There are obvious benefits to using bowtie diagrams which are a robust yet simple tool for risk assessments. That being said, there are drawbacks to using bowtie diagrams that are important to bear in mind.
It’s great to be mindful of these, as no risk assessment tool is fool proof, but this should not take away from the fact that bowtie diagrams provide a clear and easily digestible visual when it comes to understanding risk.
It does so by combining the cause and consequence analyses into a single diagram with the Fault Tree plotted sideways on the left and the Event Tree plotted sideways on the right. The major accident (AKA Top Event) is plotted as a large circle in the middle, thus making the diagram look like a Bow Tie.
Bowtie diagrams have several advantages:
- the full range of initiating events (AKA threats) are shown
- the intervening safeguards (AKA barriers) are clearly shown
- the actual way in which these combine and escalate is clearly shown
- the consequences side shows barriers in an equivalent manner
- the many possible consequence outcomes are defined
- the linkage of the barriers to the safety management system can be made explicit
For other uses of bowtie analysis read Additional Uses of Bowtie Diagrams.
Ideally these diagrams should be kept simple, as their main function is to demonstrate mechanisms and to allow staff and managers to understand how major hazard events can occur and what safeguards exist to prevent them.
Bowtie diagram limitations/disadvantages:
- can require in depth knowledge from subject matter experts for consistency on definitions and development methodology
- may take significant time to develop a meaningful bowtie diagram
- barriers identified are not independent of each other leading to a false sense of security
- confusing threats with escalations factors
- limited standards on the development and use of bowtie diagrams
- can become quite large and complicated
- does not provide a quantitative assessment of risk
The table below outlines the above limitations of the bowtie methodology and provides solutions that can be adopted to overcome these limitations.
References: Marine risk assessment by Det Norske Veritas for the Health and Safety Executive.