Well shafted

A BOART Longyear drilling team has risen to the challenge to cut a 1500mm bit to a depth of almost 170m in tough ground that included two aquifers.

Noel Dyson

Oh, and they had little more than four weeks to do it.

The team was cutting a ventilation shaft for an underground coal mine in southwest Wyoming and managed to complete the feat several days ahead of deadline.

While the technique applied to a US coal mine, it could have application in Australian hard rock operations should projects encounter similar challenges.

The Boart Longyear team used a dual-tube flooded reverse-circulation drilling technique that allowed the bit to penetrate loss circulation zones that precluded the use of a more traditional raised bore rig.

The technique pumps air through the outer tube and forces mud and cuttings upward through the inner tube, stopping them from plugging porous rock formations.

The Boart Longyear boys used a LM200 top head drive rig fitted with stabilisers and the massive bit.

To reduce the risk of a mine entry collapse the borehole was drilled off to the side of the mine tunnel and 1350mm casing with 12.5mm wall thickness was installed and cemented in place.

Underground mine crews then mined over and punched through the concrete to open up the shaft.

“What our team accomplished was absolutely remarkable,” Boart Longyear US/Mexico territory contract manager Jason Lamb said.

“Drilling such a large diameter hole through loss circulation formations in a single pass – and in less than a month and a half – is something few companies are capable of performing.”

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