Development of an Algorithm for Automated Cause - Consequence Diagram Construction

Cause-consequence analysis is one of the best tools available for a comprehensive reliability study. The cause-consequence diagram (CCD) method, like fault tree analysis, represents the failure logic of the system, but in addition the CCD also identifies the complete set of consequences following a given initiating event. While there are well-developed commercialized software packages for fault tree evaluation and construction, no satisfactory methodology has been published for automated cause-consequence chart synthesis. Hence this paper outlines the development of an algorithm for automated causeconsequence diagram construction. The algorithm builds on methods developed previously for fault tree construction, such as topology diagrams, describing how components are linked together in a system, and component decision tables which model component behaviour. Using this information rules have been developed which enable the construction of the CCD. Once constructed the diagram can be quantified to give exact system reliability. To demonstrate the construction the algorithm is applied to a simple example. Introduction In his study, Nielsen [1] developed the cause-consequence diagram as a graphical tool for the analysis and description of relevant accidents in complex process plants. The method is based on a combination of standard reliability techniques. The cause diagram is the conventional fault tree used to describe all causes of an undesired event. The consequence diagram is an event-sequential diagram (decision-tree diagram) describing the alternative failure sequences that an abnormal event leads to if one or more of the accident preventing/limiting provisions fail [1]. By using a combination of the reliability methods, the logical connections between independent accident causes and accident consequences can be established [2]. The main symbol in a CCD is a decision box which contains a component/subsystem condition. It is an identical representation of the YES/NO branches in an event tree. Following the YES/NO branches of the decision box the diagram is developed until it terminates in consequence boxes. As an example of the method consider a simple lamp circuit consisting of battery, switch and lamp. When the switch is closed power is supplied to the lamp which then lights. The CCD for the system is shown in figure 1. The first decision box condition “Switch contacts close” is considered. If the condition is not satisfied then the consequence is that there is no light (NL). If the condition is satisfied then the condition “Current reaches lamp and it lights” is considered. Once again if this is not satisfied then the consequence is NL, however if it is satisfied the consequence is light (L). As all paths have terminated in consequence the diagram is complete. The causes of the switch contacts not closing and current not reaching the lamp are developed using fault trees as shown by the arrows to the first and second