Introduction

European Project for Ice Coring in Antarctica (EPICA) Dome C drilling began in 1996/97. The set-up, drilling equipment and operation in the first five drilling seasons have already been documented (Reference Augustin and AntonelliAugustin and Antonelli, 2002). EPICA Dome C 1 (EDC1), the first hole at Dome C, ended on 20 December 1998 at 783 m depth when the drill was deemed irretrievably stuck after several attempts to free it. In November 1999, all surface equipment was moved 10m north from its original location. A new hole, EDC2, was started from the surface on 11 December 1999. By 31 January 2001, a depth of 1458m had been reached. Three more seasons were needed to reach 3270m depth, 16 m above bedrock. The challenge of the two last seasons was to drill through what we call ‘warm ice’ (unpublished data from International Partnership in Ice Core Sciences workshop, 2004). At high pressure (>20MPa) and with ice temperatures close to the pressure-melting point, water can be produced by cutting and refrozen ice appears on various parts of the drill head. This greatly affects the functioning of the drill and especially the drilling rate. Within a few metres the penetration dropped from 200m per week to a few metres per week.

Season 6: 2001/02 (1458–2871 m)

The first runs of the sixth season saw a series of breakdowns in several parts of the drilling equipment (winch, tower, drill). Malfunctioning of the embedded electronics made the risk of losing the drill too high to continue drilling. In order to improve communication with the drill, we changed the old four-wire cable (shortened by 783 m when it was cut at the surface from the stuck drill in 1998) to a one-wire coaxial cable type (Reference Gundestrup and JohnsenGundestrup and Johnsen, 2002). A three-wheeled capstan and a pre-tensioning system were used to wind the cable on the winch. This 2 day operation (1 day to set up and 1 day for winding) went smoothly. The cable was wound at a rate of 0.3 ms^–1 under a tension of 5000 N. The new cable provided improved communication, and the drill performed with no data losses. The reduced operator stress at the control console was much appreciated by the drillers. Core production was excellent, reaching 2.8m length on average, giving a weekly production of 191 m of core, working three shifts around the clock. In the most efficient week (interval 2230–2480 m), production reached 250 m. Below 2550 m depth, deposition of chips on top of the core was noticed, preventing an easy start to the run. Below 2700 m depth, runs became more difficult, shorter and irregular. On each run there was some refrozen ice on the cutters, under the penetration shoes and on the pump drive shaft.

The inclination from vertical was very good down to 2400 m, not exceeding 0.34^˚. Below this depth, it steadily increased to 3.70^˚ at 2871 m, the final depth for the sixth season. Unsuccessful efforts were made to stop this increase of inclination by sharpening cutters, monitoring more closely the weight on the cutters and improving core-barrel guidance. Seasons 5 and 6 showed the EPICA drill’s ability to penetrate well. In addition, the drill became more reliable and the pump was more efficient, leaving fewer chips at the bottom of the hole.

Season 7: 2002/03 (2871–3200 m)

Two new drill-head designs were tested at the beginning of the seventh season. The aim was to improve the fluid circulation at the cutters’ edge. It was thought that this would prevent the ice build-up experienced during the previous season. None of the designs was successful, despite good results in the laboratory in Grenoble, France. The three most obvious differences between the laboratory and the field tests were: (1) the fluid was cleaner in the laboratory; (2) the fluid pressure in the laboratory was nearly atmospheric versus 25 MPa in the hole; (3) the drill was rigidly connected in the laboratory but was at the end of a long elastic cable in the field. By experience, we know that chips and fluid should be mixed together before transportation, especially when there is only one pump for several cutters (Johnson and other, 2007). Both new designs were weak on that specific point, so the original EPICA drill head was used rather than the newer designs. At 57 rpm, with 45^˚ face angle cutters and 3 mm pitch, production averaged 20md^–1. At 3119 m, however, production suddenly dropped to near standstill. This dramatic change corresponded to a 0.15^˚C ice temperature increase to –6.28^˚C. Ice chips were frozen onto the shoes and cutters, as though the cutting process had created a mixture of ice and water, which had then refrozen just millimetres away. Several days were spent trying to overcome this problem by changing the cutters, the cutting pitch, the drill-motor rpm and even switching to a shorter version of the EPICA drill in an attempt to lessen pump losses.

After discussion among the crew on site and consultation overseas, it was decided to use ethanol–water solution (EWS) to prevent the water produced by the cutting process from refreezing. Previously, for other projects, EWS was used as the borehole fluid (Reference Zagorodnov, Kelley and NagornovZagorodnov and others, 1994). For EDC2, EWS was prepared at a concentration to be at equilibrium at ice bottom temperature. EWS was injected at the bottom with a special heated tank built on site, connected below the motor section in place of the usual lower part of the drill (chips chamber and core-barrel section). A total volume of 17 L of ethanol and 90 L of water was delivered to the bottom of the hole in 16 different runs. Each injection of EWS enabled several drilling runs. A total of 99 runs were performed in this way for 80m of core collection. Most of the time, the core was frozen inside the core barrel and electric heat guns were used to free it. In addition, the chips were frozen inside the chips chamber. All this affected the quality of the ice core, reduced the diameter and created additional cracks from the heat guns. The use of EWS was not an ideal solution to the problem of drilling ‘warm ice’.

Season 8: 2003/04 (3200 m; no.Drilling)

At the end of the seventh season, while being returned to Europe, some of our drilling equipment (drill heads, electronic section and logging equipment) was stolen. A non-drilling season was required while the missing equipment was replaced. A temperature profile of the hole was obtained by a reduced crew of two.

Season 9: 2004/05 (3200–3270 m)

A new log of the hole (diameter, inclination, orientation, pressure and temperature) showed a few spots with narrow diameter. These narrow spots were reamed with the long version of the EPICA drill (3.5m core). Seismic measurements were also performed to determine the ice thickness more accurately: 3273 m vertical depth (personal communication from J. Schwander, 2006). Dead weights were added to the short version of the EPICA drill (1.5m core) to allow a higher descent speed and more stable drilling. We decided not to use the heated tank that had been used during the seventh season to inject EWS at the bottom of the hole. Instead, taking advantage of the North Greenland Icecore Project (NorthGRIP) experience during the 2003 and 2004 summer seasons (Reference JohnsenJohnsen and others, 2007), a plastic bag filled with 1.4 L of EWS, 50% in volume, was attached, for each run, to the motor-drive shaft inside the chips chamber. The bag, scratched by a sharpened screw, released the solution at the start of drill-motor rotation.

Core recovery was improved by putting the entire lower part of the drill, disconnected from the motor section, inside a D30 fluid bath upon its return to the surface. The bath was regulated to the ice bottom temperature. Ice core and ice chips were free and easy to recover after 45 min immersion in the bath. With this gentler treatment at the surface, the core showed fewer cracks and core quality slightly improved.

An average of 8 m of core was drilled every day, working 18 hours a day. More than 60m were drilled, down to 3264.78 m, in 13 days. The temperature at this depth was –2.64^˚C. Below this depth, and within only 2m, drilling became unstable. Different changes were tried without success (shoes, pitch, cutters’ face angle, EWS concentration and fluid circulation at the start of the run). Penetration became very difficult: 5.4m for 35 runs performed from 3264.78 to 3270.20 m. The temperature was –2.34^˚C, a difference from the pressure-melting point of 0.34^˚C. We noticed more than 1 m of chips deposited on top of the core in between two runs. For some runs, the amount of chips collected inside the chips chamber was far too large for the amount of ice chips cut. On the last run, at 3270.2 m depth, the drill became stuck without warning. Pulling up to 25 000N tension on the cable did not move it. Frozen ethylene glycol pellets (2 kg) were thrown into the hole (Reference Gundestrup, Johnsen, Hansen, Shoji, Talalay and WilhelmsGundestrup and others, 2002), and the drill was free 4 hours later. We pulled it to the surface without further difficulty. To clean the bottom of the hole of ethylene glycol slush required 14 runs, corresponding to 220 L of slush, more than ten times the original amount poured in.

In response to various concerns of the EPICA steering committee (environmental issues and drill safety), the drilling activity was stopped at 3270.2 m depth, an estimated 16 m above bedrock.

Hole Measurements

At the beginning and end of every season up to 2002/03, the hole was logged with the French logger (Reference Lefebvre, Augustin and MaitreLefebvre and others, 2002). As the logger made in Grenoble was stolen in 2003, the last measurement was performed with the Danish logger (Reference Gundestrup, Clausen and HansenGundestrup and others, 1994). The maximum hole diameter of 134 mm at 328 m depth (Table 1) was obtained after use of a special, conical, tool to fish up tiny plastic pieces coming from the motor-shaft ball-bearing cage. Otherwise, the diameter was very regular, as expected for an electromechanical drill, and varied only a few tenths of a millimetre around 129.6 mm, which is the nominal hole diameter. The over-range diameter obtained in 2004/05 was a consequence of the EWS used in drilling ‘warm ice’. The inclination is very stable around 0.34^˚ down to 2400m depth, and then increases steadily to 4.7^˚ near the bottom, in spite of efforts made to reduce it (cutter changes, cutter sharpening, and changes of core-barrel polyethylene spiral guiding pieces). The reason for this inclination increase has not been clearly identified.

Table 1. EDC2 hole measurements

Drilling Data

The drilling took five seasons (including the pilot hole), and the hole was completed on 21 December 2004 after 230 drilling days. The weekly average core production was 99 m, and the highest production achieved was 250m (Fig. 1). A weekly average of 41 runs was performed, with a maximum of more than 75 runs. The average core length varied seasonally (Fig. 2). The maximum seasonal average, 2.8 m, occurred during the sixth season (1458–2870 m), when many of the cores were around 3.2 m in length. In the final season, when ‘warm ice’ was drilled, core-length average dropped to 0.58 m. Examination of core-break tensions shows a slight decrease with depth (Fig. 3). In the very last runs, core-break forces were very high. For those runs it is difficult to determine whether the drill was close to being stuck, but most likely the run was ended in a packing situation around the drill head.

Fig. 1 EDC2 hole drilling progress.

Fig. 2 EDC2 core length vs depth.

Fig. 3 EDC2 hole: core-break tension vs depth.

Drilling Fluid

The fluid used for the EDC2 hole is a two-component fluid: Exxsol D30 and HCFC-141b mixed to give a density equal to the ice density (Reference Talalay and GundestrupTalalay and Gundestrup, 2002). Some efforts were made on site to reduce the drilling-fluid consumption. The percentage of excess fluid required varied greatly from season to season. The percentage of HCFC-141b used and mixed with Exxsol D30 also increased (Table 2). The data given are based upon the total number of drums used on site for the entire drilling activity. Starting from 44% in the first season, the fluid excess consumption decreased after an effort was made to collect drilling fluid at the surface, then increased again, reaching 315% in the final season due to ‘warm-ice’ drilling activities, including use of the drilling-fluid bath. At 13.19 Lm^–1, the theoretical volume of the hole is 45 220 L. The total fluid consumption was just over 65 000 L.

Table 2. EDC2 hole drilling fluid data

Epica Dome c Drilling Equipment Characteristics