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Cobb County Pccp Assets Engineering Analysis Inspection

Cobb County-Marietta Water Authority (CCMWA) is the second largest drinking water supplier in Georgia, providing vital service to nearly 800,000 people through twelve wholesale customers. With two award-winning water treatment plants and over 200 miles of large-diameter transmission mains, CCMWA can deliver up to 158 million gallons per day. Two of CCMWA’s key objectives are to be financially viable and to reduce vulnerabilities by improving redundancy and implementing a comprehensive asset management program.

However, across the United States critical infrastructure is aging, causing utilities to see an increased number of water pipe failures. While these failures occur most commonly on small pipes – causing only minor disruptions – large-diameter mains do fail, resulting in major delays and enormous repair bills.

A large portion of CCMWA’s large-diameter pipeline inventory is made up of Prestressed Concrete Cylinder Pipe (PCCP). In order to successfully manage PCCP, the water industry has widely adopted the use of condition assessment techniques, which have a proven track record of identifying and averting PCCP failures. PCCP owners and operators continue to use these condition assessment methodologies combined with sound engineering analysis to effectively and safely manage their critical assets.

Cobb County’s Program

In 2012, CCMWA was in a similar situation to many predominant PCCP users; past failures on these critical assets had led to the decision to replace the majority of PCCP assets to avoid the risk of future failures. However, it was determined that replacing large sections of pipeline was not financially or logistically feasible.

Large-scale replacement programs are also unnecessary based on industry research, which confirms that pipe deterioration is not uniform or systematic. Specifically, electromagnetic inspection data (which identifies both the quantity and location of broken prestressing wires – the primary structural component of PCCP) collected by Pure Technologies over more than a decade indicates that less than 4 percent of pipe sections inspected have any level of wire break damage and less than 1 percent require repair – regardless of when it was manufactured.

SmartBall tool extraction

The SmartBall tool is retrieved from the extraction point.

PipeDiver retrieval

Staff remove the PipeDiver tool after the non-destructive assessment.

Therefore, by making the decision to replace entire alignments of PCCP, owners typically remove a majority of pipeline assets that are in like-new condition. A financial evaluation based on the cost of capital replacements compared with PCCP management (inspection, repair, re-inspection, and repairs) for the 48-inch diameter PCCP in CCMWA’s inventory indicates that the pipelines can be managed for approximately 10 percent of the capital replacement costs when extended over 25 years using a net present value calculation (Figure 1).

Capital Replacement vs Condition Assessment

Figure 1: Financial Evaluation of Capital Replacement vs Condition Assessment

Following a repair on a 30- and 42-inch Raw Water Line in 2012, CCMWA decided to manage its critical PCCP assets using condition assessment and engineering analysis as a proactive management strategy. In 2013, CCMWA completed its first full inline condition assessment to identify structural deterioration on its PCCP. The project focused on the 30- and 42-inch main that had previously been found to have defective joints and a deteriorating pipe wall to determine its remaining useful life.

The Inspection Program

The assessment featured two inspections – a leak and gas pocket survey and inline electromagnetic (EM) inspection – on roughly four miles of the 30- and 42-inch PCCP Raw Water Line. The subject pipeline acts as a redundant supply line from Lake Acworth to the Wyckoff Water Treatment Plant. The project also included engineering evaluations including structural analysis and remaining useful life evaluations to make management and renewal recommendations. For the prescreening survey, CCMWA used SmartBall® leak detection, a free-flowing tool that identifies the acoustic anomalies associated with leaks and gas pockets in large-diameter pipelines. Completing a prescreening leak and gas pocket survey is a prudent approach for operators of any pipe material, since leaks are often a preliminary indication of a failure location. For PCCP, leaks are usually located near the pipe joint, which is also a common failure area on PCCP. However, the inspection did not identify any leaks or pockets of trapped gas. For the more detailed structural evaluation, the PipeDiver® electromagnetic (EM) inspection platform was used. The tool uses electromagnetics to identify broken prestressing wires, which are the primary structural component in PCCP. As sections of PCCP begin to deteriorate, the prestressing wires begin to break, which weakens the pipe and makes it more likely to fail. Identifying broken wires is the most effective way of determining the condition of and preventing failures in PCCP. By completing an EM inspection on the PipeDiver platform, CCMWA was able to determine the baseline condition of the pipeline while it remained in service – a major benefit for operators who cannot remove mains from service to complete internal inspection.

The Results

For CCMWA, the inspection identified ten pipe segments amounting to less than 1 percent of the pipeline with evidence of broken prestressing wire wraps. On average, PCCP inspections across the country indicate that approximately 4 percent of the pipe segments have any level of damage. This confirms that the majority of CCMWA’s PCCP inventory is in good condition, with only a small number of pipe sections in need of immediate renewal. However, locating and renewing even one pipe section can help utilities maintain reliable service and avoid an expensive pipe failure. Beyond the prescreening and structural inspections, CCMWA was able to identify limitations in its potable water system through the planning portion of the project. The inspected pipeline is a redundant line which carries raw water to the Wyckoff Treatment Plant; the primary supply line to the plant is a 60-inch line. In order to ensure that the main was being operated safely within its limits, a hydraulic study was completed. This study found that the 30-inch section of the pipeline was incapable of supplying the treatment plant’s required operating flow rate while maintaining a safe operating pressure within the system. Operating the pipeline under the required pressures places the main at a higher risk of failure in the event that the primary raw water line is taken out of service. Based on the study, it was recommended that the approximately 1 mile of 30-inch PCCP be replaced to handle existing and future operating condition requirements of the treatment plant. This discovery allowed CCMWA to make defensible decisions about their 30-inch PCCP main and pumping station while contributing to the prevention of future pipe failures. By upgrading the 30-inch section of the pipeline, the raw water pipeline will remain a safe redundancy line for the main 60-inch line. By managing its PCCP assets using condition assessment, it has also been determined that less than 1 percent of pipe sections on the assessed main have any indication of wire break damage, which is consistent with industry standards. Additionally, the prescreening survey showed that there are no leaks or gas pockets that require maintenance. The results from the inspections will allow CCMWA to cost-effectively manage its PCCP assets in favor of completing a large-scale replacement.

 

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