On a pipeline installation near Godley, Texas, pipeline contractor Pumpco Inc. of Giddings, Texas, was contracted to install numerous 12-in. steel pipeline laterals. The job took place in the Barnett Shale Formation, southwest of Fort Worth, which is one of the most active drilling locations in central Texas.
On the last remaining bore, Pumpco was faced with several challenges: a 640-ft bore that included 35 ft of elevation changes and crossing a Santa Fe railroad track and Texas Highway 171. The challenge was further compounded by ever-changing subsurface conditions, which alternated between unconsolidated soil and rock to solid shale and limestone formations.

A similar crossing had been completed with a mud motor less than a half-mile north of the Godley bore. The crew encountered extensive fluid loss and surface frac-outs in the unconsolidated and layered formations. The frac-outs caused the inspectors considerable concern due to the threat posed to the sub-base integrity under the highway and railroad right of way.

Pumpco kept three Vermeer D100×120SII horizontal directional drills busy for months, crossing roads and creeks along the 31-mile right of way in Johnson County. Due to the fluid loss and frac-outs encountered in previous bores, Pumpco was compelled to consider an alternative pilot drilling method for the Godley bore. The Steerable Air Hammer System from StraightLine was selected.

Ron Becker, StraightLine’s lead air hammer field technician, was contacted to consult on the job. With more than 30 successful air hammer bores in layered rock formations in many of the most demanding conditions in the United States, Becker was confident the bore could successfully meet the rigorous Texas Department of Transportation (DOT) and the Santa Fe Railroad requirements.

For this bore, a StraightLine AH5.0 Air Hammer System was paired with an Ingersol Rand 1100-cfm/350-psi air compressor. Becker, knowing the bore would contain both unconsolidated and solid formations, selected this compressor for its output volume, which helped keep the hole clean throughout the duration of the bore.

The Bore

The Pumpco crew set up its Vermeer D100×120SII directional drill approximately 430 ft east of the railroad tracks, at an elevation 35 ft higher than the final exit pit, located just more than 200 ft west of Highway 171. The sloping terrain along the bore path also included a ditch on both sides of the railroad track and a steep bank on the west side of the highway nearest the exit pit.

The extremely tight bore profile was drawn up by Universal Insco, the design engineering firm on the job. As the bore progressed, measurements were taken at regular intervals with a DigiTrak Eclipse walk-over drill head monitoring system. In most cases, measurements and marks were made at each 20-ft drill pipe change-out. At critical points along the bore, measurements were taken every 5 to 10 ft. Throughout the course of the bore, the air hammer’s steerability was precise, and kept the bore path within less than 6 in. of the bore plan over the entire length of the bore.

Throughout the 640-ft bore, the air hammer maintained a near perfect left-to-right and top-to-bottom running line. It was also important to maintain the bore plan profile so that the bore hole produced would easily accept the installation of the 12-in. steel pipe that would be installed. The pilot bore and backream were completed on the third day and the 640-ft section of pipe was easily installed into the hole in a few hours the following morning.

Observations

Progress was measured each day by the number of drill rods added to the drill string. On average, a 20-ft rod was added every 30 minutes, depending on rock hardness. Interestingly, it was observed that softer formations took longer to penetrate. This performance was obvious to the Fort Worth-based DOT inspectors, with one saying: “When the bore crew hit rock, it didn’t stop the air hammer. It’s obviously designed for hard conditions. The harder the better.”

State DOT and railroad inspectors carefully observed the pilot bore over its 16-hour duration. Officials were interested in how the StraightLine Air Hammer minimized environmental risks. The inspectors’ questions were answered during the first day, as the “spoils pit,” where the bit entered the side of the rocky layer, began to capture the spoils created by the hammer. Critical to the success of any air hammer bore in rock is getting the cuttings out of the bore hole as the bore progresses. As each 20-ft section of drill pipe followed the hammer into the ground, there was an observable spray of rock fragments carried out of the hole via air and a water/polymer combination.

Based on the first day’s performance, the StraightLine AH 5.0 Air Hammer System proved its ability to pound through difficult rock and accurately navigate extreme elevation changes all with a fluid consumption of just over 1-gpm per foot. Both the performance of the Hammer System and the extremely low consumption of drilling fluids exceeded the expectations of the DOT and the railroad inspectors. The concern of subsurface damage was put to rest. When asked to comment on the nominal amount of fluids required, a Texas DOT inspector summed it up this way:

“On a traditional horizontal crossing, using lots of drilling fluid, you don’t always know what’s happening until you are done. Problems with this method often show up weeks or even months later, when the surface structure caves in because of the void created by excessive amounts of drilling fluid.”

The Outcome

The 640-ft pilot bore was successfully completed in 16 hours. Impressively, only 800 total gallons of fluid were used to complete the pilot bore. The nominal use of water demonstrated the air hammer’s unique ability to produce and maintain a pilot hole while consuming very little fluid thereby eliminating the risks of sub-base damage.

Don Cary is president of StraightLine HDD, which is based in Hutchinson, Kan.

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