For decades glaciologists have used hot water drilling for rapid access to the inner workings of glaciers. Though both hardware and energy intensive, hot water drilling provides the fastest and most efficient way to access the deep ice. Water is also the ideal drilling fluid for the IceCube application. It is readily available, the hole is self-healing since the ice returns to essentially its original condition after the deployment of the detectors, and its optical properties meet the experimental requirements. Importantly, this technique is the only drilling method capable of meeting the hole straightness requirement. Finally, components used to operate the system are standard, reliable, and readily available from industrial outlets.
IceCube holes will be drilled with the Enhanced Hot Water Drill (EHWD). The EHWD represents an evolution of the AMANDA drill, which was a research tool, to a production drill capable of drilling 16 holes per season with 60 cm diameter to a depth of 2450 m. We have used data gathered from drilling 23 holes for the AMANDA project to depths varying from 1000 to 2400 m to design a drill enabling us to drill and instrument a hole every 3.5 days.
Conceptually, hot-water drilling is as simple as it sounds. Water is pumped at high pressure through a heating system and heated to near boiling temperature. The water is then forced through a drill nozzle that directs a high-velocity stream of hot water against the ice in the hole, melting it. The drill is steered by gravity.
Hot-water drilling is energy intensive because of the large amount of energy associated with the ice-to-water phase change. This distinguishes hot-water drilling from the more energy-efficient, but much slower ice-coring method.
A hole with a 60 cm diameter and, for instance, a 1 m depth has a volume of 78 gallons. With South Pole ice at -40°C, an equal volume of 100°C water is required to melt this volume of ice. To achieve this in one minute, or drill at a rate of 1 m/min, requires 2 MW of heat. After accounting for heat losses to the ice surrounding the hole, we conclude that a 5 MW system is required to drill at an average rate of 1.5 m per minute. With this rate the EWHD can deliver IceCube holes to 2400 m depth in 40 hours with 7000 gallons of fuel. This can be seen as follows:
The relevant drilling parameters are pressure, flow and temperature. The heat delivered is the product of flow and temperature. For constant pressures this flow varies as (hose diameter)8/3 and therefore this important parameter drives the performance of the EHWD system.
The conclusions presented above are buttressed by the wealth of data acquired with experimental AMANDA drills.
While hot-water drilling is conceptually simple, meeting the power requirements for safe and reliable drilling operations can be quite challenging. Drilling holes at the rate proposed to 2400 m depth requires the design, construction and operation of a major industrial complex in a hostile environment. We list below the salient features of a hot-water drill system meeting our requirements.
Initially, the AMANDA drill was designed to provide holes to 1000 m depth for the deployment of 10 in bathyspheres and the 1.5 in diameter cable connecting them to the surface. This was accomplished with 1 MW of heat delivered through the largest commercial synthetic hose available (1.25 in). Drilling time was about 70 hr using 4500 gallons of fuel per hole.
The current AMANDA drill was designed to drill to 2000 m. It requires three hose reels to hold hose reaching this depth. The heat input to this drill was increased to 2 MW. While drilling in the upper portion of the hole with only one reel of hose connected into the system, high flow rates and rapid drilling rates were achieved. Even though the ice is about 25°C warmer near the bottom of the hole, the flow rate decreases significantly due to friction with the hose wall. With reduced flow rate, most of the heat is lost through the hose wall before reaching the drill head. Thus, in the lower part of the hole, drilling is very slow as a result of the delivery of low-pressure cool water to the drill nozzle. To drill efficiently, the heat delivery to the drill head at 2450-m depth should be comparable to the heat delivered by the drill at the top of the hole. Three holes were drilled to 2450 m with substantially increased fuel consumption and drilling times. Drilling to 2400 m required 120 hours and 13,000 gallons of fuel, demonstrating that the drill had reached a practical limit. In summary, the current AMANDA drill system is capable of drilling only four of these holes per season at these fuel consumption rates and would require 150 hours of drilling time per hole. The current drill also requires several weeks to build-up and build-down each season, limiting actual drilling to about six weeks per season.
Requiring that the heat input achieved with the AMANDA drill is doubled and a constant water flow is maintained through the entire drilling process, both of which are possible only with a single-reel design, one can extrapolate from drill data collected during the first 1000 m of AMANDA drilling. This extrapolation indicates that drilling to a depth of 2400 m can be achieved in 40 hrs while consuming about 7000 gallons of fuel per hole, corresponding to a savings of about 400,000 gallons of fuel for IceCube's planned 80 holes. Note that faster drilling is inherently more efficient because less time is spent warming surrounding ice. This extrapolation matches our earlier estimate based on the thermodynamics of the problem.
To deliver the required heat without increasing pressure, the hose size must be increased from 1.5 in to 2.5 in diameter. The hose must be mounted on a single large hose reel to eliminate the 8 hr hose reel changes currently required for each hole during drilling and drill extraction. The drill components are housed in mobile drilling structures that can be quickly towed into place and integrated, so that the drill build-up and build-down time must be reduced to a total of about three weeks, permitting the drilling of 16 holes during an approximately 75 day field season.
To improve fuel consumption we need to drill the holes quickly, particularly in cold ice. This requires more heat at the drill nozzle. The amount of heat delivered to the drill nozzle is a function of the hose diameter (given the desire to avoid hose pressures greater than 1000 psi). The velocity of the water in the hose must be kept above 10 ft/s to avoid losing too much heat through the hose wall.
The discussion of the requirements above shows that, relative to the present AMANDA drill system, the general design criteria of the EHWD are:
Additional design criteria are set by the need to:
Other objectives are:
To achieve a drill design that meets these requirements, each major subsystem must be designed accordingly. The major subsystems of the drill are:
The ways in which these drill subsystems will meet the various efficiency, speed, and heating generation and delivery requirements are detailed below.
The Heat Delivery Subsystem includes the high-pressure hose and the hose reel.
