Getting Greater Penetration When Drilling Horizontally

The residential construction near Washington, D.C., is booming with new developments. Country fields lined with streets of million-dollar homes are surrounded by pastures, spotted with Black Angus cows. The rolling hills and narrow streams are giving way to urban sprawl.

Washington Gas and contractors such as Bison Inc. — the family-owned company operated by Bob, Doris and son, Robert Ait — are feeding utility services to these monster homes as fast as the equipment allows.
The boom in the directional drilling business waned a few years ago, but quality operations like Bison have come through stronger and wiser.

When a hole needs to be drilled that others have problems with, Bison gets the call. Robert Ait operates the drill for the company and has a real feel for drilling a hole. Although he has many secrets, a lot of his success comes down to just plain experience.

“One time a guy told me he can finesse a hole. I don’t know how you do that,” said Robert Ait. For Robert, it comes down to psi, rpm, water and how and when to apply each — and the quality of the equipment they run.

“We fell in love with our little tractor,” he said, referring to his Ditch Witch 2720 drill. “And as far as I’m concerned, that 40 is the leader in the industry,” he said, pointing to Atlas Copco’s Secoroc SPX 40 tricone bit.

Bison operates mostly the 5.5-in. SPX 40 bits but also uses the 5.5-in. SPX 50, depending on the hardness of the formation. “The 50 is really all this machine can handle,” said Robert, “but what I don’t have in torque I make up for in speed [rpm].” But to protect his bit in harder rock, consistency is better.

Robert said that in this area he drills at about 30 ft an hour, or about 10 minutes per rod. “You can never go faster than what your water will allow,” he said, adding, “You can really mess things up if you go too fast.”
The more rock that is cut, the thicker the mud gets. Robert puts about 42 gals a minute in the hole. Bob Ait added, “We like to see the mud a consistency of a heavy milkshake. If it gets slushy it’s too thick.”

The size of the hole is determined by the product. For instance a 5.5-in. hole can handle three, 2-in. fiber conduits. “You can squeeze fiber in a hole, but a gas line is another story,” said Robert.

When running a gas line, specifications requires half again as much space in the hole. So an 8-in. gas line needs at least a 12-in. hole. But because the hole needs to be really clean, Robert likes to go a little bigger and use a 14-in. reamer to expand the hole. The Ditch Witch 2720 can bore up to a 20-in. hole using an Atlas Copco reamer bit in hard rock and a 30-in. fly cutter in soft material.

“The gas company is really careful with their lines, and the line can’t have gouges or scratches,” said Robert. 

Opening a Hole

Running large gas lines requires the use of a hole opener if the machine can’t drill a large enough hole in one pass. Pilot bits are available in sizes from 4.75 to 17.5 in. The strategy of sizing a drill for the most common use has worked well for Bison and the Aits can use hole openers to ream the hole when necessary.

Bison has been successful with its 2720 drill and Atlas Copco split set strategy because it gives the versatility to drill in almost any location, yet the ability to drill the necessary hole size.

Traditionally where tricone bits were used to fabricate hole openers, Atlas Copco’s split set products utilize common, random, cutting-structure bit thirds, precisely positioned to assure equal load distribution. What that comes down to is that the modern split set design has an extended the life.

Improvements in carbide shapes and grades also prolong cutting structure integrity. Bearing and seal package technology, as well as pressure compensated lubrication systems have added to bit life, but are also giving greater penetration rates.

What sets this type of split set hole opener apart is that it there is better utilization of available force, which results in better performance, and improving the penetration rate. One rotation of the cone provides complete bottom hole coverage.

For Robert, an Atlas Copco bit is the “only bit [he’s] interested in using anymore.” And as long has he continues to get greater penetrations rates and long bit life, he doesn’t see that changing.

Scott Ellenbecker is editor-in-chief of two in-house publications for Atlas Copco. He has been involved in marketing construction and mining equipment since 1995.

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