The Montara Force Main (MFM) in California was built in 1979 and is owned and operated by the Sewer Authority Mid-Coastside (SAM). It is an approximately three-mile-long, 12-in. diameter, ductile iron pipeline that conveys all sewage flow from the communities of Montara and Moss Beach south to SAM’s wastewater treatment plant in Half Moon Bay. Twenty-five percent of the wastewater flow received by SAM is conveyed by this force main and there is no redundant pipeline.
The MFM is one of three force mains in SAM’s collection system. Due to past sewer system overflows (SSOs) caused by breaks in SAM’s Granada Force Main in 2017, an agreement was made to perform proactive assessments of the other force mains to determine their condition and potentially avoid another SSO. SAM had originally approached PICA in 2020 to perform a bracelet probe assessment of its 8-in. diameter Princeton Force Main (PFM) and subsequently retained PICA for the MFM assessment. Three test pits were excavated on the PFM to expose the pipe for assessment. The pit locations were chosen arbitrarily at the beginning middle and end of the 4,000-ft long force main.
In 2021, SAM embarked on a similar evaluation of the MFM and opted to deploy PICA’s Pipers (aka Recon+) untethered assessment tool first to identify anomalies in the pipeline that might better inform the location and quantity of test pits for further assessment. The Pipers had been utilized for force main assessments before, but not with the specific goal of informing the best location for test pits, intended for further pipeline evaluation to immediately follow.
Most everyone in the wastewater industry knows how difficult it is to assess force mains. Shutdowns are difficult and sometimes impossible due to lack of redundant pipelines. Traditional CCTV/visual inspections do not provide useful information for assessment as force mains are often long and surcharged. Also, force mains rarely have pig launch/maintenance ports. This leaves few options for inspection of these often-overlooked assets.
A 2010 WERF1 study found that 26 percent of force main failures are caused by internal corrosion. The corrosion is typically caused by hydrogen sulfide (H2S) gas, malfunctioning air/vacuum release valves (AVRVs) and unintentional/unknown high points (missing AVRV’s). The Pipers screening provides a means to identify and locate potential gas pockets and high points that may be harboring corrosive gases for further evaluation. Catastrophic pipeline failures similar to what’s shown in Figure 1 can then be identified early before an SSO occurs.
Through a collaborative effort, SAM along with its engineering consultant, SRT Consultants (SRT) and PICA pre-screened the MFM with an innovative, baseball-size technology called a Piper in late February 2021. Pipers were developed by INGU for the oil and gas market. Through a partnership with PICA, the devices have been introduced into the water and wastewater sector to provide a new pre-screening option for pressure pipes. Pipers can identify and locate leaks, gas pockets, deposits, magnetic anomalies as well as alignment.
Before 2020, force main pre-screening was only offered by one vendor. Since PICA already offered external non-destructive test (NDT) scans with their electromagnetic Bracelet Probe, the entire process can now be streamlined.
SAM provided the support needed for the pre-screening inspection including its jet truck and crew, pump operations, flange removal and extraction support. PICA provided the Piper pre-screening devices, onsite support, as well as the extraction screens. SRT assisted with project planning, findings analysis, test pit location selection, NDT and geotechnical assessment/analysis and final recommendations.
The pipeline was examined for appropriate insertion and extraction points. Insertions points are generally at check valves near the pump station, but this force main had a bypass connection not far from the pump station with good access for launching the Pipers (see Figure 2). The extraction point was at a typical gravity transition manhole (see Figure 3). SAM provided a measurement of the gravity channel and a temporary bar screen was inserted to catch the Pipers.
On Feb. 25, two devices were launched at different intervals starting at 11:45 a.m. with crews already prepared with bar screens downstream at the gravity transition. The pumps were operated to propel the devices through the 14,250-ft long pipeline. The first device arrived shortly before 2 p.m. (velocity of approximately 1.75 ft/second) with the second device arrived shortly thereafter. (see Figure 4)
The PICA Pipers report was delivered within three weeks. The results were presented with elevation and acoustic data sets below the pipeline map to make it easier to choose advantageous test pit areas for further evaluation (see figure 5). A report review meeting was held to discuss the findings and choose the assessment pit locations for the Bracelet Probe work.
In mid-April, the local civil contractor will expose three, selected areas of interest for evaluation. PICA’s electromagnetic Bracelet Probescans pipes externally with no service interruption and can measure the remaining wall thickness around the entire circumference (see figure 7). During the scans, the NDT technicians will also utilize an ultrasonic gauge to calibrate and validate the findings. Findings are known at the time of testing with a full report typically delivered in less than a week. The final engineer assessment report for the MFM is due June 2021.