Framework of Methodology in This Study

With the awareness of the effectiveness of hierarchies in dealing with complexity, this study adopts hierarchies, but based on an object-oriented approach, to represent the relationships in water supply systems, and to develop frameworks for risk assessment. Meanwhile, fuzzy sets theory, evidence reasoning, and fault tree analysis are integrated with these hierarchies to generate quantitative results. This method is composed of four parts (Figure 2.6): (1) hierarchical representation of water supply systems, (2) vulnerability assessment of components and system, (3) aggregative risk assessment, and (4) risk propagations in water systems. These four parts are described briefly as follow.

2.3.1 Object-Oriented Framework for Water Supply Systems

Firstly, object-oriented approach (OOA) is proposed in this research to deal with complexity of water supply systems and to generate hierarchical structure for risk assessment. Object-oriented

Chapter 2 Literature Review

approach is a method that represents engineering systems in terms of objects. Every component in a water supply system is viewed as an object, and the overall water supply system is viewed as a network composed of sets of objects that are interconnected with each other. All risk factors about the components are considered as attributes or behaviours of objects. Furthermore, with the generalization and aggregation relationships, object-oriented hierarchical structures can be easily formed to represent the whole/part relationships and interconnections between objects in the water supply system.

2.3.2 Object-Oriented Model for Vulnerability Assessment

Practically, the influence of threats to component in water systems is a process in which the component changes its state from normal to failure due to its vulnerability. Vulnerability is the degree that a component can be influenced by threats. This mechanism can be easily described by objects states transition diagrams in object-oriented environment. Each object in a water supply system has different failure states due to the influences of different hazards or threats. Normally the vulnerabilities of an object are different according to the failure states and can be explicitly represented by its states transition diagrams. Then the vulnerabilities of objects can be integrated by following the whole/part structure to produce vulnerabilities of subsystems or the overall water system.

For a quantitative analysis, fuzzy sets theory and aggregation methods can be an option to produce quantitative results for vulnerabilities. However, study on vulnerabilities exceeds the scope of this thesis, and is proposed in further work. According to the risk definition in Chapter 1, vulnerability of any component, subsystem, or the overall water supply system is assumed unity in the following part of this thesis, which will not compromise the usefulness of the current study.

2.3.3 Object-Oriented Model for Aggregative Risk Assessment

According to Figure 2.6, aggregative risk assessment is composed of two stages, i.e., the component level and the system level.

Firstly, states transition diagrams of objects describe the relationships between hazards, object failure states, and object risks, which thus provides a hierarchical framework for risk assessment at components level. In this hierarchical framework, risk of object is at top level followed by relative failure states that are at its immediate lower level. Hazards or threats are the bottom level in this framework. This indicates that risks of object are determined by its failure states, which are in turn determined by the threats or hazards directly related to them. This research represents each hazard or threat in terms of its likelihood of occurrence and consequence that are represented by fuzzy

numbers. Risk of a component is thus an aggregative measure that is determined by aggregating the risks of threats or hazards along the hierarchical structure.

Secondly, for the risk assessment at the system level, object-oriented whole/part relationship structure is used to determine aggregative risks of water supply systems. In this hierarchical framework, the water supply system is at the top level, its subsystems and components are at relative lower levels. Therefore, risk of the overall system is an aggregative measure which is contributed by the risks of its subsystems and components along the hierarchical structure.

After the conceptual framework for aggregative risk assessment has been developed, fuzzy sets theory and aggregation method (i.e., evidence theory) are used to produce quantitative evaluations.

Figure 2.6 Research compositions of the present study

2.3.4 Object-Oriented Fault Tree Analysis

Fault tree analysis is considered in this study to represent the cause-effect relationships in the water supply system. Fault tree analysis, a deductive reliability and risk analysis technique, can answer the question of how the system could produce a failure X (e.g., contamination at some demand nodes). With the help of fault tree analysis, risk analysts will know which component in the system

Threat/Hazard

Component (e.g., pipe)

Vulnerable? Normal Yes No Component failure System analysis Vulnerable? Normal System risk No Yes

Hierarchical representation of water supply systems to identify problems

Vulnerability assessment at both component and system levels

Risk assessment at both component and system levels

Risk propagations in water supply system (i.e., what can go wrong, and

how can it go wrong?)

Chapter 2 Literature Review

is more critical and which risk scenario is more possible. Meanwhile risk contributions and uncertainty contributions can also be easily obtained to support selection of mitigation measures and asset management.

However the development of fault trees is still as much an art as a science. This research uses an object-oriented approach to generate fault tree structures via two steps. Firstly, object states transition diagram is used to generate the fault trees at component level. Then, interconnections between components in a water supply system are used to develop fault trees at system level. After fault trees have been constructed, fuzzy fault tree analysis is adopted to obtain quantitative results.