ABSTRACT
Techniques and Tools for Safety Cases Introduction This section introduces a description and discussion of a number of tools used by the safety team in developing the safety case evidence and constructs. The types of tools and techniques used include HAZOPs, Fault-trees, Event-trees, Zonal Analysis, FMEAs, SWIFT and Human Hazard Analysis. Examples will be brought out along with some demonstration of the use of graphical output from typical software tools. Many of the tools and techniques are utilised for both of the main tasks in safety case development – hazard identification and hazard . The first seeks to identify all reasonable hazards related to the system or equipment of interest, the second attempts to evaluate (qualitatively or quantitatively), organise and even prioritise the hazards. HAZOP A HAZOP is a HAZard and OPerability study, it is a technique for systematic examination of any system to assess the hazard potential due to incorrect operation of the component parts of the system. It can be further developed to analyse the consequential effects of the occurrence of the hazard on the whole system. It originated in the chemical industry, but is now used extensively in many other industry areas. Originally, the needed information items to carry out a HAZOP were process flow diagrams (PFDs) and piping and instrumentation diagrams (P&IDs), i.e. the design and construction of the system. A team of engineers and specialists were also needed covering design, operations and maintenance. The method required the system to be divided up into component parts (originally called ‘nodes’), followed by a series of guidewords applied to each node to assess what would happen if the guideword did occur. To enable efficient use of resource a standard worksheet was produced with cells and headings for each answer. To carry out a HAZOP, these resources will need to be available; the system design, the team of experts, the applicable guidewords and a method of recording the information. The guidewords used in the chemical industry are given in table 13.1 along with their chemical-based interpretations. For a non-chemical system, the
guidewords will probably have to change. This is fine, as long as a record of the actual guidewords used and the rationale for their use is kept. On completion of the HAZOP, an initial report is usually issued containing the recommended actions to be applied to the system. A further, final report is issued when all the actions have been completed and this becomes the audit trail of the study.
