Reliability assessment of corroded water distribution networks

A majority of water mains in North America are made of cast iron (CI). The use of CI pipes in water utility started at the end of the nineteenth century and many of the pipes are still functioning even after their expected lifespan. CI pipelines are susceptible to external and internal corrosion over time, which can significantly affect their mechanical strength and hydraulic performance, and consequently the water distribution system (WDS) reliability. Buried CI water mains have experienced the highest number of failures due to corrosion deterioration over time. Since most of the water pipelines are running underground, physical condition assessment of pipelines in a distributed network is often expensive and time-consuming. Existing research on WDS reliability based on statistical failure prediction methods typically assume a constant failure rate and ignore time-dependent strength change due to corrosion. This paper proposes a framework for modeling the time-dependent mechanical reliability of WDSs with CI pipes. Time-variant failure of pipelines is estimated using fragility analysis that accounts for the decrease in pipeline wall thickness due to the formation of corrosion pit on the pipeline wall. The impact of internal corrosion on hydraulic performance is evaluated using a time-variant roughness model. A hypothetical WDS is used to illustrate the proposed framework. The results of the case study show that pipe corrosion can significantly impact the reliability of a WDS as it ages. For instance, the mechanical reliability of the WDS in the case study dropped from 100% at the initial state to 16.5% at 60 years due to corrosion. The proposed framework links the deterioration of pipeline strength and hydraulic performance with system reliability and can, therefore, form the basis of a reliability-based planning for maintenance and rehabilitation.