What to do when nothing has happened?

Abstract

Often, during a layer of protection analysis (LOPA) or a quantitative risk analysis (QRA) study, historical information indicates no occurrences of a particular undesired event in the plant or system. The simple reply that the frequency or rate is zero because the event has never happened is sometimes heard by the analyst during review meetings. The analyst must then decide on an appropriate measure of the frequency of the undesired event. This article presents methods based on statistical tools for developing a measure of the frequency or failure rate of an event when the historical information includes no occurrences of the event. The article compares the various available statistical tools. Guidance is presented on methods to incorporate historical information of no failures into a LOPA or QRA study. Examples on the application of the recommended methods to deal with the problem are presented. © 2011 American Institute of Chemical Engineers Process Saf Prog, 2011

A quantitative risk management framework for dust and hybrid mixture explosions

“…When no realistic data is available or no failure has occurred in the past, mathematical tools, such as those of Freeman (2011), can generate reliable new data. Freeman (2011) developed twelve tools to estimate failure ratios of any event that has no failure history.…”

“…Freeman (2011) developed twelve tools to estimate failure ratios of any event that has no failure history.…”

“…If the best guess answer is intolerable, the confidence limit method is recommended to estimate the failure rate of devices with no failure history. Equations 2.5 and 2.6 are also recommended for explosion safety studies (Freeman, 2011). …”

“…The basic events data of the refinery-developed FTA have been generated from Equation 2.5 (the best guess method) and Equation 2.6 (the confidence limit method), as recommended by Freeman (2011) for explosion initiation test estimates.…”

“…The probability of occurrence of the FTA basic events at Semabla has been generated from Freeman's (2011) equations. Equations 2.5 and 2.7 were recommended by Freeman for such incidents.…”