Semiquantitative Risk Analysis.An EPSC Working Group

Abstract

Semiquantitative Risk Analysis (SQRA) tools like e.g. LOPA were developed for the assessment of scenarios which have previously been identified with Process Hazard Analysis (PHA) tools like e.g. HAZOP. These SQRA tools represent a scenario as chain of events with barriers preventing the propagation of the chain of events towards the final accident. Orders of magnitude are used to express frequencies, failure rates and consequences.The risk acceptance criteria are a central element of each SQRA tool and are typically represented in a matrix having an axis for frequency and one for severity. These risk matrices are defined individually by each company as part of its SQRA tool.Though some countries have defined risk acceptance criteria for individual or societal risk as part of Quantitative Risk Analysis (QRA), there is no generally accepted standard for SQRA applied to single cause a single consequence scenarios. The focus of the working group was therefore a comparison of the risk matrices of the participating companies with their individual risk acceptance criteria and application rules. These application rules a e.g. the use of modifiers or the assumed frequencies for initial events a have a significant influence on the risk level obtained by use of a risk matrix.Key results of the discussion and comparison within the working group are:Risk acceptance criteria for 1 fatality as reference scenario differ only by one order of magnitudeCompanies using or not using modifiers are equally representedCriteria used for ALARP are similar for most companiesIn some cases differences in use of modifiers and risk acceptance criteria compensate resulting in a comparable risk levelApplication of company-specific risk matrices to an example case gave the same results for most participants, thus demonstrating that most risk matrices of working group members will arrive at a comparable risk levelWhile individual companies will often not share their risk matrix in public, the results of the working group can be shared, as the comparisons have been anonymized.The matrices discussed in this study were submitted by major European companies from the chemical and oil & gas industries. As these companies are the leaders in their industries, the study will be of interest to others to compare and assess their practice.