Successful Risk-Based Phased Approach Helps REWA Prioritize and Plan Force Main Assessment
- Bill Orne, Sean FitzGerald - Hazen and Sawyer
Renewable Water Resources (ReWa) hired Hazen and Sawyer to develop and implement their force main condition assessment program and effectively analyze force main condition with appropriate technology. The force main condition assessment was divided into two phases – the first phase being the risk analysis for all force mains and the second phase being the implementation of the inspection technologies to assess force main conditions.
The risk-based analysis was conducted using a GIS approach to help develop both a consequence of failure (CoF) and probability of failure (PoF) based on factors including proximity to water, main diameter, location under busy roads, corrosion risk, etc. The combination of the CoF and PoF is called the “risk score”. We used a combination of GIS and in-house risk software to automatically generate risk scores for each 500’ segment of force main (FM) and rank each FM segment and individual components of force main based on risk. This method allowed us to identify the most critical force main segments within the system for further inspection and assessment.
The results of that analysis indicated that five force mains stood out as currently having the highest risk:
• Fountain Inn A force main
• Old Anderson force main
• Lakeside force main
• Rock Creek force main
• Huff Creek force main
The first phase of the condition assessment was a detailed desktop analysis to review specific at-risk portions of the force main due to internal and external corrosion. This analysis was used to identify specific locations to conduct a more detailed condition assessment and resulted in detailed field assessment plans for each force main.
A review of currently available condition assessment techniques was made to determine what technology would be most beneficial for each of the critical force main segments. The initial assessment for two of the force mains (the Fountain Inn A force main and the Old Anderson force main) was made with Pure Technologies’ SmartBall Pipe Wall Assessment (PWA) inspection technology.
SmartBall PWA is a detailed screening technology used for assessing the condition of metallic pipelines by identifying pipe sections (joint-to-joint) which exhibit increased levels of stress. The SmartBall tool was able to acoustically detect the presence and location of leaks and gas pockets. For force mains, gas pockets are of particular concern because they can result in internal hydrogen sulfide corrosion of pipe surface.
These force mains are currently being investigated using different techniques that include both non-destructive and destructive testing. The locations of the proposed testing were based on preliminary review of the force main routes, pipeline profile analysis, and other acoustic testing performed for two of the force mains.
The following testing techniques are being used to collect data as part of this project to determine existing pipe condition and remaining useful life:
Ultrasonic Thickness Testing (UST)
To assess the remaining wall thickness of the FM piping, Hazen is using targeted UST testing, which is a non-destructive test that uses ultrasonic waves to measure material thickness.
At each test site, the pipe is tested at sixteen points around the pipe circumference—eight evenly-spaced test points in each of two rings separated along the pipe by approximately one foot. Thus, at each test site, two measurements shall be obtained at each 12:00, 1:30, 3:00, 4:30, 6:00, 7:30, 9:00, and 10:30 clock positions (facing in the direction of flow).
Coupon Removal Pipe
A short piece of FM pipe (12 to 18 inches) is removed and replaced at specified locations. The samples are collected so that the pipe interior can be visually examined to evaluate its condition—including the condition of the lining system, which is not detected by the UST testing.
In order to remove and replace the pipe coupon, the contractor temporarily removes the FM from service. Since bypass pumping was not practical, the contractor coordinated with ReWa to select the best time of day to perform this work.
Soil Sample Collection and Analysis
Soil samples are being analyzed to evaluate the potential to corrode metal. These series of soil tests include:
• oxidation-reduction potential
• moisture content
• physical characteristics (particle size and plasticity).
Testing is currently underway with ReWa’s Lakeside force main and has revealed sections of force main that were at or near failure and validated our risk-based approach. The weeks of field testing have proven to be worth the value of assessment and system repairs are underway with the information discovered due to that testing. Hazen’s knowledge of the modes of failure and understanding of the conditions that caused those failures helped ReWa to avoid environmental impacts and fines of a catastrophic force main failure of the Lakeside force main. The project continues with the condition assessment of the remaining four force mains.
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