Issues in Setting and Using Risk Tolerance Criteria

Ultimately, decisions on process safety must be made with reference to risk tolerance criteria. Increasingly, risk analysis methods and codes, standards and regulations around the world are moving towards the use of numerical criteria, for example, with the use of Layers of Protection Analysis (LOPA) and standards for safety instrumented systems such as IEC 61511 / ISA 84. Various issues must be addressed if such criteria are to be set correctly and used meaningfully. These issues include the entity to which criteria apply, e.g. scenario, hazardous event or process risk; individual versus group risk and their relationship; suitable allocation and scaling of tolerable risk; uncertainties in risk estimates; aggregation of risk from different consequence levels; and risk qualities.

This paper discusses these issues using examples and provides guidance to avoid the pitfalls that are often encountered in using numerical criteria.