Aurorisk FR

Bow-tie: method and software

A bow-tie shows on one diagram how a hazard can be released, and which barriers prevent it or limit its consequences.

Schematic

The method

At the centre of the bow-tie is the top event: the moment control over the hazard is lost (for example a loss of containment). Threats that can lead to it are on the left; possible consequences on the right.

Preventive barriers sit between threats and the top event; mitigating barriers between the top event and consequences. Escalation factors that can defeat a barrier, and the controls that address them, complete the diagram.

Qualitative by nature, the bow-tie is above all a communication and barrier management tool: it shows everyone what must work and what must be monitored. ISO/IEC 31010 lists it among risk assessment techniques.

Hazard: flammable, toxic liquid in a tankThreatsConsequencesPreventive barriersMitigating barriersInspectionCoatingCorrosionGas detectionWater curtainToxic cloudControlRelief valveOverpressureExtinguishingSeparationFireProtectionProcedureExternal impactBundEmergency planSoil pollutionTop event(loss ofcontainment)
Example bow-tie: each line links a threat or consequence to the top event, through the barriers that stop it.

When to use it

  • To summarise and communicate major scenarios from a PHA, HAZOP or safety report.
  • To manage barriers over time: owners, testing, indicators, lessons learned.
  • As a training or audit aid for risk controls.

Watch out for

  • A barrier must be able to stop the scenario on its own: a general procedure or training is often only a supporting factor.
  • An overloaded diagram loses its value: one bow-tie per top event.
  • A bow-tie does not quantify risk; for frequency, complement it with LOPA or a fault tree.

Steps

  1. Choose the hazard and state the top event.
  2. List threats (causes) and consequences.
  3. Place preventive and mitigating barriers, checking they are real and independent.
  4. Add escalation factors and their controls.
  5. Assign each barrier an owner and define how its effectiveness is monitored.

Bow-tie software

List from the Aurorisk catalogue, links checked on 2026-09-26. Confirmed free access is listed first; unknown information is labelled.

ToolSource · licence · accessScopeDescription
Polyodos ↗Commercial publisherFree for teachingInternationalBow-tie alongside the other industrial methods.
BowTieXP (Wolters Kluwer) ↗Commercial publisherInternationalReference software for bow-tie diagrams.

Standards and references

Related methods

Related course : How to run a technical or industrial risk analysis FR

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