Operability analysis devoted to the development of logic trees

This paper illustrates the execution of an Operability Analysis (OA) directed to the elaboration of logic trees. It is shown that both Fault Trees and Event Trees descend almost automatically from a well-structured OA, though it is also pointed out that this transition is facilitated by the intermediate construction of an Incidental Sequences Diagram (ISD), a logic tree that provides a clear indication of all the links between primary events and their unwanted consequences. As far as the OA is concerned, a very innovative procedure has been devised to allow filling up of the form to the point of systematic and definite completion of the analysis by means of a recursive mechanism. In addition, direct extraction of an ISD from the OA permits an otherwise impossible check to be made of the congruence of the analysis. The method is illustrated with the aid of flow diagrams and is fully capable of computerization. The application relates to a Pressure Regulating Installation on a city gas mains. (C) 1997 Elsevier Science Limited.