Use Maximum-Credible Accident Scenarios for Realistic and Reliable Risk Assessment

here have been many methodologies proposed for the risk assessment in the chemical process industries (CPI). Among them, the most notable ones are quantitative risk analysis, probabilistic safety analysis, worst-case methodology for risk assessment and optimal risk analysis. A critical review of these methodologies is available (1). The key points of these methods are: Quantitative risk analysis (QRA) — This method is comprised of four steps: hazard identification, frequency estimation, consequence analysis and measure of risk. The first step answers the question: What can go wrong? This is the most important step because hazards that are not identified will not be quantified, leading to an underestimated risk (2, 3). The techniques used for hazard identification include hazard indices, hazard and operability (HAZOP) studies, failure mode and effect analysis (FMEA), what-if analysis and checklists. After the hazards are identified, the scope of a QRA is defined. The second step asks: How likely is the occurrence of each accident? Answering this question means quantifying of the probability of each accident scenario. The third step aims to quantify the negative impacts of the envisaged accident scenario. The consequences are normally measured in terms of the number of fatalities , although they can also be determined by the number of injuries or value of the property lost. The last step of a QRA is to calculate the actual risk. Probabilistic safety analysis (PSA) — Different techniques can be combined to carry out PSA (4, 5, 6). PSA provides a framework for a systematic analysis of hazards and quantification of the corresponding risks. It also establishes a basis for supporting safety-related decision making. The methodology and the procedures followed for the PSA of a typical chemical installation handling a hazardous substance can be outlined in seven major steps: (1) hazard identification; (2) accident-sequence modeling; (3) data acquisition and parameter estimation ; (4) accident-sequence quantification; (5) hazardous substance-release-categories assessment; (6) consequence assessment; and (7) integration of results. Worst-case methodology for risk assessment — An excellent work on hazard and risk screening based on worst-case methodology is presented by Hirst and Carter (7). The method does not discuss use of worst-case scenarios in detailed risk assessment, but rather outlines the development of ready-made and easy-to-use risk indices, such as the risk integral, scaled risk integral and approximate risk integral, for early planning T Posing various possible incidents — rather than just the worst-case one — illuminates those …