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

All the joints were made by butt fusion by PNM technicians. The trenching rate limited the pipe installation rate despite the relatively rock-free soil. Even considering that there were delays while waiting for trench, the entire 2.5-mile pipeline was installed and connected in about four working days by a five-man crew, including the trench machine operator.

Cost Comparisons

In a 2002 survey of 35 gas utilities located around the United States, we received total installed cost (TIC) information about both steel and plastic pipe installations of various lengths and diameters. The data revealed a range of TIC for pipe, excavation, burial, inspection and other typical line-item components of a gas pipe construction job.

The data revealed that in 2002 the typical TIC for 2-inch steel gas pipe ranged from $14-41 per foot based on 2001-2002 steel pricing. The range was representative of various soil types, climates and job mileage. At the same time we learned that 2-inch plastic pipe generally costs about $5-7 per foot to install after it is delivered to the job site if it is installed using sticks.

At PNM it was concluded that the cost of installation of pipe from long coils is about $3 per foot. The 2-inch SDR-11 (Standard Dimension Ratio used to describe pipe wall thickness) PA-11 pipe used for the projects at Nashville Gas and Questar Gas cost about $10 per foot. Two-inch SDR11 PA-11 pipe used for the PNM installation was supplied for about $8 per foot. This puts the TIC range of a PA-11 pipe project at $11-17 per foot which is very competitive with steel costs.

The economics of coiled PA-11 pipe can be illustrated using the examples of the three utilities that have installations. Nashville Gas and Questar performed stick and coil installations at the same time using their own personnel and conventional equipment. The most obvious advantage of coiled pipe is that there are far fewer fusions in a coiled installation. In the same 500-foot installation made with 40-foot sticks there would be about 12 fusions.

Each butt fusion takes about 15 minutes including cooling before handling and inspection, resulting in approximately three hours of technician labor devoted entirely to fusing pipe. Nashville Gas installed a 500-foot coil of 2-inch SDR11 PA-11 pipe in about an hour.

The experience at Questar was similar and benefited from technique developed during the coiled PA-11 pipe installation at Nashville Gas. Each 500-foot coil was deployed and fused onto the adjoining segment in about 40 minutes, taking less than three hours to install the 2,000 feet of coiled pipe. In contrast, construction of the same length of pipeline from 40-foot sticks took about 10 hours. Because of the experience gained at the Questar and Nashville coiled-pipe installations, PNM could deploy and fuse a 1,000-foot coil in about 45 minutes.

Plastic pipe is manufactured in a continuous process that lends itself well to making a coiled product. Finished coil diameter is the principal factor that limits the pipe diameter for coiling to product availability. Larger diameter pipes must be coiled around larger diameter hubs than smaller diameter pipes to avoid excessive ovality in the product. As the pipe and hub diameter increase, the coil diameter approaches the maximum dimensions for shipping to the job site without special permits and routing; at some point so little pipe can be put on the coil that it loses any installation cost advantage.