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Tube & Pipe

A weld runs through it

By Nick Wright

Above: A piece of pipe moves through production at Northwest Pipe Co. in Denver.

Using the weld-after-backfill method, crews keep Colorado waterline project on track

May 2015 - The Arkansas River, one of the main tributaries of the Mississippi River, winds 1,469 miles from the Rocky Mountains in Colorado to Arkansas, providing drinking water for some of the communities along the way. As of March, Colorado Springs, Colorado, joined those communities that are increasingly using the watershed as crews finished a 50-mile pipeline that brings water to the growing area. 

Called the Southern Delivery System (SDS), the pipeline originates from the Pueblo Reservoir in Pueblo, Colorado, where the Arkansas River flows through the Pueblo Dam. From there, the SDS runs north to Colorado Springs, roughly parallel to Interstate 25 between the two cities. The completed pipeline, which began in 2010, is phase one of two; the second is intended to add reservoirs and pumping capacity as needed long term. It’ll serve the communities of Fountain, Security and Pueblo West as well. The project is one of about a dozen water infrastructure projects either in planning or under construction in the Southwest.

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On the ground level, crews assembled the pipe from more than 7,000 individual pieces of mostly 66-in. diameter pipe supplied by Northwest Pipe Co., which is headquartered in Vancouver, Washington, and has a fabrication facility in Denver. Other inner diameters of the pipeline include 36-, 42-, 54-, 72- and 90-in. clear after cement mortar lining. The steel for the pipe was formed through a continuous cast process, says Neal Keleman, sales representative for Northwest Pipe Co.

Sheet or plate is rolled into a coil, spiral welded and fabricated into individual pipe lengths, generally 50 ft. to conform to American Water Works Association (AWWA) C-200 standards. The wall thickness varies based on the internal pressure in the pipeline, as well as other design considerations.

While there are different technical standards for water pipelines than, say, crude oil, there are still water sustainability and projected water demand challenges. But the project, which has thus far come in under budget, is expected to serve about three quarters of a million residents by 2040. The short- and long-term jobs created, combined with low interest rates for project financing, are expected to give SDS a return on investment after 2020—as well as a supply of drinking water.

“Colorado Springs Utilities and our project partners estimate completing SDS on time and forecast a savings of about $150 million, reducing the final project cost from almost $1 billion to $841 million,” says John Fredell, SDS program director.

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Weld-after-backfill

Among the permits needed to install such a pipeline is the Pueblo County 1041 Land Use Permit, which concerns land disturbance for big municipal water and sewage systems. What that meant for laying and welding pipe was sequencing its installation. Because crews could not leave more than 400 feet of trench open in one day, they had to carefully sequence work. This made digging a trench, laying the pipe, and backfilling a challenge to maintain schedule. To help meet this 1041 requirement, they would join the pipe with a heat shrink sleeve, then go back and weld the pipes from the inside later on.

This method of welding pipe from the inside is called weld-after-backfill (WAB). From an access point, a welder would return to pre-installed sections fit together via bell and spigot ends, and weld them. Because the pipe is already backfilled and the joints covered with a shrink wrap sleeve, the heat dissipates into the ground and with it goes the risk of warping around the heat-affected zone. The whole process lets crews backfill the pipe and weld separately from the open trench installation—a key method for satisfying rules of the permit.

WAB is the go-to method for lap-welded joints, the most common for water pipelines. The SDS pipeline incorporates single- and double-lap joints as well as full penetration butt-welded joints, according to Colorado Springs Utilities Project Manager Joseph Rasmussen.

The ends of each joint have no factory lining or coating on the interior of the pipe—that’s where the weld takes place. “Hand-applied cement mortar completes the interior lining of the joint area” once welding is done, says Rasmussen. 

The cement mortar lining conforms to AWWA C-205 and National Sanitation Foundation 61 standards, according to Kelemen.

Welders on the pipeline trained on WAB procedures before starting on site; that included field demonstrations with welders performing WAB jobs similar to the SDS job.

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“The demonstration had to account for variables including multiple pipe wall thicknesses, different welders and weld methods, as well as the use of native and controlled low-strength backfill materials,” says Rasmussen. Also known as flowable fill, controlled low-strength material is any variation of a runny concrete mix.

As far as how deep the pipe is buried, the minimum cover for the pipeline is 5 ft.; in some cases, it’s 25 ft. Garney Construction, Kansas City, Missouri, one of three contractors on the SDS, used a Caterpillar 390 to dig the trenches. The CAT 390 is one of the biggest excavation machines that Caterpillar makes. Garney used three of them at the site. Because it was bigger and more powerful than other available equipment, the 390 helped them maintain schedule.

But there are some places where the pipe is even deeper, such as the Fountain Creek tunnel crossing, where the pipeline runs underneath I-25 for about one mile. There, it’s about 85 ft. underground, and was constructed using trenchless technology (tunnel boring). The WAB method was used in lower pressure reaches of the pipeline, not at deep tunnel segments.

Moving water

Harnessing Arkansas River water and moving it 50 mi. north is no small feat. According to the SDS, among the biggest challenges was sequencing the work activities. At the height of pipeline installation, about one mile of pipe was being installed per week, with multiple contractors active in separate areas along the pipe. 

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“Since there was a limited supply of approved welders that had gone through the WAB training, the construction teams collaborated to share resources and shift work activities to allow welding to catch up to pipe laying production,” says Rasmussen. “Welding of the pipe is fairly standard, with the biggest challenges being related to safety such as trench shoring for outer diameter welds, fall protection for vertical sections and air monitoring in confined spaces.”

The infrastructure project also features three pump stations that lift the water about 1,500 ft. from Pueblo Reservoir to a new treatment plant in Colorado Springs. There are three inline triple-offset valves and four flow meters, as well as several blowoff structures for discharging water. To mitigate water surges, crews installed a combination of air vacuum release valves that either draw air into the pipe or release it depending on hydraulic conditions, according to Rasmussen.

Phase 1 of the project is nearly complete as construction on the three pump stations and water treatment plant are winding down. The project is on schedule to start flowing water in 2016. The SDS will supplement aging water delivery systems in place. As Colorado Springs and nearby communities grow, they are guaranteed water for future generations. Meanwhile, residents in cities like Wichita, Tulsa and Little Rock, through which the Arkansas River flows, are unlikely to notice a drop of water gone the other way. FFJ

Sources

  • Northwest Pipe Co.
    Vancouver, Wash.
    phone: 360/397-6250
    www.nwpipe.com
  • Southern Delivery System
    Colorado Springs, Colo.
    phone: 855/737-4968
    www.sdswater.org
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