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Bow-Tie Diagram

What is a bow-tie analysis? 

Bow-tie diagram is a qualitative visual risk analysis tool, that can be used to communicate and analyse risk scenarios. To use the bow-tie, you first start with visually analysing plausible incident scenarios that could exist around a certain hazard. Second the bow-tie represent what an organisation does to control those scenarios by identifying safety barriers. Barriers are then divided into 2 groups; Prevention and Mitigation. Preventive barriers are placed on the left of the top event and Mitigation barriers on the right.  

How do we use a Bow-Tie Diagram? 

In the bow-tie diagram you have a hazard, which creates the top event. On the left side of the diagram, the threats are placed. Threats are those events that ‘’can’’ happen. How do we prevent them? The answer is barriers. Barriers has the function to prevent, control and mitigate treats or consequences. Now we talked about what could happen, and how to prevent it. But what if it happens, and the barriers does not prevent the threat. If you look at the diagram, you will see consequences on the right side of the diagram. Consequences is those events that happen, if the selected barriers are not durable.  

All the different bow-tie specific elements are described in detail below, followed by a diagram as an example: 

Hazard – The ‘hazard’ is an operation, activity or material with the potential to cause harm. Hazards are part of normal business and are often unavoidable. Some may even be necessary to run an operation (e.g., flammable gas). Some examples of hazards are toxic materials (e.g. paints and solvents) high pressure gases (e.g. oxygen, propane, acetylene) radioactive materials (e.g. NORM)  

The “top event” – The top event is the moment when control over the hazard or its containment is lost, releasing its harmful potential. It represents the turning point in the risk analysis, separating prevention from mitigation. This is visually represented in the bow tie diagram by the central ‘knot’ — hence the bow tie analogy. Even though it is undesirable for the top event to occur, there may still be time for barriers to act to stop or limit the consequences. The term top event derives from another type of risk analysis called fault tree analysis, which has similarities with the left side of the bow tie model.

Consequences (bow-tie) – Consequences are unwanted outcomes that could result from the top event and lead to damage or harm. Consequences can be described in terms of safety, environment, asset / property damage, and reputational losses, although the scope of the analysis will determine this. A single top event usually has multiple consequences although typically only the most significant consequences (in terms of quantifiable loss) are included in the analysis.

Threats – Threats are potential reasons for loss of control of the hazard leading to the top event. For each top event there are normally multiple threats placed on the left side of the diagram. Threats have some of the same characteristics as both hazards and barrier failures (fx. having the potential to cause harm) but they are defined separately in the context of bow tie analysis.

Barriers – Barriers are physical or non-physical measures to prevent or mitigate unwanted events. They are the ‘meat on the bones’ of the Bow Tie diagram. Barriers are so-called because each has the capability on its own to interrupt a sequence of events. 
A barrier is placed on the bow tie diagram where it delivers its function or effect; either prevention (threat side) or mitigation (consequence side). Prevention barriers prevent the top event from occurring. Mitigation barriers are employed after the top event has occurred to help prevent or reduce losses and to regain control once it has been lost. 

Degradation factors – The degradation factor is a condition that can reduce the effectiveness of the barrier to which it is attached. A degradation factor does not directly cause a top event or consequence, but since it degrades a barrier on a main pathway, the likelihood of reaching undesired consequences will be higher. A degradation factor can apply to barriers on either side of the bow tie diagram. Degradation factors are sometimes referred to as escalation factors, i.e., Lack of/poor training which makes people unable to activate barriers correctly. 

Degradation controls – Degradation controls do not directly prevent or mitigate the sequence of events, as that is the role of the main pathway barriers. Degradation controls provide greater confidence that the barrier will do its job effectively. Degradation controls are frequently human and organisational factors concerned with the management of risk and barrier assurance (fx. competence, scheduled maintenance). 

Below is an example of a simple bow-tie diagram. In the sources there is a link to the specific software used to make this bow-tie.

An example of a very simple bow-tie diagram.

Sources

  1. CGE Risk, the wikipedia for risk management, has a great article explaining, in great detail, the bow-tie method. Check it out here.
  2. The software can be found here.

About the author

Mikkel K. Nyegaard

mn@rocconsult.eu

Aspiring risk manager studying Disaster & Risk management at University College Copenhagen. Currently at an intern position at RoC Consult ApS.


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