Critical large-diameter water transmission mains frequently run beneath city streets in busy urban environments. Like the majority of water infrastructure across North America, these pipes are reaching the end of their design life, causing an increased number of leaks and pipe failures.
Although these assets are beneath the ground, the U.S. Environmental Protection Agency estimates that buried pipelines represent two-thirds of a utility’s value. Despite this, the majority of capital investment has historically been focused above the ground. Although this has changed recently after several industry studies highlighted the growing need for pipeline renewal, water and wastewater service providers still must employ innovative solutions to manage their buried pipelines.
While it is important for utilities to manage their entire inventory of buried infrastructure, pipelines in urban environments pose a significantly greater risk and challenge for water utilities.
The Urban Challenge
These pipelines are high-risk because of their high consequence of failure. If a pipe beneath a busy downtown street fails, the repair costs can quickly escalate and the failure causes a massive disruption to businesses and commuters. In some cases, failures in urban environments have cost utilities upward of $5 million to remediate. A failure not only carries a high repair bill, but also contributes to a negative public perception of the utility, which can harm consumer confidence and lead to negative public relations.
Leaks on urban pipelines are also challenging since they can be more difficult to locate. The environment also makes accurate location of these leaks critical in order to reduce repair times.
With such high risk, utilities often prioritize these mains ahead of those with lower consequence of failure. However, because these mains are located in high-traffic areas, assessing them is far more challenging than assessing a linear main in a rural area.
Dealing with above-ground obstructions, commuter delays and a lack of access points means that operators need to have close control over inspection technologies. In addition, the technology must provide the best possible information to allow for accurate repair and excavation decisions.
Like other major metropolitan areas, the City of Montréal has aging pipeline infrastructure that runs through its downtown core. In Montréal — one of the oldest cities in North America — this infrastructure is old and beginning to reach the end of its design life.
Inspections Under Way
In a proactive effort to identify problem areas in its prestressed concrete cylinder pipe (PCCP) assets, the City is in the midst of an inspection program using advanced non-destructive technologies. In total, the City will assess the condition of more than 60 km of PCCP by 2015.
To 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.
In the majority of cases, assessing the condition of assets to identify problem areas has high value for utilities, since the majority of pipelines have remaining useful life, despite their age. This allows for selective rehabilitation in favor of full-scale replacement. This is particularly important in urban areas, since excavation costs are higher and more disruptive in urban environments.
For a large portion of the condition assessment, the City is using the PureRobotics electromagnetic (EM) platform, since it is ideal for challenging urban environments. The tool remains tethered to the surface during inspection and is controlled by an operator. It also features live high definition video to observe internal pipe conditions. These features allow the City to see internal pipe conditions and closely verify areas with potential problems.
In addition, the tool identifies broken prestressing wire wraps in PCCP. As PCCP ages, the prestressing wires, which make up the main structural component, begin to break due to a number of factors.
The presence of broken wires in PCCP is the main indication that the pipe will eventually fail. Unlike metallic pipe materials that typically fail after a long period of leakage, PCCP is prone to sudden failures when too many wires break in one area. The diagram below demonstrates how PCCP typically fails.
Recently, the City completed the assessment of more than 17 km of its urban PCCP assets with diameters of 600, 750 and 900 mm (24, 30 and 36 in.). Of the 2,798 pipe sections assessed in this 17 km, only 97, or 3.5 percent, have shown evidence of distress. This is slightly below the industry average of 5 percent of pipe sections with distress.
Using condition assessment, the City has been able to identify isolated distress on its critical urban mains, while leaving pipeline assets with remaining useful life in operation.
After completing the initial phases of condition assessment, the City has excavated certain sections of pipe for validation of the inspection results, as well as repair of any damage.
Both the excavation locations and presence of distress have been very accurate. This has allowed the City to repair isolated pipe sections, which restores the overall condition of the pipeline. This will help to prevent failures that would significantly disrupt day-to-day life in the city.
The City has also successfully reduced leakage on its urban pipelines in recent years. In February 2012, the City completed a leak detection survey on the Pine Avenue transmission main, which is made of 80-year-old, 34-in. cage and cylinder Bonna-type pipe, which is a variation of Reinforced Concrete Cylinder Pipe (RCCP). The pipeline is a critical supply of potable water for the western portion of a major sector in Montreal.
Based on previous non-invasive leak detection inspections, the City was aware that leaks were present on the 1.3-km stretch of pipeline. Given the importance of the pipeline, locating and repairing the leaks was made a priority to ensure service reliability, reduce water loss and avoid a potential pipe failure.
The City used the Sahara leak detection platform, which is a non-destructive tethered leak detection tool that is pulled by the water flow by a small drag chute.
After the survey on Pine Avenue, nine leaks were identified in the 1.3 km of inspection. City staff was surprised at the number of leaks identified, as other leak detection technologies suggested that there would be only one major leak and possibly another minor one. The use of the tethered system qualified the nine leaks as being from small to large in size.
In August 2012, all nine leaks identified in the inspection were excavated for repair by the City. The leak locations had been precisely identified and marked, and all leaks were found within 1 m of the marked location, though most were within 0.5 m. All of the located leaks also had a size that corresponded with the estimates made by the tethered system.
Through the use of advanced leak detection and condition assessment technologies, the City has been able to reduce water loss on an important water main, while gaining a baseline condition of the pipelines it assessed with EM technology. This helps in the development of future capital planning for monitoring or re-inspection.
By proactively assessing its transmission mains, Montréal is taking steps to reduce water loss and prevent pipe failures, while allowing for more fiscally responsible asset management in the future.