Preliminary safety analysis

Abstract Various major safety studies are carried out at appropriate stages during a project. Many companies do some form of preliminary analysis at points between initial project concept and when the process design is completed. These studies aim to ensure that the decisions on process design and site selection take full account of process safety requirements and related risk and environmental constraints. Methods have been incorporated and developed during this work to take account of best industrial practice for such safety studies. These are listed under the general heading of preliminary safety analysis (PSA) and are carried out from the time of the concept safety review until such time as reasonably firm process flow diagrams or early P & I diagrams are available. The methods included are as follows: • concept safety review (CSR) • critical examination of system safety (CE) • concept hazard analysis (CHA) • preliminary consequence analysis (PCA) • preliminary hazard analysis (PHA) These have been developed from a model of the plant and its interpretation as part of an incident scenario. The emphasis throughout is on utilizing the best points to start the search to identify undesired events contributing to the development of accidents. For the main method described, preliminary hazard analysis, this search has as its starting point and fulcrum the ‘dangerous disturbances of plant’ which arise at a point in the incident scenario just after emergency control measures have failed to control the situation. The study should be conducted using risk evaluation sheets which model each stage of the incident scenario and allow for a short-cut assessment of risk when this is desired. The above methods are demonstrated by part of a simplified case study. The methods function well and provide not only a good model of incident scenarios but are readily developed into fault and event trees and operating procedures. They are invaluable for the development of safety reports for regulatory authorities. Furthermore, by not imitating HAZOP methods they strengthen the effectiveness of the search process.