Design Modifications for Recovering 4-Meter Ice Cores with the DISC Drill
|Abstract:||The initial conceptual design of what is now called the Deep Ice Sheet Coring (DISC)
Drill envisioned a drill capable of drilling a 3-meter core with ongoing design
improvements leading to cores of greater length (Eustes et al., 2003). As the
configuration of the drill was finalized during the design process (Shturmakov et al.,
2007; Mason et al., 2007), the combined core barrel and screen (chip collection) section
was set at approximately 11 meters. Based on the experience others had gained in
operating other drills, the ratio of core length to length of chip collecting was calculated
and the core length for the final configuration was estimated to be approximately 4
meters with the remaining 7 meters being needed for chip collection.
During the summer of 2006, the DISC Drill was tested in Greenland (Johnson et al., 2007). During the test, it was found that the practical limit of the core length was approximately 2.5 meters although a core of 3.1 meters was collected. The chips were not packed in the screen section as tightly as predicted and the inability to collect more chips limited the core length. During the test, the drill fluid consisted only of Isopar K rather than the Isopar K - HCFC 141b mixture that would be used during production drilling at WAIS Divide. ICDS personnel believed that chip packing and core length would improve with the increased density of the drill fluid. It was also thought that increasing the screen section length would allow for more chip collection and improve the core length in production drilling.
The DISC Drill was first used in a production drilling mode during the 2007-2008 Antarctic field season at WAIS Divide. Core length did improve somewhat at WAIS Divide with lengths of approximately 2.7 meters being consistently achieved; the maximum core length drilled was approximately 3 meters. ICDS engineers determined that the greater density of the drilling fluid had minimal effect on increasing core length and that adding length to the screen section yielded diminishing returns.
Because increased core lengths would save total time required to drill a deep hole by reducing the number of trips in and out of the hole, ICDS started to consider modifications to the drill that would allow core lengths approaching the 4-meter design target. Several, separately or in conjunction with others, appear to be promising and are being pursued as resources allow.