Bow-tie diagrams

Introduction

Bow-tie diagrams are powerful risk management tools that are clear, simple and visually describe the dynamics of a major incident.

This tool can be used in all industries and sectors.

What is a bowtie?

A bow-tie diagram summarises the risks associated with a particular hazard in a pictorial form. It gets its name from the bowtie shape of the diagram, which separates proactive and reactive risk management.

The starting point is to map out the incident using a hazard and the ‘top event’, or point at which the hazard becomes active. An example is shown in the centre of the diagram below.

Diagram courtesy of Fiona Boyle, R4Risk

On the left of the diagram above, the causes of the incident are mapped, and on the right, the outcomes.

The second step is to insert preventative controls on the left of the diagram, to lessen the likelihood of the triggering causes (as shown below). This summarises a proactive approach to preventing the incident.

Diagram courtesy of Fiona Boyle, R4Risk

The final step is to add mitigative controls on the right of the diagram to summarise the reactive response to the incident, as shown in the following diagram.

Diagram courtesy of Fiona Boyle, R4Risk

For the bow-tie to be effective, the validity and completeness of the controls and information included needs to be carefully considered.

Applications

Bow-ties can be used to demonstrate ALARP and SFAIRP (as low as reasonably practicable and so far as is reasonably practicable) assessments, and to facilitate semi-quantitative risk assessments.

They are also a good way of identifying critical controls to prevent and mitigate incidents and communicating hazards to workers. They can also be useful in training and auditing applications.

Benefits and limitations

The benefits of bowties are their simplicity, usefulness as a communication tool and applicability to a wide variety of industries and applications. Software is also available to use bowties in reporting and risk register creation.

They do have some limitations. One of these is the challenge of balancing the level of detail with the ease of reading the diagram, as greater levels of detail can make reading difficult (as shown below).

Diagram courtesy of Fiona Boyle, R4Risk

Bow-ties do not provide a methodology to assess the effectiveness of controls, unless details are added (as in the diagram above).

Other challenges are defining the scope that the bowtie will cover, keeping the diagram to a manageable size, and differentiating between onsite and offsite risks.

In some hazardous applications, the large number of bow-tie diagrams required to detail risks can limit their practicality. For example, a refinery would require hundreds of bowties for the many hazards present.

It is also important to consider whether the controls included add value to the risk management process, whether they are generic or in-depth, and if they are independent.

Application in Safety Cases

Bow-ties have been used in Safety Cases to link hazards, controls and consequences. They have also been generated and used to review and validation of existing major incidents and controls.

Control assessment using bowties

Bow-ties have been used in the assessment of site material risks for non-major hazard facilities (MHF).

This application concentrates on high consequence potential events and demonstration that they have been reduced to ALARP.

They can also help workers gain an understanding of the site and awareness of hazards.

A summary of the process to use bow-ties in control assessments is shown in the diagram below.

Diagram courtesy of Fiona Boyle, R4Risk

Sources:

The information on this page was primarily drawn from:

•   Presentation titled “Bowtie diagrams – Looking at the Bigger Picture” by Fiona Boyle, Risk Consultant, R4Risk, Risk Engineering Society Conference 2012