The bow tie methodology, originally developed by ICI in the late 1970’s, has seen increasing industry adoption over the last 20 years. Bow tie diagrams build on the “Swiss Cheese Model” of hazard escalation and causation popularized by James Reason. They provide a powerful means for visualizing how a company controls major accident hazards in a structure that is readily understandable to non-experts. It illustrates the relationships between the causes of events, the potential consequences, the barriers in place to prevent the event from occurring, and the barriers to mitigate consequences. 

The figure below, taken from the new from the new CCPS book (Bow Ties in Risk Management), illustrates the format and basic elements of a bow tie diagram.

The key elements of the bow tie diagram are as follows:

1. Hazard: the operation, activity, or material with the potential to cause harm
2. Top Event: the loss of control of the hazard which releases its harmful potential
3. Consequences: the unwanted outcomes that could result from the top event and lead to damage or harm
4. Threats: the potential reasons for loss of control of the hazard leading to the top event
5. Prevention Barriers: a barrier that on its own is capable of preventing the top event from occurring (left side of the diagram). It has properties similar to an independent protection layer (specific, auditable, independent, and dependable)
6. Mitigation Barriers: a barrier that acts after the top event has occurred to prevent or reduce losses and which helps to regain control once it is lost (right side of the diagram)
7. Degradation Factors: a condition that can reduce the effectiveness of the barrier to which it is attached
8. Degradation Controls: help defeat the degradation factor and provide greater confidence that the associated barrier will do its job effectively 

The figure below shows what a typical Prevention side of a bowtie diagram might look like. Prevention barriers are employed to prevent the Hazard “Volatile hydrocarbons under pressure in pipeline” from leading to the top event “Loss of Containment”. One of the prevention barriers, “External corrosion protection system”,can be degraded by the factor “Jointing in field disturbs factory coatings”. To alleviate the impact of the degradation factor, two degradation controls are shown; “Install extra cathodic protection anodes” and “Recoating with increased inspection program”.

Summary – Benefits of Bow Ties

• Provides a graphical format for visualizing risk management information (hazards, potential consequences, barriers, degradation factors and controls)
• Demonstrates how hazards are controlled and illustrates the direct link between risk controls and elements of the safety management system
• Supports the identification and prioritization of actions to strengthen and maintain barriers and degradation controls  
• Helps operation and maintenance teams to focus on barriers and the degradation controls which are relied upon to prevent major accident events (MAE)
• Drives accountability by providing a way for barrier owners to be identified, who are then responsible for tracking performance
• In advanced applications can be used for real-time monitoring and display of barrier status
• Are easier for the non-expert to understand than other risk assessment methods such as QRA, LOPA, FTA, and ETA

Reference

“Bow Ties in Risk Management: A Concept Book for Process Safety” CCPS in association with the Energy Institute, 2018.

“Bowtie Analysis for Alarm Management”, D. Hatch, and A. Geddes, IChemE Alarm Systems and Controls Seminar & Tyneside Process Safety Forum 2018.