Coiled polyamide-11 high pressure gas pipe costs less to install and operate than steel stick pipe

Pipeline & Gas Journal, Dec, 2006 by Dennis Jarnecke

For rapid and economical installation it is difficult to beat coiled plastic pipe. For higher pressures in the range of up to 250 psig it is now possible to install coiled polyamide-11 (PA-11) gas pipe instead of steel in diameters up to two inches. In a study (1) done by Arkema Inc. and Gas Technology Institute (GTI) it was concluded that polyamide-11 pipe could be installed by all of the conventional methods including plowing and directional boring in addition to the more common open-cut trench methods. Coiled pipe can be used in all of these methods and offers cost advantages over long lengths of plastic pipe fabricated from butt-fused sticks.

Although detailed and robust fusion procedures are available for the various plastics used as gas pipe, including PA-11, it is a reality that each fusion represents an opportunity for a problem to develop despite the best efforts of well-qualified technicians and inspectors. Although fusion problems are rare in plastic pipelines assembled by qualified personnel, it makes good sense to avoid constructing pipelines from stick pipe joined by fusion methods when coiled pipe is available.

The cost savings, in the form of reduced installation costs as well as the elimination of a cathodic protection system, have been realized by natural gas distribution companies while installing high-pressure systems. These installations have occurred over the past two years as part of a development program designed to test the suitability of PA-11 gas pipe in a variety of pressures, climates, and soil types.

Nashville Gas, Questar Gas, and Public Service Company of New Mexico (PNM) each installed coiled PA-11 pipe in pressures from 175-200 psig to test the assumption that it would be more economical than a conventional steel pipeline constructed of 40-foot sticks.

Demonstration Projects

Working closely with GTI, the utilities obtained state and federal waivers for the PA-11 1 installations because the operating pressures would be higher than the 125 psig allowed for plastic pipe in Title 49 Code of Federal Regulations Part 192.123. Additionally, a design factor of 0.4 would be used for designing the pipe instead of the 0.32 design factor required in Part 192.121.

Since these installations, Arkema Inc. has petitioned the U.S. Department of Transportation (USDOT) to amend the pipeline safety regulations to permit PA-11 pipe to be used at pressures up to 200 psig, limited by its hydrostatic design basis rating, and with a design factor of 0.4 without the need for waivers. Federal action is pending.

Coiled PA-11 pipe requires a straightener/rerounder for installation because the pipe takes a set on the coil after extrusion. Although the force required to uncoil the pipe is small, if a straightener is not used, the uncoiled pipe will tend to elastically re-coil, like a spring, which is both inconvenient and a safety hazard.

The April 2003 Nashville Gas installation was the first use of coiled PA-11 pipe. It was part of a much larger project that also used about 5,000 feet of PA-11 pipe in 40-foot sticks. A PLCS-coiled pipe trailer and 2-inch pipe straightener were used to uncoil the coiled PA-11 pipe at Nashville Gas. One 500-foot coil of 2-inch PA-11 was secured on the unwind stand of the trailer.

One end of the pipe was directed through the straightener and secured to a cable. (Figure 1) A backhoe was used to pull the pipe from the unwind stand, through the straightener, and off the trailer. (Figure 2) The straightened pipe was then laid in the trench. The use of the coiled pipe greatly increased the speed at which the pipe was installed. Butt-fusion joints are only required every 500 feet instead of every 40 feet as with the stick pipe.

All the pipe in this project was installed by open-trench technique and all the joints were made by butt fusion. Nashville Gas personnel made fusion joints after certification training given by the GTI. The PA-11 pipeline operates at 175 psig in a Class 3 location. There are now eight residential services connected.

In June 2003 Questar Gas installed about 4,200 feet of PA-11 pipe as two parallel 2-inch SDR11 pipelines serving a single industrial customer and operating at 200 psig. (Figure 3) Questar Gas obtained waivers from the State of Utah and the USDOT for the same reasons as Nashville Gas. Two 2-inch pipelines were specified because the desired single 4-inch PA-11 pipe was not yet available.

One of the pipelines consisted of 2,000 feet of coiled PA-11 pipe using four 500-foot coils. Another 2,200 feet was installed using 40-foot sticks. As with the Nashville Gas installation, the joints were made by butt fusion by the utility's own construction crews. Again, a PLCS-coiled pipe trailer and straightener were used for the coiled-pipe installation.

The PNM installation was accomplished in November 2004. It consists of 2.5 miles of 2-inch SDR11 pipe operating at 175 psig. This was the first installation to use coiled PA-11 pipe exclusively for the distribution pipeline. There were seven coils of nominally 1,000-feet (Figure 4), and 12 coils of nominally 500 feet laid into open-cut trench dug with a tracked trenching machine.