Can It Handle the Pressure? Condition Assessment, Structural Evaluation, and Repair of an Existing 72-Inch PCCP Pipeline
Since its formation in 1951, the North Texas Municipal Water District (NTMWD) has been charged with developing a reliable water supply source for its 13 member cities and 46 other customers (some direct and some indirect), including more than 1.6 million people in portions of Collin, Dallas, Denton, and Kaufman Counties and all of Rockwall County. The NTMWD’s Lake Texoma Water Supply Project was designed in the late 1980’s and completed in 1991 to convey raw water from Lake Texoma to Lake Lavon via a discharge point at Sister Grove Creek approximately 30 miles to the south of Lake Texoma. Sister Grove Creek then flows into Lake Lavon to the south. The City of Sherman, Texas’ water treatment plant is also supplied from the 72-inch pipeline. The NTMWD had never performed a condition assessment of the pipeline, so a condition assessment utilizing interior visual inspection, Remote Field Transformer Coupling (RFTC), and external pipeline inspection at areas where the other inspection methods identified potential issues was performed in 2009. The PCCP was designed to meet the requirements of the American Water Works Association’s 1st Edition of the M-9 Concrete Pressure Pipe (1979). The thrust restraint design sections of M-9 underwent a major change in the 2008 edition that recognized additional stresses that are experienced in the steel cylinder of PCCP and the need to potentially increase the cylinder thicknesses to resist these stresses, especially where welded joints are used for thrust restraint. This revision of M-9 combined with the Engineer’s experience with PCCP failures at welded bends on past projects prompted an in-depth structural analysis of the existing pipeline bends and fittings at the 125 MGD flow rate. This analysis included finite element analysis, geotechnical analysis of in-situ soil conditions at 26 bends, and follow up structural analysis. After the analysis phase, several bend strengthening options were considered at the 21 bends that did not meet the criteria in M-9 at the increased flow rate. This paper will summarize the procedures and results of the condition assessment, results of the structural analysis of the PCCP bends, and the repairs necessary to strengthen the bends before increasing the flow.
Pipelines 2012 – innovations in design, construction, operations, and maintenance, doing more with less, August 19-22, 2012, ASCE