THE APPLICATION OF HAZOP STUDIES TO INTEGRATED REQUIREMENTS MODELS FOR CONTROL-SYSTEMS

This paper discusses the relationship between and the potential integr ation of three mature disciplines, hazard identification, structured a nd formal methods for control systems specification. In particular the methods used are Hazard and Operability Studies (HAZOPS), Ward and Me llor (W&M) Essential Models and the Calculus of Communicating Systems (CCS). Ward and Melter models not only provide the required control fl ow and transformation extensions to capture the essence of control and protective systems but we also suggest that the modelling philosophy, underpining the Ward and Mellor development method, provides an appro priate model to which the HAZOPS of such systems can be successfully a pplied at the requirements stage. Specifically, we can demonstrate how the model can be effectively partitioned to provide the necessary vis ibility of the systems requirements that will enable an efficient and effective HAZOP meeting. The paper shows, with appropriate guide words and interpretations, how meaningful deviations from the required inte nt can be developed. We also show how the interpretation of the model can significantly effect our ability to produce meaningful deviations and what impact this has on the HAZOP process. A particular concern ha s been that these techniques should be seen as an extension of current good practice within the relevant engineering disciplines. Our approa ch is illustrated by a small but realistic industrial case study.