Using Bowtie Analysis for Identifying and Controlling Major Accident Hazards
Every high-risk workplace — whether it’s a chemical plant, oil terminal, construction mega-project, or heavy manufacturing line — carries the potential for a single error or failure to escalate into a catastrophic event. Safety professionals call these Major Accident Hazards (MAHs): events with the potential to cause multiple fatalities, widespread damage, or long-term environmental harm.
Traditional risk assessments list hazards and controls, but often leave people struggling to see how everything fits together. Bowtie Analysis changes that. It’s a simple yet powerful visual technique that maps how threats lead to an undesired event and how controls (or “barriers”) prevent or mitigate the outcome.
This article will give you a practical, step-by-step guide to using Bowtie Analysis to identify and control MAHs. You’ll see why it works, how to build one from scratch, and how to embed it into your audits and training programmes.
What Is Bowtie Analysis?
Bowtie Analysis is a hybrid of fault tree analysis (causes) and event tree analysis (consequences). When drawn, it looks like a bowtie:
- Left Side (Threats): All the ways something could go wrong.
- The Knot (Top Event): The point at which control is lost — for example, a gas leak, a scaffold collapse, or a fire starting.
- Right Side (Consequences): What could happen if that event occurs.
- Barriers: The controls you have in place on both sides — preventive on the left, mitigative on the right.
Because the diagram is visual and intuitive, it’s easy for managers, engineers and front-line workers to understand their roles in preventing a major accident.
Why Use Bowtie Analysis for Major Accident Hazards
- Clarity Across Levels: It translates technical risk assessments into something everyone can grasp at a glance.
- Barrier Management: Shows exactly which preventive and mitigative controls exist and where gaps or weaknesses lie.
- Training Aid: Bowties can be used in inductions and toolbox talks to make hazard pathways real for workers.
- Audit Roadmap: Auditors can follow the diagram to check each barrier’s condition and effectiveness.
- Regulatory Alignment: Bowtie Analysis supports requirements under regimes like COMAH, Seveso, OSHA Process Safety Management, and ISO 45001.
Step-by-Step Guide to Building a Bowtie Analysis
1. Define Scope and the Top Event
Pick a process or activity with major accident potential. Define the Top Event clearly. Examples:
- “Uncontrolled release of chlorine gas.”
- “Collapse of tower crane.”
- “Explosion in dust collection system.”
Be specific. The more precise your Top Event, the clearer your barriers will be.
2. Identify Threats (Left Side)
List all plausible causes leading to the Top Event:
- Equipment failure (pump seizure, valve leak)
- Human error (operator mis-step, over-loading)
- External events (earthquake, severe weather)
Use historical data, incident reports, HAZOP findings, and frontline experience to build a complete list.
3. Map Preventive Barriers
For each threat, identify the measures that prevent it from causing the Top Event:
- Design features (pressure relief valves, guards)
- Maintenance programmes (regular inspections, calibration)
- Training and procedures (operator competence, permits)
- Monitoring systems (alarms, sensors)
Evaluate each barrier’s reliability, independence, and test frequency.
4. Identify Consequences (Right Side)
List what could happen if the Top Event occurs:
- Multiple injuries or fatalities
- Environmental contamination
- Asset loss or business interruption
- Community impact or regulatory sanctions
Think in terms of worst credible outcomes.
5. Map Mitigative Barriers
For each consequence, list what reduces severity:
- Emergency shutdown systems
- Fire detection and suppression
- Evacuation plans
- Secondary containment or spill kits
- Medical response and mutual aid agreements
These are your last lines of defence when prevention fails.
6. Consider Escalation Factors
Barriers themselves can be weakened by “degradation factors.” Examples:
- Deferred maintenance
- Poor training
- Out-of-date procedures
- Ageing infrastructure
- Unauthorised modifications
Identifying these gives you an extra layer of insight for audits.
7. Draw the Bowtie Diagram
Use software like BowTieXP, Sphera, or even PowerPoint/Visio to create the diagram. Place threats on the left, consequences on the right, with preventive and mitigative barriers between them and the Top Event. Use clear symbols and colours for readability.
8. Assign Ownership
Every barrier needs an owner — a person or department responsible for inspection, testing, maintenance, and training. This turns a static diagram into a living management tool.
Example: Bowtie for a Gas Storage Facility
- Top Event: Catastrophic LPG Tank Rupture
- Threats: Corrosion, overfilling, valve failure, vehicle impact
- Preventive Barriers: NDT inspections, overfill alarms, safety valves, impact protection
- Consequences: Fireball, explosion, off-site impact
- Mitigative Barriers: Fire water systems, emergency shutdown valves, public warning systems, evacuation plan
- Escalation Factors: Delayed inspections, sensor calibration drift, untrained contractors
When you audit this facility, the bowtie diagram becomes your checklist: check inspection records, test alarms, review training, and confirm emergency drills.
Integrating Bowtie Analysis into Safety Audits
One of the biggest advantages of bowties is how well they integrate with audits:
- Pre-Audit Planning: Use the bowtie to identify “critical barriers” you must inspect.
- On-Site Verification: Follow the diagram to check each barrier’s presence, condition, and documentation.
- Testing and Drills: Simulate alarms or emergency shutdowns to verify functionality.
- Gap Analysis: Identify weak or missing barriers and rank them by risk.
- Follow-Up: Update the bowtie after corrective actions or changes to the process.
This approach turns audits from a generic checklist exercise into a targeted assessment of real risk controls.
Best Practices for Successful Bowtie Analysis
- Involve a multidisciplinary team: operations, maintenance, safety, engineering, and contractors.
- Keep diagrams clear and readable; split complex scenarios into several bowties if needed.
- Link barriers to real-world KPIs (inspection overdue, alarm test frequency, training attendance).
- Use bowties in induction and refresher training to build hazard awareness.
- Treat bowties as living documents, updated after incidents or changes.
Common Pitfalls to Avoid
- Focusing only on preventive controls and neglecting mitigation.
- Listing “paper” barriers that exist only in procedures but are not implemented or maintained.
- Over-complicating diagrams so they become unreadable.
- Failing to assign ownership, so no one maintains the barriers or updates the diagram.
- Not integrating bowtie findings into maintenance plans, emergency drills, or management reviews.
Advanced Tips: Quantifying and Monitoring Barriers
While bowtie diagrams are primarily qualitative, you can enhance them with semi-quantitative data:
- Assign probability of failure or reliability scores to each barrier.
- Track degradation indicators (number of overdue inspections, test failures).
- Combine with Layer of Protection Analysis (LOPA) for high-consequence scenarios.
- Develop dashboards that show barrier health in real time.
This elevates bowtie analysis from a static picture to a dynamic management system.
Training and Communication Uses
Bowties are not just for engineers. They make great training tools:
- Inductions: Show new employees the key hazards and how their tasks fit into barrier management.
- Toolbox Talks: Focus on one threat and its barriers each week.
- Incident Investigations: Use bowties to map where barriers failed and how to prevent recurrence.
Benefits of Bowtie Analysis for Major Accident Hazards
- Improved Hazard Awareness: Everyone sees the same risk picture.
- Stronger Barriers: Weak spots are identified before incidents occur.
- Better Audits: Auditors focus on what matters most.
- Regulatory Confidence: Demonstrates a systematic approach to MAH control.
- Cultural Shift: Moves the organisation from reactive to proactive safety management.
Conclusion
Bowtie Analysis is more than a diagram. It’s a mindset that recognises hazards as pathways, not isolated points. By mapping threats, top events, consequences, and barriers — and then maintaining and auditing those barriers — you can drastically reduce the likelihood and impact of major accidents.
For safety professionals, adopting bowtie analysis is a powerful way to strengthen your risk assessments, focus your audits, and communicate effectively with everyone from operators to senior management.
For more on occupational safety management systems and training, visit the official NEBOSH website which outlines globally recognised qualifications for health and safety professionals.
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