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Hammer Time!
Air Hammer Technology Proves to be the Driving Force in Hard Rock HDD Project

Hammer Time

Making a living in directional drilling is never easy, however, once in a great while a bore so challenging arises that it causes every potential bidder to take pause. One by one they decline to bid, yet something deep down inside an accomplished contractor causes him to say YES, we can do that. While most saw eminent failure, one saw the challenge of a lifetime. This story is about such a bore.

Drilling in northern Pennsylvania can present a number of challenges. Steep valleys cutting through layers of hard rock and then quickly shifting to soft rock making steering tough, and borehole stability even tougher. Throw in a high-quality cold water fishery (mountain stream) and a state highway crossing and this pipeline installation to provide water to nearby gas wells presented many challenges.

The challenges presented were: a 1,100-ft bore length, bore path complexity including a 60-ft descent, a RH turn of more than 50-degrees followed by an ascent of 180 ft up to the exit. The bore path crossed a Pennsylvania state highway followed by a creek that is protected by state law under The Clean Water Act of Pennsylvania and then up the mountainside that was covered by dense undergrowth, limiting travel in the slope. And to get it all started, the entry area was loose gravel covering 10 ft of concrete bridge footer debris and another 5 to 10 ft of boulder and stream gravel.

After studying the bore for nearly a year, Pennsylvania-based Directed Technologies Drilling (DTD) president Dan Ombalski, determined that the job could be completed with the proper tools and onsite technical support from the manufacturers.  

The first challenge was the bore profile. The bore path’s initial heading was nearly perpendicular to the target in order to meet Pennsylvania DOT criteria for the highway crossing and to avoid a bridge footing. The bore would commence at a minus 17 percent slope, taking it on a 60-ft descent that would include crossing a highway and then finding its way to level as it passed from under the creek. Once the bore cleared the creek, it immediately began steering to the right to accomplish a 50-degree change in direction while
simultaneously beginning the 180-ft ascent up the mountainside.

The next challenges were the ground conditions. The subsurface conditions were varied and began with a totally unconsolidated mix of concrete, river rock and gravel for the first 100 ft. This transitioned into moderate to very hard sedimentary rock, shale and sizeable boulders through the remainder of the bore. “We obviously needed something that was both durable and highly maneuverable,” noted Ombalski.

Next, were the environmental challenges presented by crossing a Pennsylvania state highway and the ecologically-sensitive, high-quality cold water fishery protected by state law under The Clean Water Act of Pennsylvania. Due to the protected status of the creek, extreme care had to be taken to prevent drilling fluids from entering the creek. This fact alone eliminated the most obvious tool — the mud motor — from consideration and all but eliminated the rod-in-a rod style drilling system, as well.

Distance was also a factor. DTD project manager Jason Yablonski noted that the remote mountainside exit, required upward of an hour to reach, traveling via 4x4 truck and a final run down the mountain on an ATV. “The difficulty reaching the exit provided a number of logistical problems, perhaps biggest being the inability to handle drilling mud on the exit side,” Yablonski said. The remote setting combined with the curved nature of the bore path presented a significant challenge.

The final wrinkle to the job was the dense vegetation that covered the mountainside. The vegetation consisted of thick Rhododendron and Mountain Laurel — the Pennsylvania state flower. The bore would need to be completed without damaging the native vegetation. Even absent the overgrowth, the intense mountainside terrain, with loose rocks and rugged outcroppings, made it nearly impossible for a man with a traditional walkover locator to navigate the bore path. It was clear that a wireline guidance system would be required to accurately track the progress of the bore.

Air Hammer to the Rescue
With the challenges assessed, the DTD crew’s first order of business was to identify the right combination of equipment. The priority was to find tooling suppliers that could address the steering requirements and maintain control of the bore hole. The StraightLine HDD Air Hammer System emerged as an early contender.
Ombalski contacted StraightLine HDD in May 2010 to discuss two rock projects. “At the time, in addition to the Pennsylvania project we were also prepping for an upward-inclined, casing advanced bore in the mountains of Colorado,” recalled Ombalski. Ombalski was put in touch with StraightLine Air Hammer specialist Ron Becker and operations chief Joe Phillips to hash out the details.

The StraightLine RockEye Hammer system rose to the top of the list of candidates by virtue of its down-hole geometry and precision two-point directional control; characteristics proven to be extremely responsive and steerable. StraightLine is known for its technical abilities in assisting contractors with engineered tooling solutions. Ombalski also knew the StraightLine Air Hammer system had a growing list of successful bores in a number of very technically challenging conditions.

In addition to the steering issues, there were two additional considerations: hard rock and frac-out potential. While there were no borings across the length of the bore path, it didn’t take a geologist to see that the bore would travel through a lot of rock. After entry, the bore would pass through concrete debris and boulders before getting into 180 vf of near horizontal layers of siltstone, shale and hard sandstone. Some of the sandstone was extremely hard. Finally, the rock over the last 300 to 400 ft was essentially a pile of school bus-size boulders.

“The risk of frac-out in these conditions was extremely high,” Ombalski observed, “So we felt the StraightLine Air Hammer was the ideal choice for the Pennsylvania project — due to the unconsolidated substructure of the project.”

StraightLine agreed to provide Becker to work alongside of the DTD crew for the duration of the bore.

Guiding the Hammer
Once the RockEye hammer was selected, the DTD team turned its attention to the question of the guidance system. “The tree cover and dense underbrush was so thick, it took the survey team two entire days to plot the 1,100-ft bore,” recalled Becker. Using a walkover guidance system would be impossible under such conditions.

In a HDD industry first, DTD drilling supervisor James Ditto paired a StraightLine RockEye Hammer system with wireline guidance equipment from Sharewell. Explaining the decision, Ditto said, “Sharewell had never paired its tool with an air hammer in an HDD river crossing, but [it was] the only company willing to try. It was a good match that worked well.”

Sharewell technicians worked alongside DTD to assure that the planned bore path was maintained.

Putting the Hammer Down
Directed Technologies Drilling teamed its CMS 6015 with the StraightLine AH5.0 RockEye air hammer system and an Ingersoll Rand air compressor, providing 1,200 cfm and 350 psi.

The air hammer entered the ground on April 15, 2011. The first 100 ft of the bore was cased with 12-in. steel
with the assistance of TT Technologies. The casing was set at the precise angle required to achieve the necessary depth at the creek, stabilize the bore and provide a passage for air to escape to the surface without eroding the sub structure of the road and footings of the nearby bridge.

Halfway across the creek, the hammer achieved a depth of 38 ft deep, and was at a minus 5 percent. The next two drill rods leveled the bore in preparation for angling the hammer up the mountainside.

Becker was onsite to guide the hammer on its next test. “After navigating the highway and creek crossing, the hammer was at the base of the mountain, nearly 65 ft below the surface,” he explained. “The real test for the hammer was the simultaneous 20-degree up turn and the beginning of the 50-degree steer to the right through 180 vf of layers of siltstone, shale and hard sandstone.”

The majority of the steering to the right was accomplished in the first 500 ft after emerging from the creek beginning at the base of the mountain. There was significant wait time in between each drill rod to get precise feedback from the locating equipment, insuring that the hammer was on target. The final 300 to 400 ft of the bore encountered numerous school bus-size boulders.

The hammer emerged from the exit location on May 2, 2011, on target. Actual hammer time was just under 60 hours. “The hammer did its job and the rate of penetration was fast in very hard rock, we learned many things that will help us on our next bore,” Ditto said.

Don Cary is president of StraightLine HDD.
Courtesy trenchless Technology
 
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