Cinergy Corporation’s natural gas distribution system includes over 478,000 customers throughout Ohio, Kentucky, and Indiana. The Cincinnati, OH based electric and gas utility is one of the leading diversified energy companies in the United States. In order to maintain a service area of that size, the company is continually looking at ways to increase efficiency in the field. Utilizing trenchless technology, such as pipe bursting, for mainline replacement is one way Cinergy is meeting its efficiency goal.
The Cinergy gas distribution system is made up of over 6,500 miles of gas pipeline. Upgrading is being done yearly to the system, portions of which were installed as long ago as 1873. Other portions of the system were installed in the mid to late 1950s. According to Cinergy Senior Design Technician Dan Schuler, approximately 20 miles of pipeline a year are scheduled for renewal. Schuler said, “With the help of various software programs and a point criteria we developed in house, we are able to prioritize and designate sections of the system for replacement. It’s based on age, diameter, wall thickness, pressure on the pipe and past leak history.”During the 2000 construction season Schuler chose a 295-foot section of main line to be replaced with 6-inch Medium Density Polyethylene Pipe (MDPE) through pipe bursting. The 6-inch MDPE would serve as a sleeve in which a 4-inch MDPE line would be inserted to serve as the actual carrier pipe. The existing 295-foot cast iron line, installed in the mid-50s, ran along side a busy two-lane road and under several driveways; so a trenchless option was preferable. Schuler said, “We use several trenchless techniques including dead-insertion and directional drilling. Over the last few years, we have looked at using pipe bursting and for this job bursting was well suited.”According to Schuler a majority of Cinergy’s exposure to pipe bursting has been with static bursting systems. That was the type of system initially chosen for this project. The static unit, however, was unable to overcome the difficult mechanical joints along the length of the cast iron pipe. Cinergy contacted gas, sewer and water pipeline contractors, Waterworks, Dayton, OH. Waterworks would attempt the burst with a pneumatic GrundocrackPCF 145 from TT Technologies, Aurora, IL.
Waterworks has been in the pipeline rehabilitation and replacement business since 1968. In addition to commercial plumbing and industrial services, the company provides hydro-excavation, vacuum services, pipeline cleaning, video inspection and high pressure cleaning up to 40,000 PSI. According to Vice President Ron Wilkes trenchless technology did not play a big factor in their work until recently. Wilkes said, “A majority of the work we performed until a few years ago was open cut, and we’re still seeing a lot of entities that want to open cut projects. The biggest stumbling blocks to trenchless methods like pipe bursting, are unfamiliarity and lack of exposure to the method.” The biggest potential stumbling blocks for the Cinergy burst were the mechanical joints the Waterworks crew would have to break. With several years and several thousand feet of bursting experience, Wilkes was confident it could be done.
A crew from Cinergy performed a majority of the prep work. They dug one 4-foot by 12-foot launch pit at the beginning of the run and another 3-foot by 6-foot exit pit 295 feet away at the end of the run. All services were exposed and tested before bursting.After the pits were dug, the crew prepared to bypass the existing line. A 300-foot section of 2-inch HDPE was used as the bypass. The crew ran the line along the ground and through culverts under driveways to reduce disruption to homeowners. Once the bypass was in place, the crew purged the host pipe and was ready to begin bursting.
For this particular burst the Waterworks crew used a 5.75-inch diameter Grundocrack PCF 145 pneumatic pipe bursting tool, equipped with a 7-inch rear expander. A 10-ton Grundowinch was positioned at the exit pit. The winch guides the bursting tool through the cast iron host pipe.According to TT Technologies Pipe Bursting Specialist Ben Cocogliato the Grundowinch played a key role. Cocogliato said, “The Grundowinch provides constant tension at variable speeds. That was especially important for this project because of mechanical joints. When the bursting tool reaches a mechanical joint it slows down as it works its way through. After the joint is burst, there is less resistance and the tool speeds up quickly. The Grundowinch compensates for this change in speed and prevents slack from developing in the line.”Also essential to the success of the job was the use of a guide head or “schnozz.” The guide head is attached to the front of the pipe bursting tool and adds needed tool length. It allows the tool to get into the pipe and break it at its weakest point, the inside. Cast iron pipe tends to break several feet in front of a standard pipe bursting tool. This causes the potential for two major problems. First, the shards of cast iron pipe are sharp and can actually cut winch lines. Second, pipe fragments can build up making the host pipe impassable. This can cause the tool to change direction and veer off course. Besides adding tool length, the guide head helps protect the winch line. In many configurations, the winch line is connected to the front of the guide head instead of the front of the tool. This keeps the line ahead of the actual bursting and out of the way of shards of pipe. Cutting blades were also used in conjunction with a guide head. The blades were welded directly to the guide head. The blades focus the percussive action of the tool and greatly enhance overall bursting power. This allows the tool to successfully burst difficult host pipes like cast iron, as well as the mechanical joints.
While many bursting applications require sections of the new HDPE or MDPE to be fused on site, a coil of MDPE was used on this project. The coiled MDPE was attached to the PCF 145 with a threaded PE pipe pulling link. After rodding the winch line through the host pipe and connecting it to the bursting tool, the Waterworks crew lowered the tool into the launch pit and into the host pipe. Then they started bursting.After a few minutes the tool reached the first mechanical joint. Wilkes said, “We were all wondering what would happen as the tool hit the first joint. Its forward progress slowed considerably as it sat there and hammered away. About 20 seconds later the joint burst and the tool continued on to the next one. We thought it would be a lot harder to break the joints than it turned out.”Cocogliato said, “The combination of percussive power, the guide head and the cutting blades made bursting the mechanical joints possible. It just wouldn’t have been possible without those elements.”As the tool moved through the existing line, its 7-inch rear expander displaced the pipe fragments into the surrounding soil while the new 6-inch sleeve was pulled into place. It took approximately one hour and 12 minutes to complete the 295-foot run.
Once in place, cuts were made in the 6-inch sleeve to allow for the service connections. The Waterworks crew then pulled in the 4-inch carrier line with the help of the Grundowinch. Final service connections were made by Cinergy crewmembers using the existing service lines.Schuler said, “Prior to any of the bursting we took the existing service lines out of commission and air tested them. That way there were no surprises when it came time to reconnect the lines. We knew before hand if we needed to replace the service. We try to get the residents taken care of as quickly as possible. Our goal is, if we take you out of service that day, we want you back in service that night.”Service connections to the new main were made using electro-fusion saddle tees with stab fittings to tap the line.
All parties were extremely pleased with the results of the burst. Wilkes said, “Projects like this are good for everyone. We successfully completed a difficult burst, which is great. Cinergy replaced their line without major disruption. But more importantly, we were able to help prove the capabilities of the method and grew the market for bursting.”According to Schuler, now that they have gained more experience with pneumatic pipe bursting they will continue to utilize it when they can. He said, “We are planning on increasing the amount of pipe replacement we do in 2001. We are always going to be looking for the right situation to use bursting. It’s on a case-by-case basis.”
by Jim Schill
Pipeline & Gas Journal, January 2001, Pages 32-34