Water & Wastewater Treatment Magazine
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24 | NOVEMBER 2019 | WWT | www.wwtonline.co.uk The Knowledge Pipeline management For example, it assumes that the deepest internal and external corrosion pits will coincide spatially on the pipe wall. Then, the critical residual wall thickness is calculated using Barlow's formula, even though this formula relates to membrane stresses, and not to the stresses in the vicinity of a pit. These inaccuracies can, of course, lead to conservative estimates of residual life, but that cannot be relied upon to compensate for ignoring the other, oen more likely, failure modes. Common causes of failure The most common mode of failure for small diameter (<300mm) cast-iron pipes is circumferential fracture. This type of failure is caused by longitudinal bending forces applied to the pipe resulting in a failure propagating around the circumference of the pipe. This is as a result of the high circumferential stiffness, coupled with a relatively low longitudinal stiffness. Longi- tudinal bending is relatively easily analysed using standard beam theory, although a major difficulty exists in establishing the support conditions along the bottom of pipelines. Longitudinal fracture is more likely to be found in larger diameter cast iron pipes (>600mm). Longitudinal fractures occur axially along the pipeline as a result of excessive ring bending forces. The failure mode can be due to internal water pressure, crush- ing forces acting on the pipe or possibly due to longitudinal compressive forces acting on the pipe. Longitudinal fracture can extend along the full length of the barrel of the pipe. The high longitudinal stiffness and low ring stiffness of larger diameter pipes means they are unlikely to experience circumferential fracture. As previously mentioned, circumferential fracture as a result of longitudinal bending is relatively easily analysed using standard beam theory. It can be difficult to establish the support conditions along the invert of pipelines. Assump- tions relating to the length of pipe that is unsupported are critical as the applied bending moment increases in proportion to the square of the unsupported length. The pipes' ability to resist the ap- plied bending is related to its residual wall thickness, which can be determined through condition assessment. For larger diameter pipe- lines, where structural failure is more likely to occur in the form of longitudinal fracture as a result of ring bending, predicting the remaining life becomes more taxing. Predicting, and subse- quently preventing, failure in large diameter pipelines means that the high cost of assessment and analysis is oen easily outweighed by the reduction in risk of a major burst event. Predicting the point at which the pipe will fail can be done by comparing the applied loading with the residual load-bearing capacity. Determining the loading applied to a pipeline can be done following the established pipeline structural design procedures. Calculation of the load-bearing capacity requires a more detailed understand- ing of the behaviour of buried rigid pipes, and the loss of wall thickness due to corro- sion. By comparing the current applied load with the residual load-bearing capacity of the pipe-soil system, it is possible to determine the current factor of safety against failure. By ex- trapolation of the loss of wall thickness, it is also possible to predict the point at which the loading will exceed the capac- ity and failure will occur. Targeted interventions yield greater investment value Maximising the life of the pipelines within a water distribution network relies on timely interventions. Improv- ing the method by which the timing of the intervention is planned is a significant factor in getting the most value from investment by targeting inter- ventions at the most effective locations. Whilst there are acknowl- edged challenges in undertak- ing the type of structural anal- ysis outlined above, especially in smaller diameters, adopting this approach can yield signifi- cant benefit over and above the established remaining life analysis approach.