The engineering challenges involved in drilling Antarctic ice

The Antarctic is the most untouched land mass on the planet. Like rings of a tree truck, layers of snow and ice laid down over millennia enable scientists to peer back in time. But first they need to find a suitable site that has the necessary prehistoric ice reserves. Analysis of ice allows scientists to look back through history at the various ice age cycles and the effect that human produced CO2 has had. From sediment records it's known that in the past the planet was warmer and it's now thought that the planet will get warmer once again in the future. Julius Rix, a drilling engineer with the British Antarctic Survey, explained the challenge. He said: "We are looking for a continuous record of ice from the present day all the way back one million years. Unfortunately, once the ice at the bottom of the core has melted the data is lost, which is what happened at Dome C over a decade ago. That limited the data age to 800,000 years. Dome C is a French-Italian research station on the high east Antarctic plateau. Its average surface temperature is close to -55°C. Research teams only visit during the polar summer when temperatures are least harsh. "We'd like to get to ice from 1.5million years ago but we don't know if that's even possible," said Rix. "The ice begins to melt at the bottom, so we were losing the oldest ice because of heat produced from geothermal activity of the earth, and ice on top acting as an isolator, melted it." Part of the problem is that Antarctica is twice the size of Australia and remains largely uncharted. While satellite data and computer models have revealed fresh insight, geothermal activity and hence temperatures deep below the thick ice sheets still remain a mystery. Finding the oldest cores means finding deep ice still intact. BAS has typically used large 5-inch diameter drills with a hollow core that enables samples to be collected in the bore of the drill. This is a slow process that takes months to get to the necessary 3km depth of one million year old ice. So choosing a site that has a relatively high geothermal temperature could spell disaster and ruin many months of effort. However, to make sure they get it right, the British Antarctic Survey is taking part in a project that will drill six separate 3-inch holes, 600m down. The aim is to assess geothermal activity and help the selection of a future main site to hunt for age old ice. "By drilling shorter holes quickly and putting in temperature sensing cables we get a profile for the top 600m," said Rix. "Then the modellers will be able to extrapolate the rest of the information. "We use an 8m drill on the end of a cable, it's known as wire line drilling. The drill has two cutting blades, a bit like a conventional drill, and these are attached to an outer barrel that spins. As the outer barrel spins the spiral of the drill moves ice chippings [akin to swarf] upwards." Helping with the design and specification of the motor and gear unit for the drilling rig was Paul Williams, a senior sales engineer at Maxon Motors. He said: "Conventionally you drill a core, take it out and then move on to the next one. With this, we are trying to move quickly, so it looks more like a conventional drill and we take the chippings. We know the depth where they come from so we can measure the age of the ice." The normal concept is to use a heavy motor and gearbox to enable the 5-inch bore holes to be drilled out. However, as the team is going to be drilling smaller holes to a relatively shallower depth, a motor and gearbox needed to be re-specified. However, as the team had not done this before, it was difficult to know what was actually needed. So part of the work was to calculate the forces and torques required. In order to get the power, it uses an epicycle gear module from Maxon Motor. This holds the planet gear stationary, with the sun and annulus turning, enabling the outside of the barrel to turn at the necessary speed and torque. The actual drill remains stationary allowing chippings to travel up where they can be collected. "You pull out the whole drill, empty it, and you get chippings," said Rix. "The chippings might be mixed up in the spiral, and we will not get that fine resolution of mm you get from a core. It is every 0.5m or so we will get an average reading. However, we can then compare that rough data with accurate data of other cores and find out its age. Then the climate modellers will be able to map the geothermal surface activity and determine the best location to drill a full core and look for the million year old ice." BAS contacted Maxon Motor as it had very specific requirement of a powerful motor, with high torque, in a small diameter to fit in the drill. They needed to be able to vary the speed but get constant torque, even at a slow speed. Initial calculations suggested that a motor in the kW region would be required. So Maxon Motor initially spec'd a 42mm 1.2kW motor to meet the requirement. However, it then looked at an alternative, a 250W motor in combination with a gearbox. "What we found was we didn't need the torque we thought we needed," said Rix. "The major thing affecting it was the cutter design, so we tried different drill bits and found the design used for ice fishing was orders of magnitude better than anything else, and that allowed us to use a conventional off the shelf motor." Maxon Motor's off-the-shelf combination has proved to be more than capable and did not even need a different lubricant at the extreme temperatures the drill is exposed to. Power at the drill sites is generated using a standard Honda 5kW generator as it is able to work well in the low temperatures and high altitude. "Climate change is going to affect all of us," said Rix. "With this research we can be more confident of what the effects of climate change are and how to react to them. "Finding ice greater than one million years old is a massive international undertaking, and we're contributing to that by identifying the best possible location to drill. And, I haven't been to that part of Antarctica, so it is an exciting prospect. Getting invited to go to the high east Antarctic plateau doesn't happen very often."