Yehuda Kleiner

Comprehensive review of structural deterioration of water mains: statistical models

Abstract This paper provides a comprehensive (although not exhaustive) overview of a large body of work carried out in the last 20 years to quantify the structural deterioration of water mains by analysing historical performance data. The physical mechanisms that lead to pipe failure often require data that are not readily available and are costly to obtain. Thus, physical models may currently be justified only for major transmission water mains, where the cost of failure is significant, whereas statistical models, which can be applied with various levels of input data, are useful for distribution water mains. The statistical methods are classified into two classes, deterministic and probabilistic models. Sub classes are probabilistic multi-variate and probabilistic single-variate group processing models. The review provides descriptions of the various models including their governing equations, as well as critiques, comparisons and identification of the types of data that are required for implementation. In some cases, a brief description of the methodology is provided where a decision support system was developed based on a specific statistical model. A companion paper “Comprehensive review of structural deterioration of water mains: physical models” helps to complete the picture of the work that has been done on the subject of water main deterioration and failure.

Comprehensive review of structural deterioration of water mains: physically based models

Abstract This paper provides a comprehensive (although not exhaustive) overview of the physical/mechanical models that have been developed to improve the understanding of the structural performance of water mains. Several components have to be considered in modelling this structural behaviour. The residual structural capacity of water mains is affected by material deterioration due to environmental and operational conditions as well as quality of manufacturing and installation. This residual structural capacity is subjected to external and internal loads exerted by the soil pressure, traffic loading, frost loads, operational pressure and third party interference. Some models address only one or a few of the numerous components of the physical process that lead to breakage, while others attempt to take a more comprehensive approach. Initial efforts were aimed mainly towards development of deterministic models, while more recent models use a probabilistic approach to deal with uncertainties in defining the deterioration and failure processes. The physical/mechanical models were classified into two classes: deterministic and probabilistic models. The effect of temperature on pipe breakage is discussed from three angles; the first deals with temperature effects on pipe-soil interaction, the second deals with frost load effects and the third provides a brief review of various attempts to statistically quantify influence of temperature on water main failure. This paper complements the companion paper “Comprehensive review of structural deterioration of water mains: statistical models”, which reviews statistical methods that explain, quantify and predict pipe breakage or structural failures of water mains.

Probabilistic risk analysis of corrosion associated failures in cast iron water mains

Abstract This paper proposes a method using probabilistic risk analysis for application to corrosion associated failures in grey cast iron water mains. External corrosion reduces the capacity of the pipeline to resist stresses. When external stresses exceed the residual ultimate strength, pipe breakage becomes imminent, and the overall reliability of a water distribution network is reduced. Modelling stresses and external corrosion acting on a pipe involves uncertainties inherent in the mechanistic/statistical models and their input parameters. Monte Carlo (MC) simulations were used to perform the probabilistic analysis. The reduction in the factor of safety (FOS) of water mains over time was computed, with a failure defined as a situation in which FOS becomes smaller than 1. The MC simulations yielded an empirical probability density function of time to failure, to which a lognormal distribution was fitted leading to the derivation of a failure hazard function. A sensitivity analysis revealed that the contribution of corrosion parameters to the variability of time to failure was more significant than the combined contributions of all other parameters. Areas where more research is needed are identified.

Long‐term planning methodology for water distribution system rehabilitation

The most expensive component of a water supply system is the distribution network. Deterioration due to aging and stress causes increased operation and maintenance costs, water losses, reduction in the quality of service, and reduction in the quality of water supplied. In this paper an approach is proposed in which the water distribution network economics and hydraulic capacity are analyzed simultaneously over a predefined analysis period while the deterioration over time of both the structural integrity and the hydraulic capacity of every pipe in the system is explicitly considered. The cost associated with each pipe in the network is calculated as the present value of an infinite stream of costs. In Kleiner et al. [this issue] a methodology is presented to implement this approach into a decision support system that facilitates the identification of an optimal rehabilitation strategy.

Modeling failure risk in buried pipes using fuzzy Markov deterioration process

Numerous models have been proposed in the last two decades for the deterioration of buried pipes. The most prominen t approach has been the Markovian deterioration processes (MDP), which requires that the condition of the deteriorating system be encoded as an ordinal condition state. This encoding is based on numerous distress indicators obtained possibly from direct an d indirect observations, as well as from non-destructive tests. To date, few buried pipes have been inspected and their condition assessed. In addition, the encoding of distress indicators into condition states is inherently imprecise and involves subjecti ve judgment. Furthermore, the consequences of failure for buried pipes are often difficult to quantify precisely due to lack of data. In this paper, a new approach is presented to model the deterioration of buried pipes using a fuzzy rule-based, non-homogeneous Markov process. This deterioration model yields possibility of failure at every point along the life of the pipe. The possibility of failure, expressed as a fuzzy number, is coupled with the failure consequence (also expressed as a fuzzy number) to o btain the failure risk as a function of the pipe age. The use of fuzzy sets and fuzzy techniques help to incorporate the inherent imprecision and subjectivity of the data, as well as to propagate these attributes throughout the model, yielding more realist ic results. At the time of submission, adequate and sufficient data to validate the model were not available.

Estimating risk of contaminant intrusion in water distribution networks using Dempster–Shafer theory of evidence

Intrusion of contaminants into water distribution networks requires the simultaneous presence of three elements: contamination source, pathway and driving force. The existence of each of these elements provides ‘partial’ evidence (typically incomplete and non-specific) to the occurrence of contaminant intrusion into distribution networks. Evidential reasoning, also called Dempster–Shafer theory, has proved useful to incorporate both aleatory and epistemic uncertainties in the inference mechanism. The application of evidential reasoning to assess risk of contaminant intrusion is demonstrated with the help of an example of a single pipe. The proposed approach can be extended to full-scale water distribution networks to establish risk-contours of contaminant intrusion. Risk-contours using GIS may help utilities to identify sensitive locations in the water distribution network and prioritize control and preventive strategies.

Scheduling Renewal of Water Pipes While Considering Adjacency of Infrastructure Works and Economies of Scale

Much research effort has been dedicated to the development of optimal strategies for rehabilitation and/or replacement of water mains. Some of the methods are intended for high-level planning of groups or cohorts of pipes, while others address low-level scheduling of individual water mains. This paper focuses on the latter aspect. An approach is proposed for the efficient scheduling of individual water mains for replacement in a short to medium predefined planning period and subject to various budgetary constraints. This approach also accounts for economies of scale considerations as well as harmonization with other known infrastructure works. A multiobjective genetic algorithm scheme is used as a tool to search a vast combinatorial solution space, comprising various combinations of pipe replacement schedules.

State-of-the-Art Review of Technologies for Pipe Structural Health Monitoring

Advances in electronics, sensor technology, information science, electrical and computer engineering give rise to emerging technologies, some of which could be applied to the inspection, monitoring, and condition assessment of buried water mains. This paper presents a state of the review of sensor technologies used for monitoring indicators pointing to pipe structural deterioration. The potential for multi-sensor system and sensor data fusion for condition-based maintenance are also discussed.

IMPACT OF SOIL PROPERTIES ON PIPE CORROSION: RE-EXAMINATION OF TRADITIONAL CONVENTIONS

Soil corrosivity is not a directly measurable parameter and pipe corrosion is largely a random phenomenon. The literature is replete with methods and systems that attempts to predict soil corrosivity and resulting metallic pipe corrosion from soil properties (e.g., resistivity, pH, redox potential and others) surrounding the pipe. This paper describes research that endeavors to gain a thorough understanding of the geometry of external corrosion pits and the factors (e.g., soil properties, appurtenances, service connections, etc.) that influence this geometry. This understanding would lead to the ultimate objective of achieving a better ability to assess the remaining life of ductile iron pipes for a given set of circumstances. Varying lengths of ductile iron pipes were exhumed by several North American and Australian water utilities. The exhumed pipes were cut into sections, sandblasted and tagged. Soil samples extracted along the exhumed pipe were also provided. Pipe segments were scanned, using a specially developed laser scanner. Scanned data were processed using specially developed software. Statistical analyses were performed on three geometrical attributes, namely pit depth, pit area and pit volume. Various soil characteristics were investigated for their impact on the geometric properties of the corrosion pits. Preliminary findings indicate that the data not always support traditional conventions.

FATIGUE FAILURE OF LARGE-DIAMETER CAST IRON MAINS

Water utility engineers have reported on large diameter water main failures that occurred suddenly without warning, no signs of prior leaks and no visual evidence of corrosion on the fracture surfaces. Often these failed mains had been operating without major problems for over 80 years. A possible explanation may be attributed to alternating or fluctuating stresses such as those caused by heavy traffic and cyclical operating water pressure with occasional occurrences of transients. These fluctuating stresses are accentuated if the pipe impinges on an object with sharp geometry and high stiffness like a rock or a stone. Fatigue of cast iron has been extensively studied in the context of cast iron bridges, structural elements such as columns, engine blocks, etc, but not in the context of buried, grey cast iron water pipes. Fatigue analysis methods developed over the years involve a lot of empiricism and combine engineering principles with experimental observations. Consequently, it is fair to say that fatigue analysis results should be taken more as a guide than as precise or accurate answers. A mechanistic approach to explain fatigue failures of buried cast iron pipes had not been previously explored. This paper explores the application of the fracture mechanics approach (LEFM) to explain some failures in cast iron pipes that occur through the fatigue mechanism. It endeavors to provide insight into the plausibility of fatigue failures in grey cast iron pipes when and if subjected to alternating (also often referred to as repeated or variable) stresses due to surface traffic loads, operating pressure variations and transient pressure occurrences. It is important to note that the proposed analysis refers to grey cast iron pipe type, with carbon in form of flake graphite, which is the predominant material of existing iron trunk mains in North America and Europe.