Don't Give Up When Things Get Tough to Perform Your Critical Force Main Inspections

The City of Oceanside California operates and maintains three critical ductile iron force main installed during the 1970's and 1990's, the 42' Combined Sewer Line, the 24' Mission Ave Force Main, and the 24'/36' Land Outfall. The six-mile-long Land Outfall conveys effluent from the San Luis Rey Water Reclamation Facility (SLRWRF) at the northern end of the City to the La Salina Wastewater Treatment Plant (LSWWTP) near the ocean outfall. The two other pipelines are approximately 3 miles long and begin near the intersection of Mesa Drive and Garrison Street, run parallel to the Land Outfall, and flow toward the SLRWRF. Past failures on the Land Outfall prompted the City to create a condition assessment project to evaluate these mains as well as construct improvements on appurtenances and create access points for inspections. Due to the topography, portions of the force mains are in gravity flow conditions, which complicated the condition assessment plans along with the fact that portions of the mains run along and under the City's major roadway, Oceanside Boulevard, and through culturally protected, and biologically protected lands. The condition assessment team, consisting of Arcadis and Infrastructure Engineering Corporation, utilized multiple technologies in a phased approach to collect the most relevant data using a cost-effective approach. Initialization and selection of technologies included the following. - Development of Base Maps. The team compiled all existing information on the pipelines into a base map - Up-Front Capital Improvement Projects. The team, in conjunction with the City, identified several items to prioritize as an immediate need for replacement. - Development of Condition Assessment Plan. With baseline information and identification of up-front capital improvement projects, the team proposed a Condition Assessment Plan that included the following assessment types: - Acoustic screening using Smart Ball to identify gas pockets and the potential for internal corrosion failure. - Broadband Electromagnetic (BEM) testing to establish baseline wall thickness data, inspect locations in high corrosivity soil locations, and inspect high points. - Internal Electromagnetic testing using PipeDiver on high consequence areas for more detailed wall thickness data. - Additional soil corrosivity samples from in the trench adjacent to the pipelines. As part of the condition assessment plan for the internal acoustic and electromagnetic testing, flow conditions were analyzed on paper and specific actions were detailed regarding operations of the Mission Avenue Lift Station and the SLRWRF effluent pumps to maintain 2 feet per second of full pipe flow during the inspections as well as depressurizing and repressurizing the Land Outfall for the insertion of Pipe Diver. Flow diversion plans were also created to restrict an industrial customer and the desalination plant that discharge directly into the Land Outfall. There were intentions of performing a dry run to simulate the required flow conditions prior to performing the Pipe Diver inspection as well as having all of the air release valve repairs completed, but due to weather impacts on the construction and the inconvenience of customers having to hold their flows, the dry run flow test was not performed. During the Land Outfall inspection of the river crossing section, the condition assessment team was able to successfully control flow at 2 feet per second for the two Pipe Diver runs after some trial and error of partially closing a valve downstream to fully fill the pipe after it was depressurized. The testing lasted a day longer but was successful. However, the Land Outfall section at the railroad crossing was more challenging and the team was not able to turn the valve and consistently maintain full flow at 2 feet per second and the test was abandoned. Analyzing the data collected from the successful inspections on the Land Outfall provided an inconclusive determination of the condition of the railroad crossing portion of the force main. BEM testing at the beginning and end of the railroad segment performed during the construction of the insertion and retrieval access points did not show any wall loss internally at the crown or any external corrosion. However, various soil analysis performed along the full 6 mile force main length did not correlate with some of the observed external pitting, and some pipe segments were found to have polywrap to protect the pipe from external corrosion and some did not. Even more concerning, the results from the river crossing Pipe Diver inspection showed many small areas of corrosion up to 50% wall loss that seemed to be from corrosive soil hot spots and or deteriorated polywrap or no polywrap present. It would have been easy to give up and say that based on the two BEM test points the railroad crossing was in good condition but since it was such a large crossing and included passenger trains, the team felt it was worth further exploration to prevent a failure in the area and associated social disruptions. The City and condition assessment team worked closely together to perform additional hydraulic evaluations, perform a dry run to get the required flow conditions and hold them prior to conducting the inspection and writing up a detailed protocol to follow on the inspection day. With this advance effort, the Pipe Diver was successfully deployed and provided meaningful data. The data showed multiple localized areas of corrosion up to 70% wall loss. Approximately 1,500 feet of pipeline was recommended for immediate replacement. The planning level cost opinion for the replacement, assuming the replacement is installed in parallel in the same general alignment as the existing Land Outfall is $1,454,700. Lessons learned are that you should do all possible up front planning and modeling, including dry runs if flow conditions may be difficult to achieve, to support a successful internal condition assessment, and selecting point inspections based on base map data may not provide a full picture of pipeline conditions if external corrosion hot spots exist and the pipeline has not been consistently protected. Detailed assessments should be conducted for high consequence of failure force mains to prevent high profile failures. Additional inspections are recommended for Mission Avenue Force Main and the Combined Sewer line. At this time, the City is evaluating if the Mission Avenue and Combined Sewer Force Mains may need increased capacity or be taken out of service as part of decommissioning the LSWWTP. Additional inspections will be planned in conjunction with parallel pipeline construction, depending on the outcome of this evaluation.