The bow tie: a risk analysis method

To prevent the occurrence of major accidents, manufacturers carry out risk analyses. Experience shows that major industrial accidents are generally the consequence of a chain of undesirable events combined with safety barrier failures. To analyze such accidents, it is necessary to have sufficiently detailed analysis methods to identify all accident sequences, without ruling out any a priori. The bow-tie enables us to meet this need by providing a detailed tree structure capable of explaining the chronological sequence of an accident.

There are four key steps to a successful bow-tie analysis:

• Define the dreaded event that forms the focal point of the bow tie.

• Construct the fault tree (see Hazard analysis: identify the external interests to be protected [FIC 0535] ) to highlight all the combinations of causes that could lead to the feared event.

• Build the event tree (see Hazard studies: event tree (ADR detailed risk analysis method) [FIC 0536] ) to highlight all possible consequences, depending on whether or not the mitigation barriers are fulfilling their safety functions.

• Quantify the bow-tie, i.e. evaluate the frequency of occurrence of the consequences at the output of the event tree.