However, some issues are arising as the methodology is deceptively simple and it is possible to incorrectly construct the barrier diagram – and therefore derive a faulty risk picture. The diagram is founded on fault tree methodology (every barrier is an AND gate combining a demand combined with barrier fails) and just as there are formal fault tree rules, there need to be equivalent barrier diagram rules. Many current bow ties do not conform to fault tree requirements. These include a requirement for independence between barriers. Also many bow ties include controls belonging in the barrier decay mechanism and not on the main pathway – leading to an incorrect overestimate of barrier depth. Finally there are overarching safety culture or management influences that can lead to dependence between barriers that are normally considered independent.
CCPS is considering developing a bow tie barrier diagram guidance document, but this will not be ready for 2 years at the earliest and the objective of this paper is to establish some interim guidance.
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