Elementos de la obligación tributaria

Object-oriented approach is a method that can naturally represent real-world entities and phenomena in terms of objects and classes (Booch, 1994; Martin and Odell, 1995, 1998; Embley et al., 1992). In an object-oriented modelling paradigm, object and class are two key concepts with which analysts can effectively manage complex engineering systems. These two concepts are also effective to organise risk information in a water supply system.

(1) Objects

Objects are model constructs used to represent real-world entities which can communicate with one another (Booch, 1994; Martin and Odell, 1998). This communication consists of messages exchanged between objects. Messages represent the transfer of information, materials, or energy. Components (e.g., pipe, pump, etc.) and subsystems (e.g., water source, water distribution, etc.) in a water supply system can be viewed as different objects. These objects are interconnected together to form a system of supplying water to customers. Water flow is the message exchanged between any two objects in a water supply system.

When an object receives a message, it gives responses by altering its internal state and important characteristics or attributes, and generating output messages to other objects in the model. The way in which the object responds to messages depends on its internal processes and states. For a pipe in a water supply system, road loading can be viewed as an external message/hazard which is potentially influencing the pipe. If the road loading is too high, the pipe will be broken and change its state from normal operation to failure state, which consequently introduces risk to the water system.

One of the most important characteristics of object is encapsulation. This means that the attributes and behaviours of a component or subsystem are entirely encapsulated within the confines of a self-contained object. Attributes define an object’s state and behaviours describe an object’s functionality. The entire system is thus viewed as the combination of individual objects with different functionalities. Meanwhile the individual objects communicate with one another in a way that faithfully replicates their interactions in the real-world (Booch, 1994). In a water supply system, for example, a river can be viewed as an individual object encapsulating the attributes of raw water. A water treatment plant can be viewed as an another object which encapsulates the behaviour of removing the impurities from raw water in order to meet the standards of drinking water. Similarly, a pipe in the water system can be viewed as an object which encapsulates the

Chapter 3 Object-Oriented Risk Assessment of Water Supply Systems

attributes, such as length, diameter, age, and roughness factor, and behaviours of delivering water and reducing water head due to roughness. The overall water supply system is thus a composite object composed of interconnected individual objects (including river, water treatment plant, pipes, etc.). In the object-oriented environment, the system object has functions (e.g., hydraulic models) that control the communications among individual objects in a way that faithfully replicates the interactions in the real-world water supply system.

(2) Classes

In a real engineering system, there are many objects of a specific kind. It would be extremely inefficient to redefine the same methods in every single occurrence of that object. Thus the concept of class is proposed in the object-oriented approach. A class is a template or blueprint that defines the methods and variables included in a particular kind of objects. The methods and variables that make up the object are defined only once in the definition of the class. The objects that belong to a class, commonly called instances of the class, contain only their own particular values for the variables. This concept is also applied to a water distribution network with large numbers of pipes. Although different pipes have different values of length, diameter, and material, all the pipes share some common attributes (e.g., diameter, length, age, material, etc.) and behaviours (e.g., delivering water, introducing head loss, deteriorating with age, etc.). With respect to this, mechanical engineers always use the same template/blueprint to produce pipes that have similar properties; and water engineers use the concept of pipe class to represent different pipe instances in the system by extracting their common features. Normally associated with the pipe class, many models are developed to describe its behaviours in real water supply processes. This process of obtaining classes is usually called abstraction or generalisation in practice. Even though it is only an abstract concept which has no physical counterpart in real world, class plays important roles in helping people organise complex information in the system.

Inheritance is an important characteristic of class. It is one of the fundamental rules supporting abstraction and generalisation in object-oriented approach. During the generalizing process, the more general class is called base class, and the relative less general class is viewed as derived class or instance. Inheritance allows the derived class or instance to inherit the attributes and behaviours defined within the base class. In a distribution network, pipe instances are usually viewed as derived classes, and the general pipe class is viewed as their base class. Attributes (i.e., diameter, length, age, material, roughness coefficient, etc.) and behaviours (i.e., introducing head loss, deteriorating with time, etc.) are defined within the base class. These attributes and behaviours are inherited by the pipe instances. This mechanism of inheritance facilitates the process of risk assessment in water supply system by developing common risk models for pipe class, while repeatedly reusing these models in different pipe instances.

Chapter 3 Object-Oriented Risk Assessment of Water Supply Systems

(3) Applications of object-oriented approach

Applications of object-oriented approach have covered various areas in practice. Object-oriented paradigm has represented a major achievement in software engineering that facilitates modelling complex real-world problems (Martin and Odell, 1998). When properly applied, it can yield robust models consisting of reusable, easy-to-maintain components in different kinds of engineering systems (Booch, 1994; Solomatine, 1996; Ross et al., 1992; Black and Megabit, 1995). Meanwhile, object-oriented approach has also been used to solve engineering problems, which includes development of framework for decision making (Liu and Stewart, 2003), surface water quality management (Elshorbagy and Ormsbee, 2006), management of river system and water resources (McKinney and Cai, 2002; Simonovic, et al., 1997, Reitsma and Carron, 1997; Tisdale, 1996), reliability and risk assessment (Wyss, et al., 2004; Wyss and Durán, 2001; Black and Megabit, 1995; Matsinos, et al., 1994), material failures (Roberge, 1996), uncertainty of early design (Crossland, et al., 2003) etc. The effectiveness of using object-oriented approach to deal with complexity is also specifically illustrated by many researchers (Booch, 1994; Wyss et al, 1999; Weber and Jouffe, 2006;). However, its potential in risk assessment of complex system has not been specifically considered in the existing research.

From the above discussion and applications of object-oriented approach, it is identified that one of most important power of objects and classes is their effectiveness in organising complicated information of engineering system (Martin and Odell, 1998). Firstly, all engineering systems, including water supply systems, are designed, constructed, operated and managed in terms of objects. For a water supply system, its performance is determined by the performance of the components/objects. That is risks of individual objects contributing to the risks of the overall water system. Secondly, most of the knowledge about the engineering system focuses on objects. For an example of analysing pipe deterioration, many models (including physical and statistical models) have been developed to represent the deteriorating process with time. Even though these models are different with their applications, they are all related to specific pipe objects in a water system. These models can be viewed as the behaviours or methods of pipe objects. Thirdly, generalisation of classes is a straightforward way to avoid repeated work, which thus makes it effective in managing the common features in a complex system.

The above discussion shows the possibility of using object-oriented approach as an effective tool to organise complex risk information in water supply systems. This awareness motivates this study to adopt object-oriented approach to develop frameworks of risk assessment.

Chapter 3 Object-Oriented Risk Assessment of Water Supply Systems