Gas Main Extraction: Percussive Pipe Ramming Pulls (and Pushes) Its Weight
Pipe extraction is not a new process, but one that has been refined over the last 20 years. Through the combination of several techniques and tools, including pneumatic pipe rammers, contractors have become skilled at removing significant lengths of pipe from the ground.
On a recent project in Ohio, a 12-inch diameter steel gas main that had been in service for many years, began showing signs of internal and external problems. The owner had no choice but to replace 2,400 feet of aging pipe. The right-of-way corridor was very congested, however, and no room was available for a new installation. The only viable option was pipe extraction. The project contained a high degree of risk and required technical expertise.
Pipe Ramming Specialist Rick Melvin from trenchless equipment manufacturer TT Technologies, Aurora, IL, provided technical consultation on the extraction project. According to Melvin modern pipe extraction was in part developed through pneumatic pipe ramming HDD Assist methods. He said, “You’ll find pipe rammers on HDD projects all the time. Typically they are being used in an HDD Assist method to install the pipe and overcome hydrolock. But pipe extraction has also been part of this process. For example, when a drill stem breaks a ramming tool with a specially fabricated sleeve can be used to extract or retrieve that stem by applying percussive force.
“The process is a bit more complicated for a full pipe extraction, especially one that’s 2,400 feet long and has been in the ground for a long time. However, for the project, Midwestern Contractors [Elburn, IL] was able to extract the 2,400 feet of heavy wall pipe, weld and hydro-test a new pipe string and pulled it in behind the old in about four weeks. That’s impressive.”
Midwestern Contractors has been installing piping systems for the oil and natural gas industry for over 60 years. During that time, Midwestern Contractors has grown and kept pace with the industries it serves. Midwestern Contractors has built a reputation for quality work, an experienced workforce and quick response. The company has been successful in staying ahead of the fast paced changes in the natural gas and refined products industry. Projects like the pipe extraction in Ohio demonstrate the contractor’s expertise and skill.
Pipe Extraction Process
Extracting 2,400 feet of 12-inch steel pipe required significant planning. According to Melvin, the process is somewhat of a balancing act.
Melvin said, “Tension by itself would not be able to move the steel pipe or overcome the friction forces that hold the pipe in place. That’s why on projects like this, extraction is about combining forces in a way that meet the requirements of the jobsite conditions. First, you have pulling force on the front end of the pipe which can be generated in a number of ways, through a directional drill, excavators, etc.
“Second, you have percussive force typically generated by a large pipe rammer or combination of pipe rammers. The percussive force can be applied on the back end of the pipe, the front end of the pipe or both. Balancing the percussive and pull forces is the key to successful extraction.”
Pull force prevents the pipe from reverting to its original position and allows any movement to be maintained when applying percussive force. This combination of a point force combined with a steady pull actually imparts movement to the pipe mass and once in motion it becomes easier to maintain the movement.
Each extraction project is unique with many variables determining the amount of pulling force and the requirements for the size of the pipe rammer. Melvin said, “There are a variety factors that you need to keep in mind when planning a pipe extraction. What is the quality of the existing pipe? What’s the diameter, wall thickness, grade of steel? Is there a pipe coating? What type of soil are we dealing with. Are there ground water issues? That’s just the beginning. You have to breakdown the jobsite and anticipate issues in advance.”
For the Ohio extraction project two 24-inch diameter pneumatic pipe ramming tools, one on each end of the pipe string, two five-part steel block and tackle units on sleds, and two D8 CATS, applying a combined force of over 186,000 pounds of force were utilized to extract the pipe.
On the Job
There were space constraints on each end of the project. On the push end of the pipe string or launch pit, the pneumatic pipe rammer was supported by track hoes and guided towards the entry point for the first 40 feet into the launch pit. The launch pit was excavated in a way to serve two purposes. The first was to allow for the needed alignment between the pneumatic rammer and the angle of the pipe. The second was to create a containment pit for bentonite drilling mud to coat the pipe as it entered the ground.
On the pulling end, or exit pit, the pneumatic rammer was supported and positioned at the optimum angle by two track hoes moving in conjunction with the pipe, as the D8 CATS and the 10:1 blocks applied pulling force. The space was extremely tight and only allowed for the extraction of one 40-foot section of pipe at a time when the 10:1 blocks were used, and 90-foot section of pipe when the pipe was pulled directly with the two CATS.
The 10:1 block system provides smooth and incremental pulling and allowed for continuous monitoring from a load cell. Once the existing pipe was free of the initial friction force, pipe movement began and the extraction force rapidly diminished to 52,000 lbs. as the pipe was withdrawn.
Melvin said, “The pipe rammers were only used at 25-30 percent of full power. The pneumatic pipe rammer can be varied from 50 to 180 strokes per minute. Throttling the frequency of the pneumatic rammer applied percussive force to the pipe and transmitted energy through the pipe and into the old drilling mud and soil. The percussive force helps to activate the drill mud and reduce friction between the pipe wall and soil interface. Combined with the constant pulling force the pipe overcame the wall friction and started to move.”
At the 40-foot mark in the exit pit, the rammer was disconnected. The section of old pipe was cut and removed. The rammer was brought back to the stub end of the pipe, reconnected and the process was repeated. At the launch pit, the process was similar as the pneumatic rammer was disconnected from the pipe. A new joint was welded on, the rammer was reconnected to the new joint and the process repeated.
At the point where 160 feet of pipe had been extracted, the pneumatic pipe rammer was removed from the pulling end and the D8 CATS pulled directly on the string and removed it in 90-foot sections. The force required to pull the entire pipe was steadily decreasing from about 200,000 psi to 86,000 psi. and then to 52,000 psi. At the launch pit, when the new string was welded on the pipe rammer on that side was no longer needed. The process was repeated until all 2,400 feet of new pipe was in place.
Melvin said, “This was a particularly challenging extraction. Midwestern Contractors operated at a level that was exceptional. Their knowledge and skill made this project a success.”