Dual Technologies Used in Unique Gasline Installation
Pneumatic Tool Pipe Ramming Assists Directional Boring

The Conecuh- Monroe Counties Gas District in southern Alabama planned a 440 foot crossing of Interstate 65 just south of Evergreen, Alabama. They wanted to lay in a four inch steel gasline to transfer natural gas burn off from an oil well to a transmission line. The best route called for going under Interstate 65. With the permits for using directional boring for installing the line secured from the Alabama DOT, the crew was on their way.

Pulling the casing under the interstate highway made the project quite challenging because of the 20,000 pound weight of the 10 inch diameter, 440 foot steel pipe assembly. Simple math told the General Foreman for Conecuh- Monroe Gas, Mike Presley, that his D-24 Vermeer Navigator would be tested to the limit.

"In the eight months that we had used our directional boring machine," Presley said, "The longest bore we had done previously was pulling 4 inch PE 720 feet. We knew this job would be tougher. Most of this installation was in Alabama red clay with some sandy pea gravel sections. We thought that this clay would hold the bore well, but in order to pull this long string of medium diameter steel pipe, we lubricated the bore plenty during the bore, pre-reaming and pullback."

To make sure they would have a successful installation of the steel pipe in the red clay conditions, the crew planned to have a backup system available. If needed, they would use the impact force of a HammerHead Mole to push on the end of the pipe as it was being pulled through.

Because of Alabama DOT rules for boring under its highways, the crew set its entrance pit on the west side of the highway, a full 75 feet back from the westernmost boundary fence for the interstate. Since the land elevation on the eastern exit side of the bore was eight feet higher than the entrance point, the crew dug an exit pit eight feet deep to meet the bore and feed in the steel casing for pullback.

The crew planned the bore carefully so that the bore path was never less than 5 feet deep under the surface even at the lowest point in the landscape at the bottom of the road ditches. The maximum depth of the bore path reached twenty feet below the surface. Longitudinally and laterally, they were going for as straight a line as possible. They calculated that with this much load on the reverse thrust motor of the directional rig, the bore path had to be as straight as possible in order for the pullback to be successful. The more turns or corrections to the bore path, the tougher time they would have.

Presley commented on the careful planning for the bore. "When you pull back any steel, " he explained," The steel will bend a little. But the larger the diameter and the longer the pipe string, the more difficult it is to make a correction and not overcorrect or exceed the bend radius. When you exceed the bend radius, you multiply the tension factor on the side of the pipe. So it's imperative that you have as straight a bore as possible."

To help the welded pipe assembly travel in as straight an elevation as possible, the crew dug a slit trench 50 feet back from the exit pit into which the pipe entered during the pullback phase. They used a backhoe to move this long pipe assembly into position.

The first day the crew made the pilot bore, and on the second day pre-reamed the bore, and attempted the pullback. Unfortunately the first attempt ended in frustration when the swivel attached to backreamer broke 140 feet into the pullback. Realizing that this was the end of this particular bore, the crew pulled the pipe back out and reentered as closely as possible to the existing bore hole and pre-reamed it twice the next day. Pre-reaming was done with two 14 inch diameter wing cutters.

On the fourth day of the operation, the crew dug the slit trenches for the pipe assembly and lined up the pipe. With water brought onto the site by a water truck, they successfully lubricated the bore with water and a bentonite/polymer mix and pulled in the 10 inch casing 310 feet when the unit reached its limit and stopped. " It's moments like this, that you can't rush things," said Presley. That's when we were glad we had the impact tool to push the pipe.

The crew inserted a 5 3/4 inch HammerHead Mole pneumatic tool into special ramming collets at the back of the pipe assembly. This is the same method used to push steel casing under roads and railbeds. Powered by an air compressor, the pneumatic tool locked into the center of the collets and transferred its horizontal force to the pipe.

When this ram assist was set up, the crew resumed pulling in the pipe, alerting the crew via radio at the compressor to start the HammerHead Mole when the directional rig pulled, and to stop the ramming action when a drill rod needed to be removed. The pneumatic tool slid along on the ground locked into pipe never entering the bore hole. By utilizing the HammerHead's impact force, they succeeded in simultaneously ramming and pulling in the remaining 130 feet of the pipe assembly this way.

According to Presley, "The use of pipe ramming to assist directional boring, proved to be a a terrific backup system. "I wouldn't hesitate to use it again. There was a good chance we would not have made it without the ramming force of the pneumatic tool. It really saved the bore."

Provided by: Vermeer Manufacturing Company