Water pipelines - is there life after design life?
Understanding the residual life of pipelines is crucial to making effective investment decisions, writes Guy Cleveland, pipelines specialist lead at Stantec.
The bulk of water supplies are carried by major supply aqueducts and distribution pipelines, which suffer deterioration with age. The water companies owning these ageing assets will firstly look to explore how they can enhance and improve their existing asset before considering investment in complete new-build projects.
This is where asset management, maintenance and renovation to promote additional design life becomes an attractive proposition.
Cast-iron pipes have been used for many hundreds of years for the conveyance of water. Despite iron no longer being the predominant material for the construction of new pipelines, it still accounts for the greater part of the existing water supply networks. Maximising the benefit derived from the enormous asset value of these existing cast iron mains is essential to the successful management of distribution and transmission systems.
Proactive renovation must be properly targeted if all the benefits are to be realised. This targeting requires a reliable estimate of the remaining asset life. The service life of cast iron water mains is dependent on the type of cast iron, the type of soil the pipe is buried in, and the loads imposed on the pipe during its service life.
Whilst many cast-iron pipes used for UK water supply have already comfortably exceeded their intended design life, it is vital to understand their residual life in order to make appropriate decisions on rehabilitation or replacement. The primary cause of degradation of cast-iron water pipe is corrosion of the pipe wall.
The starting point for the prediction of the failure of a pipeline must be identification of the possible failure modes. For iron pipelines, these include through wall perforation, bursting, crushing, longitudinal bending and joint leakage. Most dangerous amongst these other failure modes are circumferential fracture in smaller diameter pipes, and longitudinal fracture in larger diameters.
Resistance to all of these, other than joint leakage, depends on the residual pipe-wall thickness, but whereas for most failure modes the relationship is linear, in the case of the crushing mode, resistance depends on the square of wall thickness, and the effect of corrosion is thus greatly amplified.
Unfortunately, the reliability of what seems to have become the traditional approach to condition assessment can be questioned, on the grounds that it addresses only one of the potential, and unlikely, failure modes of cast iron water mains.
The current, and widely used, remaining asset life calculation consists only of looking at the risk of through wall corrosion. Not only does this ignore other failure modes that are often observed, but it seems frequently to be done in a questionable way.
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, often 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. Longitudinal 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, crushing 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. Assumptions 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 applied bending is related to its residual wall thickness, which can be determined through condition assessment.
For larger diameter pipelines, 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 subsequently preventing, failure in large diameter pipelines means that the high cost of assessment and analysis is often 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 understanding of the behaviour of buried rigid pipes, and the loss of wall thickness due to corrosion.
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 extrapolation of the loss of wall thickness, it is also possible to predict the point at which the loading will exceed the capacity 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. Improving the method by which the timing of the intervention is planned is a significant factor in getting the most value from investment by targeting interventions at the most effective locations.
Whilst there are acknowledged challenges in undertaking the type of structural analysis outlined above, especially in smaller diameters, adopting this approach can yield significant benefit over and above the established remaining life analysis approach.
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