Life-Saving Underground Mission
While the February 2008 Now You Know discussed tunnel boring, it seemed appropriate to look at deep drilling in light of the recent Chilean miner rescue. It is especially appropriate because without thermal processing, the rescue would have taken much longer or might not have been possible at all.
The definition of a deep hole is anything more than four to five times its diameter. Needless to say, the more than 2,000-foot hole required to reach the Chilean miners was deep. There are several techniques for drilling deep holes. The choice depends on hole size and economics.
When the news broke that it might be Christmas before the miners could be rescued, folks from Center Rock (CR) in Pennsylvania knew they needed to get involved. Having provided equipment and manpower for the Quecreek rescue in 2002, they were familiar with what needed to be done. Although there were many unknowns, they knew they could reach the miners long before we were celebrating the holidays. CR worked around the clock to prepare equipment, and UPS shipped it for free.
Unlike the BP Gulf oil spill, where the U.S. government turned away help from other countries, Chile welcomed CR’s help. Led by the CR team, who were on site for 33 days of drilling, miners were pulled to safety on Oct. 13 after 69 days underground.
The type of drill used in Chile is called the LP drill. LP stands for “low-profile,” and it was developed by CR about five years ago –
since Quecreek. Inside the 28-inch canister – made by CR – there are four air hammers with 250-pound pistons and drill bits
(as shown in the photo) that move in tandem to dig through the rock. As the piston strikes the rock, it is fractured, and the air moves the rock chips to the surface. The drilling rig stands almost 150-feet high, and it can bore through as much as 130 feet of rock in a day.
With a mission as important as this one, equipment must be up to the task. That’s where thermal processing comes in. Most of the material used in the construction of the canister and the pistons is quench-and-tempered 4140/4340. The drill piston is carburized to create good wear and improved fatigue properties. Pistons are designed to survive 10-15 bits. They are also shot peened to improve the fatigue strength.
Companies like CR rely on commercial heat treaters like Rex Heat Treat in Bedford, Pa., to help their tools meet the demanding requirements of applications like deep-hole drilling.
The “expendable” bits/inserts are tungsten carbide (WC) or diamond-impregnated WC. For a 6.5-inch hole, an insert can last 1,000-25,000 feet, depending on the rock type. In Chile, the rock was both hard and abrasive. The WC inserts are produced using hot isostatic pressing (HIP) followed by sintering. The diamond bits make use of a tetrahedral press. A WC insert is used with polycrystalline diamond powder. Temperatures of 1800-2500˚F and pressures to 1.5 million pounds fuse the diamond to the carbide, which improves the wear resistance of the insert 50 times.
Industrial Heating extends its gratitude to Center Rock for this information, the accompanying photographs and mostly for their unselfish work, which resulted in the rescue of 33 trapped miners in Chile. IH