When a critical pipeline in a populated residential area has a high consequence of failure, it makes sense to assess its condition with the highest resolution technology available.
That’s the proactive route taken by Moulton Niguel Water District (MNWD), which provides water, recycled water and wastewater service to approximately 170,000 people within its service area, located in Orange County, California.
In 2015 MNWD contracted Pure Technologies U.S. Inc. (Pure) to perform a condition assessment of the Central Intertie Pipeline, a large diameter steel pipeline running through residential areas and busy transportation corridors. Constructed in 1994, the pipeline is a potable water transmission main that travels under Oso Parkway in Mission Viejo, California and is comprised of 54-inch cement mortar lined and coated welded steel pipe.
To date, the pipeline had not experienced any reported failures or leaks. While the probability of failure was low, MNWD felt it was critical to conduct a thorough assessment of such an important component of its water delivery system.
Pipeline failure would cause major disruption to traffic and water supply
The Central Intertie Pipeline is MNWD’s largest pipeline and typically delivers half of its potable water supply. A failure of this pipeline would cause major disruption to traffic and to water supply, which contributes to the high risk nature. As such, a confident management strategy is essential to manage this asset, which includes gathering high resolution condition data on the entire length of pipe and evaluating the results using advanced engineering analytics.
The assessment project included a total of three months for risk assessment, on-site preparation and repairs. Because of the criticality of the pipeline in a busy urban area, much of the work was conducted at night, when traffic flows were reduced.
PureMFL tool collects high resolution pipe wall thickness data
The project approach included a risk evaluation, collecting actual condition data along the length of the pipeline, and evaluating that data to make short and long term management decisions.
The overall purpose of the assessment was to identify locations in the asset with pitting and metal loss, recommend and perform strategic steel patch repairs to extend the life of the pipeline, determine the remaining useful life and recommend future actions to manage the pipeline.
The assessment utilized PureMFL (magnetic flux leakage) technology to determine the current condition of the asset by measuring the relative wall thickness of the pipe wall. Collecting high resolution wall thickness data over the length of the pipeline is the most confident way to understand the actual condition of the pipeline.
In order to perform the MFL inspection, it was necessary for Pure to remove pipeline appurtenances, dewater low spots in the pipeline and measure the internal diameter of the pipeline.
Prior to the internal inspection, Pure evaluated the design of the pipe under current loading conditions and current design standards, followed by a preliminary risk prioritization of the pipeline.
The preliminary risk assessment was completed pre-inspection to aid in the prioritization of repairs if required. The window of opportunity to perform the shutdown, inspection and repairs was limited.
Inspection results established a true baseline condition
Data obtained during the inspection determined that 212 pipe segments out of 395 segments had MFL anomalies consistent with wall loss ranging from 10 percent to 70 percent.
The engineering corrosion assessment showed operating pressure is well under burst pressure throughout the pipeline, at all defects, giving MNWD some confidence to proceed, knowing that not all defects needed repair.
31 locations selected for steel patch repairs
In total, 31 repair locations were selected for steel patch repairs, which were directed and overseen by Pure. These 31 repair locations encompassed 37 metal loss defects due to metal loss defects being in close proximity of one another.
From the MFL data, Pure also reported an estimation of the remaining useful life to failure, using Monte Carlo simulation. This statistical model indicated the average useful life of the pipeline at well over 100 years.
Overall, the results of the condition assessment indicate that the Central Intertie Pipeline is in good condition, which was confirmed after the Pure MFL inspection established a true baseline condition. The analysis is helping MNWD understand when the pipe should be planned for capital improvements and guide re-inspection and leak detection recommendations.
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