Contribution of the Thesis

In this thesis, reliability assessment method of water mains is developed accounting

for the physical failure mechanism of pipelines. A method for overall reliability assessment

of complex WDN is then developed using the parameter from graph theory. Risk

assessment methods for WDN are also developed considering the effect on individual pipes

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- Develop a framework for mechanics-based reliability/risk assessment for WDN

A method for prioritizing water mains for maintenance considering failure

probability and risk of pipeline system is developed. For reliability assessment, a

mechanics-based model of corroded water main is used, which would be useful for

municipalities with no available prior break data. The pipe failure probability model is

established based on a stochastic degradation analysis using MC simulation to develop a

Weibull failure distribution function. A method is proposed for updating the model using

expert opinions on the pipe conditions after an inspection and/or maintenance

(repair/replacement) work without any requirement for data mining exercise. The model

would thus be useful for existing and repaired/renewed pipe components. A risk assessment

method is proposed for calculation of risk for small pipeline systems. The system failure

of the pipeline network is defined using FTA applicable for small network.

Thresholds for the reliability and risk are found to govern the decision process in the

reliability and risk-based approaches. In this regard, a careful determination of the

thresholds is required to optimize the decision process considering both the reliability and

the risk. To this end, a relation has been developed between the risk and the reliability for

a system defined using FTA. The thresholds can rationally be chosen based on this relation.

- Development of a mechanics-based failure assessment model

For mechanics-based reliability assessment of water mains, a remaining strength

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intake pipeline are available in AWWA design code, a method for assessment of corroded

water main is not available. Remaining strength assessment methods available for energy

pipelines are examined here to develop a stress calculation method for corroded water main

due to internal pressure. Three existing burst pressure models for corroded energy pipeline

are first revisited for a range of pipe sizes and material strength grades. A series of FE

analysis is conducted to develop a database that is used to revisit different coefficients and

exponents of the models (model parameters). An optimization algorithm based on

differential evolution (DE) method is used to determine the model parameters through

minimization of model errors.

Existing models for energy pipelines are examined with the results of FE analysis to

revisit the existing models. FE analyses are then used to develop burst pressure models for

corroded cast iron water mains.

- Effects of water main breaks on overall network

Studying the disconnection probability of WDN is important for optimizing

repair/maintenance prioritization planning. The conventional method of finding a pipe or

combination of pipes causing network isolation is very complex and requires extensive

computation. This study presents a novel method for finding minimal cut-sets using

Algebraic Connectivity (AC) determined by solving Eigenvalue problems of the Laplacian

matrix.

It is observed that the decrease of AC due to a pipe break indicates a reduction of

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network. A failure probability estimation framework is developed using this property of

the AC to find the minimal cut-sets. This method is found to provide significant saving of

computational time in determining the cut-sets.

It is also revealed that the importance level of pipe to the network corresponds with

the change in AC, and the pipes in the network can be ranked according to this importance

level. The critical and high betweenness pipes identified using the AC based method are

validated through comparison with the results of the Shortest Path method.

- Risk assessment of WDN due to water main breaks

A framework for risk assessment of complex WDN is developed for prioritizing plan

for maintenance. For determination of consequence of water main breaks, the use of

Algebraic Connectivity (AC) from complex network analysis is proposed as the topological

consequences of redundancy reduction and network isolation due to water main breaks.

Pipes in a WDN are ranked using the changing amount of AC before and after break has

occurred. This approach was found to reasonably correspond to hydraulic importance-

based ranking, particularly for the most critical pipes, indicating that the change in AC can

be related to the topological consequence. A pipe diameter related consequence is proposed

to account for the economic consequence. The Fuzzy Inference System is proposed to

aggregate different consequences to the final consequences for risk assessment. A risk map

for prioritizing water mains is developed for decision making based on the risk level of

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decision maker for selecting the risk threshold knowing the corresponding failure

probability.