1What is a Bow-Tie?
The Bow-Tie (Bow-Tie) place l'top event at the centre of a bow-tie-shaped diagram: on the left, the causes that may lead to it, each intercepted by preventive barriers ; on the right, the consequences that may result, each mitigated by protective barriers. It is essentially a visual combination of a fault tree on the cause side and an event tree on the consequence side, focused on barriers rather than calculation.
Its strength is communication : a Bow-Tie diagram is much easier to read and present in a meeting than a HAZOP table or raw fault tree, while retaining equivalent analytical rigour.
Two detail levels are available: a basic mode with cause and consequence barriers but no calculation, for quick communication, and an advanced mode with logic gates, probabilistic quantification, and minimal cut sets, for a full fault-tree-equivalent analysis on both sides.
2Method steps
- Define the top event — the scenario's tipping point (e.g. loss of containment).
- List the causes on the left, often taken from an existing PHA, HAZOP, or safety report.
- Position preventive barriers between each cause and the top event.
- List the consequences that may occur on the right.
- Position protective barriers between the top event and each consequence.
- (advanced mode) Quantify — cause probabilities, barrier PFDs, resulting probabilities, and minimal cut sets.
A barrier counts only when it is truly independent and effective for that specific scenario. Copying the same generic barrier list to every top event indicates that the analysis lacks depth.
3Example completed in Polyodos
Section to be completed — step-by-step import of a PHA safety-report scenario and positioning of barriers, with Polyodos screenshots in basic and advanced modes.
4Common mistakes
| Mistake | Why it is a problem |
|---|---|
| Counting the same barrier twice | A barrier shared by several cause or consequence paths must not be counted as if every path had its own independent protection. Double counting overstates actual risk control. |
| Mixing preventive and protective barriers | A preventive barrier acts before the top event, preventing escalation from the cause; a protective barrier acts after it, limiting the consequences. Placing barriers on the wrong side of the diagram distorts its meaning. |
| Confusing basic and advanced modes | Basic mode contains neither probabilities nor logic gates. Expecting a quantified risk calculation from it is a misunderstanding, not a limitation of the tool. |