Case study - Trinity River Authority verifies condition of C303 Pipeline

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Courtesy of Pure Technologies

  • Service: Assess and Address
  • Client: Trinity River Authority of Texas
  • Project Date: November 2012 - ongoing
  • Location: Trinity River, TX
  • Type of Pipeline: Water Transmission Main
  • Diameter: 30-inch (750mm) and 54-inch (1350mm)
  • Pipe Material: Bar-Wrapped Pipe and PCCP

Trinity River Authority of Texas (TRA) owns and operates 8.5 miles of 30-inch BWP and PCCP that supplies raw water from Lake Arlington to the Tarrant County Water Supply Project Water Treatment Plant in Euless, Texas. The 30-inch pipeline, in conjunction with a parallel 54-inch pipeline, conveys raw water to the Authority’s 87 mgd WTP. Treated water produced at the WTP is supplied to five cities in the mid-cities region between Dallas and Fort Worth including Bedford, Colleyville, Euless, Grapevine and North Richland Hills.

TRA's Challenge

TRA had originally planned to replace this pipeline, but chose to assess and selectively rehabilitate the pipeline by finding solutions that could identify the most distressed areas. The pipeline spans about 8.5 miles and is made up primarily of BWP, although there are some sections of PCCP. It was constructed in 1973.

Results at a Glance

  • The Assess & Address cost was roughly 4% of the capital replacement estimate of $25 million
  • SmartBall survey identified 4 leaks and 3 air pockets
  • TRA verified and repaired 3 high-risk BWP sections
  • Only 1.1% of BWP pipe sections identified as distressed

What was the Solution?

In November 2012, TRA began a condition assessment program that included transient pressure monitoring, acoustic leak and gas pocket detection, internal electromagnetic inspection, and structural condition assessment including finite element analysis.

For the leak and air pocket assessment, TRA used SmartBall® technology. The SmartBall inspection tool is a non-destructive, free-swimming technology that measures the acoustic activity associated with leaks and gas pockets in pressurized pipelines. When acoustic anomalies are present, the data is analyzed to determine if it is a leak, gas pocket, or just an external sound.

Regular leak detection inspections can help utilities identify leaks that may not be visible at the surface. By repairing leaks, utilities can reduce their Non-Revenue Water and prevent pipeline failures, as leaks are often a preliminary indication of pipeline deterioration. Location and elimination of air pockets is also beneficial as it reduces pressure on pumps that are attempting to pump water past a gas pocket.

For the structural inspection, TRA used PipeDiver®, a free swimming electromagnetic tool that identifies wire breaks in PCCP and bar breaks and broad areas of cylinder corrosion in BWP using PureEM™ technology. The tool operates while the pipeline remains in service.

Although BWP looks similar to Prestressed Concete Cylinder Pipe (PCCP) in cross section, their design and materials are significantly different. PCCP is a concrete pipe that remains under compression because of the prestressing wires, with the thin-gauge steel cylinder acting as a water membrane. With BWP, the cylinder plays a much larger role in the structural integrity of the pipe. BWP is essentially designed as a steel pipe with mild steel used to manufacture the steel cylinder and steel bars.

As a result, the bar in BWP and wire in PCCP respond differently to environmental conditions that facilitate corrosion. The high strength steel wire in PCCP is smaller in diameter and wrapped under higher tension, therefore corrosion makes it quite vulnerable to breakage. The mild steel bars in BWP are thicker in diameter and wrapped under less tension, therefore corrosion takes significantly longer to lead to breakage.

The engineering services portion of the project was completed to identify optimal operating conditions for the pipeline and determine the structural performance of the pipe materials. This included creating performance curves for TRA’s BWP, as well transient pressure monitoring. The BWP structural performance curves allowed TRA to determine which pipe sections to excavate and verify.

By determining the bar break yield limit for the specific pipe material, TRA was able to identify which pipe sections should be immediately addressed and which could remain in safe operation.

Results

The SmartBall survey identified four leaks and three gas pockets. Although the four identified leaks were small (less than 2 gallons per minute), one was located in the front yard of a brand new church building and could have caused significant water damage had it not been repaired immediately by TRA. Water from this leak was visible at the surface 325 feet away from the actual leak location.

The structural inspection using PipeDiver identified four PCCP pipes with electromagnetic anomalies resembling wire breaks. The inspection of the BWP identified 14 pipes with bar break damage and 72 pipes with electromagnetic anomalies resembling cylinder defects out of 1,284 inspected pipes.

TRA has verified and repaired three sections of BWP that were beyond the yield limit determined by the structural performance curves. Upon verification, TRA and Pure determined that distress areas identified in the inspection were accurate and the excavated pipe sections had bar breaks and corrosion.

Through the use of condition assessment, TRA was able to selectively rehabilitate its assets for roughly 4 percent of the estimated $25 million replacement cost. The project has also allowed TRA to increase service reliability for customers in the region.

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