Layers of protection analysis in the framework of possibility theory

•Uncertainty in LOPA method is considered in the framework of possibility theory.•Fuzzy consequence frequency is derived from fuzzy initiating event frequency and fuzzy PFD.•Fuzzy consequence is compared to a tolerable risk level using possibility and necessity measures.•The required risk reduction is obtained by resolving a possibilistic decision-making problem An important issue faced by risk analysts is how to deal with uncertainties associated with accident scenarios. In industry, one often uses single values derived from historical data or literature to estimate events probability or their frequency. However, both dynamic environments of systems and the need to consider rare component failures may make unrealistic this kind of data. In this paper, uncertainty encountered in Layers Of Protection Analysis (LOPA) is considered in the framework of possibility theory. Data provided by reliability databases and/or experts judgments are represented by fuzzy quantities (possibilities). The fuzzy outcome frequency is calculated by extended multiplication using α-cuts method. The fuzzy outcome is compared to a scenario risk tolerance criteria and the required reduction is obtained by resolving a possibilistic decision-making problem under necessity constraint. In order to validate the proposed model, a case study concerning the protection layers of an operational heater is carried out