magnetic (linear variable differential transformers (LVDTs) and resolver) sensors and five processors/controllers with analog-to-digital (A/D) and digital-to-analog interfaces. A LVDT interface allows operators to control each individual actuator (Reference Mortensen, Johnson and ShturmakovMortensen and others, 2014a).

The new replicate coring control module is presented in Figure 11. Figure 12 shows the drill inclinometer board; the actual inclinometer accuracy during the production drilling was ∼±0.01°. Figure 13 shows the 3DM-GX1 system navigation sensor made by MicroStrain. 3DM-GX1^® combines three angular rate gyros with three orthogonal d.c. accelerometers, three orthogonal magnetometers, multiplexer, 16-bit A/D converter, and embedded microcontroller, to output its orientation in dynamic and static environments.

Fig. 11. Replicate coring control module.

Fig. 12. Inclinometer circuit board.

Fig. 13. Navigation sensor.

5.3. Smaller-diameter core barrels and new cutting heads

The replicate coring drill employs new smaller-diameter core barrels (137.0/151.5 mm inner diameter (ID)/outer diameter (OD) DISC drill vs 127.0/139.7 mm ID/OD RCS) along with smaller-diameter 3-cutter coring heads (Reference Gibson, Johnson, Shturmakov, Mortensen and GoetzGibson and others, 2014). The smaller-diameter core barrels and cutter heads are necessary to allow the deviation to be made. The modular ice chip screens for the replicate coring drill have the same 108.0 mm ID and 119.1 mm OD as the base DISC drill.

The set of heads used for the replicate coring drill consists of a coring head (Fig. 14), a milling cutter head with shoes (Fig. 15), a broaching head (Fig. 16), a conical retrieval head (Fig. 17) and a magnetic retrieval head (Fig. 18). The replicate coring drill assembly with a milling head is presented in Figure 19. Each of these accessories serves a very specific purpose. The procedure was developed to start the deviation from the parent borehole and to perform the replicate coring itself; this procedure and the use of the different cutter heads is described in detail by Reference Gibson, Johnson, Shturmakov, Mortensen and GoetzGibson and others (2014). The new milling head includes the radial and axial shoes and provides an improved geometry for axial cutting. The head prioritizes axial cutting over radial cutting and minimizes cutter contact with the borehole wall. The axial feed rate is up to 50 mms⁻¹, and the minimum controlled axial feed is 1 mm per tooth. The cutters have a 30° rake angle, a 9° relief angle and minimized cutter height. A milling cutter head and a broaching cutter head are utilized to start a deviation, and then a newly designed coring cutter head is used to retrieve replicate cores. A conical tool and a magnet head are the emergency recovery tools needed to ‘clean’ the replicate borehole of metal and non-metal debris or broken cutter head parts.

Fig. 14. Coring head.

Fig. 15. Milling head.

Fig. 16. Broaching head.

Fig. 17. Conical head.

Fig. 18. Magnetic head.

Fig. 19. Replicate coring drill with a milling head.

5.4. Borehole camera

One of the new subsystems that were designed and built before the 2011/12 field season at WAIS Divide is the borehole camera (Fig. 20). This battery-powered camera has a LED light ring at the front and is designed to internally record video data during borehole trips (Fig. 21).

Fig. 20. Borehole camera mounted in the core barrel.

Fig. 21. Images of parent and replicate boreholes.

5.5. System characterization, validation and field testing

The replicate coring system was ready for the test at WAIS Divide in January 2012. Three major issues were identified as a result of this test: excessive sonde flexure, non-optimal cutter geometry and uncontrollable axial ‘stick–slip’ of the drill.

In order to be ready for a successful replicate coring production season in 2012/13, IDDO decided to design and build special testing fixtures that would allow performance of extensive modular and system level testing in order to characterize the system and to calibrate the drill in the laboratory (Reference Johnson, Mortensen, Gibson, Goetz and ShturmakovJohnson and others, 2014). The first fixture (Figs 22 and 23) allowed for simulation of the borehole and for measurement of the drill deflection along with the forces applied to the cutter head.

Fig. 22. Drill deflection simulator.

Fig. 23. Oval borehole simulator.

The second fixture (Figs 24 and 25) was built to test and develop the optimum broaching and milling cutter designs that would be used in similar conditions in the field.

The laboratory tests let the design team characterize the system performance for various borehole shapes, validate finite-element models, measure and simulate axial and radial forces available at the cutter head, and determine the forces required to initiate a side-wall cut. They also allowed IDDO to test and debug the electronics and software. Based on the tests, IDDO extensively revised the replicate coring procedure, outlining every step of deviation drilling, from creating an initial deviation to the recovery of the last ice core taken from the deviation borehole.

Fig. 24. Cutter testing simulator.

Fig. 25. Testing in ice.

Additional system testing took place at WAIS Divide before production drilling was initiated. This final testing provided IDDO with confidence that the deviation coring concept was correct, and that replicate coring would be successfully completed.

5.6. DISC and replicate coring production drilling

All aspects of the DISC and replicate coring production drilling are summarized in Reference SlawnySlawny and others (2014).

1. Major system performance data:

2. Drill descent speed: up to 1 ms⁻¹

3. Drill ascent speed: up to 2.5 ms⁻¹

4. Average core length 3.2 m

5. Maximum core length 4 m

6. Cutter feed rate ∼0.5–1.2 mm per tooth

7. Cutter teeth quantity: 4

8. Cutter speed 80 rpm

9. Typical time to take a full core: 15–20min

10. Percentage of core recovered vs attempted: 100%

11. Cutter life: cutters sharpened after every ∼200 m of core taken

12. Three drill motor sections used in creating 3405 m DISC parent borehole plus 285 m of replicate deviation borehole

The parent borehole inclination data for the last coring run are presented in Figure 26.

Fig. 26. Parent borehole (WDC06A) inclination, last coring run of 2011/12 season (31 December 2011).

The highlights of the ICDS/IDDO multi-year drilling endeavor using the DISC/replicate coring drill at WAIS Divide include:

1. Accomplishing drilling goals over eight field seasons from 2005/06, when the camp was established and the drill and core-processing arch were erected, to 2012/13 when the replicate coring was successfully completed (Fig. 27)

2. Drilling 3405 m of ice, a new US depth record for ice coring and the second-deepest ice core drilled by any nation

3. New ‘3 for 1’ and ‘2 for 1’ techniques of drilling brittle ice, allowing two or three consecutive ice cores to be drilled and broken before bringing them to the surface and eliminating the need to cut the brittle ice cores on the surface for shipping (Reference SlawnySlawny and others, 2014)

4. First-ever robotic deviation drill

5. Replicate coring: five azimuth and depth-controlled deviations from four target depths (3008 m, 2421 m, 2227m, 1957m) (Fig. 28)

6. 285 m of replicate core (first core 17 December 2012)

7. Excellent ductile, brittle, warm and replicate ice-core quality

8. Success is the result of not only good planning and design, but being prepared for the unexpected

Fig. 27. Ice-drilling progress for borehole WDC06A, parent borehole.

Fig. 28. Ice-drilling progress, replicate borehole.

5.7. Future system development

The American and international science communities are interested in recovering 1 Ma old ice in East Antarctica at an altitude that will likely be >3000 m. The ice thickness would be <4000m. The DISC/RCS appears to be a very good candidate for this mission.

At the time of writing, the drilling site for this endeavor is undetermined and no science proposals for drilling have been submitted. As a result of IDPO (Ice Drilling Program Office) and IDDO’s long-range planning, IDDO will initiate analysis of cryogenic properties of the entire system since the drill sonde will need to operate at –60°C if used in East Antarctica. IDDO also plans to improve the drill system based on its experience at WAIS Divide. Current planning anticipates that the system will be field-ready by the 2018/19 season in Antarctica.