Forensic systems analysis linking pipe failures and operating states in water distribution systems

Water distribution (WD) systems throughout the world are deteriorating, most commonly due to external corrosion ‘pitting’ on cast iron pipes. Replacing an entire aging pipe infrastructure is likely to be prohibitively expensive so it is worthwhile to seek incremental improvements in these systems. By developing a deeper understanding of WD operational states associated with water pipe failures, system managers will be better informed to propose operational improvements that extend asset life and reduce long term costs. A novel framework was developed that describes the process of forensic systems analysis (FSA) to investigate how system operations are associated with large diameter trunk-main failures. FSA utilises a water utility’s pipe failure database and its historical supervisory control and data acquisition technology to analyse operating states at the failure time. Using FSA about 313 large diameter (≥250 millimetres) trunk-main failures were investigated in three real urban WD subsystems in Newcastle, Australia. Significant insights into system operations association with failure were developed resulting in discovery of a previously unknown system failure mode called ‘pump off-peak (POP) increasing static pressure’. POP was characterised by high speed pumping (greater than 80% of the maximum pump rotational speed) or fixed speed pumping from the central pump stations in low demand periods, and with limited pressure relief from downstream reservoirs. POP was shown to be the dominant system failure mode in two of the three investigated subsystems. Frequency analyses of POP conditions associated with failure were conducted. It was found in one of the investigated subsystems that between 2002 and 2012 in the off-peak period between 10:00pm and 6:00am, trunk-main failures were four to five times more likely to occur in the presence of POP conditions. Inexpensive operational changes were investigated to reduce off-peak system pressure and hence, by association, reduce the frequency of POP failures. FSA can assist in identifying activities and characteristics of the WD system that increase the risk of discontinuity of water supply. Furthermore, it was shown that system failure mode diagnoses of trunk-main failures can be used to prioritise pipeline rehabilitation. FSA has been successfully tested on three WD subsystems suggesting that practicing WD engineers can now utilise FSA in the investigation of recent and historical trunk-main failures.