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Case Study

Milwaukee Metropolitan Sewerage District

The Milwaukee Metropolitan Sewerage District (MMSD) takes a proactive approach to water management initiatives, as evidenced in the condition assessment of the Franklin-Muskego Force Main.

Ownership of the pipeline is shared between the City of Muskego and MMSD, the government agency that provides water management services for about 1.1 million people in 28 communities in the Greater Milwaukee Area.

In 2015, Pure Technologies (Pure) worked closely with MMSD to perform a detailed condition assessment of the approximately 25-year old ductile iron pipeline. The purpose of the assessment was to identify the structural condition of the metallic force main, and included pressure monitoring, a SmartBall® leak and gas pocket detection survey, and a PipeDiver® electromagnetic inspection of the pipeline.

Project Details

Services
 SmartBall® Leak and Gas Pocket Detection
PipeDiver® Electromagnetic Inspection
Transient Pressure Monitoring
Structural Engineering
Timing
 2015
Pipe Material
 Ductile Iron
Inspection Length
 2.9 miles (4.7 kms)
Diameter
 20-30 inches (500-750mm)
Transmission Type
 Wastewater

Project Highlights

Inspection identified 13 pipe sections with electromagnetic anomalies

Defects ranged from 20-55% wall loss

Transient pressure monitoring indicated pipeline operating within design capacity

Challenge
The Franklin-Muskego Force Main carries wastewater along approximately 3 miles of 24-inch and 30-inch ductile iron pipe (DIP). One of the challenges in assessing DIP is determining if the pipe has undergone any wall thickness loss due to internal or external corrosion, which are the primary causes of failure. DIP in water service with a cement mortar lining generally has fewer internal corrosion failure rates, unless damaged during handling and installation, or later as a result of 3rd party damage. This is not the case when DIP is used in a force main, where internal corrosion is the primary cause of failure.

Gas pockets are of significant concern as concentrations of hydrogen sulfide gas within wastewater may cause corrosion and eventual breakdown of the pipe’s exposed surface. In a force main, identifying internal areas with potential corrosion is challenging, as traditional gravity pipeline inspection techniques are often not applicable to in-service pressurized pipelines.

One method for assessing gas pockets is to locate air release valves (ARVs) or other high points along the alignment and conduct test pit investigations in those areas. While this is a valid method for locating potential gas pocket locations, additional gas pockets may occur due to differential settlement, improper installation or non-functioning ARVs. Desktop surveys may not identify and locate all gas pockets along a pipeline, which is why Pure recommends other more precise survey methods.

Solution
To evaluate the condition of the Franklin-Muskego force main, Pure recommended in-line condition assessment. This included inspecting for the presence of gas pockets, using electromagnetics for assessing the condition of the pipe wall and structural engineering to evaluate the significance of defects found.

In October 2015 Pure performed a SmartBall leak and gas pocket detection survey and a PipeDiver electromagnetic inspection of the Franklin-Muskego Force Main. The SmartBall platform is a free-swimming tool that uses acoustics to detect leaks and gas pockets while the pipeline remains in full service. Pure’s flexible, free-swimming PipeDiver tool collects electromagnetic (EM) data that is used to measure the relative wall thickness of the cylinder – the main structural component of the pipeline. With electromagnetics onboard, PipeDiver can identify localized areas of wall loss in the cylinder of the pipe, and broken bar wraps in BWP, all while the pipeline remains in service.

Results
The results of the C150 design check showed that the pipe’s nominal wall thickness is sufficient for current loading conditions. Transient pressure monitoring indicated that over the period of monitoring, the pipeline operated within its design capacity.

Through the PipeDiver inspection, 13 pipes were found to have a total of 16 electromagnetic anomalies consistent with localized wall loss, ranging between 20 percent to 55 percent wall loss. At the time of writing, MMSD was making plans to excavate and repair one pipe section with three areas of pipe loss ranging from 35 percent to 55 percent wall loss. The results of the condition assessment indicate that the Franklin-Muskego Force Main is in good condition.

While the assessment recognized several areas with an increased likelihood of failure, overall the data was good, and coupled with Pure’s engineering recommendations, gave all stakeholders confidence in the health of pipeline for the near foreseeable future.