Estimation of Calendar-Time- and Process-Operative-Time-Hazardous-Event Rates for the Assessment of Fatal Risk

Abstract: ISO/IEC Guide 51 recommends to estimate and to evaluate the safety-related risk in order to reduce it to a tolerable level. For the reasonable estimation and evaluation of risk reduction made by a final protection layer (FPL), it is necessary to understand the relationship between failures of FPL, demands and hazardous-event rates (HER). In the present paper, firstly, generic models for risk assessment are classified into three types by use of state-transition diagrams involving a fault state of an FPL, demand state and fatal harm state. And new generic Markov models are introduced and analyzed to estimate the HER in a steady state. Moreover, relationships between HER and the mean calendar-time to a hazardous event, MTH, are examined. And it is explored that HER equals the reciprocal of MTH, and HER can be calculated by use of the reciprocals of MTH even if the overall system has two process-inoperative states.