Rescuers face tough challenge to save Chile miners
The plan to rescue the 33 men trapped 700m (2,300ft) underground in the San Jose copper mine in Chile is a complex undertaking that could take engineers until the end of the year to achieve.
In a similar operation in 2002, American rescuers spent two days drilling a hole just wide enough to fit a man to rescue nine miners trapped underground.
The Americans had to drill down just 74m. By comparison, the plan to rescue the 33 men in Chile nearly three quarters of a kilometre underground is a much greater challenge. But, says John Urosek, who took part in the 2002 Quecreek mine rescue in Pennsylvania, it is not "mission impossible."
"I would put this at the tough end of things. It's not mission impossible but it's a difficult mission," says Mr Urosek who is now chief of mine emergency operations for the US Mine Safety and Health Administration.
The key to the operation is the use of a specialist drilling machine, designed to bore deep narrow holes through any rock to a depth of just over a kilometre.
How rescuers will drill to save miners
1. First, a 33cm pilot hole is drilled down to the miners' location
2. Next, the hole is "reamed" with a wider drill to a diameter of between 60 and 70cm. Debris falls down the hole to the bottom
3. Thirdly, a rescue capsule is lowered down to the men and each is slowly brought to the surface
South African mining company Murray & Roberts had one such machine in Chile for a separate mining contract. Called a raise drill, the 28.5 tonne behemoth was brought up to the San Jose mine on Tuesday.
Heny Laas, a managing director at Murray & Roberts, says the machine can drill up to 1,100m depending on the rock conditions, so "this depth is well within the capability of this machine".
However, rescuers will have to adjust the way the machine is usually operated because of the particular conditions of the mine.
Normally, engineers drill a narrow pilot hole down to a cavity underground, attach a special drill bit from inside the mine and then to ream or drill back up the hole creating a larger shaft.
Since the rock collapse that trapped the miners has blocked access to their refuge, this won't be possible at the San Jose mine which is located under a barren mountain in northern Chile's Atacama Desert.
Round the clock
"It's going to be a two stage operation," says Mr Laas. "We will drill the pilot hole from the surface, then we will have to ream it from the surface and we will probably have to fabricate a new component to do so."
Working round the clock, Mr Laas says this will take a minimum of two months, but possibly as many as three. That, of course, assumes everything goes to plan.
Mr Urosek says that in 2002 there were many delays when trying to dig the men out of Quecreek.
"In an emergency, you drill a number of holes and things can go wrong. In Quecreek, the drill bit broke and we had to go in and fish it out. It's a really tough process," he says.
Mr Laas believes his machine is more robust and is not expecting any breakages. But designing and making a bespoke part for the machine is just one of the adjustments the rescuers will have to make to reach the men.
First of all, it needs to be positioned plumb centre above the point where the shaft emerges. Secondly, the rescuers are relying on the rock being solid enough not to require support or reinforcement. There is also the question of how the debris that falls down the hole as it is reamed will be removed.
"When you drill holes of 700 meters, you get deflection and the drill bit wanders so you may not end up where you want to be," Mr Laas explains.
Computer controlled directional drilling takes care of that by minutely adjusting the bit to keep a straight line bore. But the drill has to be dead centre above the exit point. This can only be established from expert surveying of the mine interior, a job that will take several days to complete.
Once determined, engineers will need to build a special platform for the Strata 950 machine to sit on. Once drilling begins however, it's simply a question of keeping going.
However, the last few meters are a delicate point in the drilling process and pose another challenge.
"When you drill a big hole through the roof you have to be careful you don't collapse it," says Mr Urosek. "The last few hundred feet or so is very slow going."
Not a pleasant ride
Once the pilot hole breaks through, the reaming out of the shaft can begin. Again, this throws up another obstacle to overcome.
Whereas all the debris is pulled to the surface when drilling the narrow bore, when the pilot hole is reamed to make it wider, all the debris will fall down the shaft to where the miners are located.
"There will be a pile of rubble at the bottom of the hole so the miners themselves may have to be involved in removing this," says Mr Laas.
Once through however, the process of bringing the men to the surface should be fairly straightforward, Mr Urosek believes. When the nine men were pulled up through the 74m long rescue shaft in Quecreek, it took 15 minutes for each miner to be brought to the surface.
They were hoisted to safety using a standard rescue capsule, a metal tube shaped cage just wide enough for a man that is sometimes called a bullet.
"I'm sure it's not a pleasant ride," says Mr Urosek. "It's not something that you want to do but if you are trapped in a mine and want to get out you can endure it while you are being pulled to the surface.
"Miners are a pretty tough breed and in the end they are going to get these people out and these guys will do just fine."