Bowtie Diagrams began circulating as far back as the 1970s, but did not become widely used until the 1990s. Today, they are an invaluable tool across a wide range of industries including Oil and Gas, Petrochemical, Aviation and even Cyber Security. They are a lens through which to visualise risk. Bowtie diagrams focus our attention on the barriers we put in place to prevent incidents from occurring, or limiting the consequences if they do.
The bowtie method is a risk evaluation method that can be used to analyse and demonstrate causal relationships in high-risk scenarios. The method takes its name from the shape of the diagram which looks like a men’s bowtie.
A bowtie diagram takes shape from the Top Event in the middle, or ‘knot’, the point at which control of the hazard is lost. The two sides branch out to form a clear picture of the barriers in place to prevent an incident or mitigate its consequences. Let’s introduce the elements of a Bowtie Diagram and their purpose.
A Hazard is a source of potential harm and is shown top and centre of the bowtie diagram, above the Top Event. It may be an operation, activity or a material. Examples of hazards may be an object stored at height, hydrocarbons under pressure or valuable data being stored on a server.
The Top Event is the point at which control of the hazard is lost, but no consequences have been realised yet. Examples of a top event may be, object is dropped, flammable hydrocarbons release to atmosphere, or data excursion.
Consequences are the undesirable outcomes of the top event, are located on the right-hand side of the Bowtie Diagram. The purpose of a bowtie diagram is to stop these consequences from occurring, should your preventative barriers fail.
Threats are located on the left side of a bowtie diagram, and are factors that could contribute to the top event happening. Threats themselves can be the cause of a failure, an external influence or operational issues. There are many types of threats, including system failures, human error or malicious attacks.
Barriers are the steps taken to prevent an accident scenario from occurring. These may be physical equipment or actions carried out by people. The barriers along the line are the steps we put in place to prevent a threat from leading to the top event, or to prevent or limit the consequences of the top event.
There are two main types of barriers (see escalation barriers below for the third), preventative and mitigative. Preventative barriers are located between the threats and top event, and aim to prevent the top event from occurring. Mitigative barriers are situated between the top event and consequences, and aim to reduce the severity of the consequences once the top event has occurred.
No barrier is 100% reliable. Escalation factors allow us to visualise what can make a barrier’s performance degrade or make it fail. They do not directly cause an incident but may mean that a barrier may not function when called upon. For example, if a Barrier is ‘Emergency Exits’, an escalation factor may be ‘Emergency Exits blocked’.
Escalation barriers prevent an escalation factor from causing a barrier to fail. From the above example, Escalation barriers may be ‘Inspections to ensure that Emergency Exits are not blocked’ or ‘Training for all staff not to leave items near emergency exits’
The major aim of process safety management is to develop plant systems and procedures to prevent unwanted releases, which may ignite and cause toxic impacts, local fires, or explosions in plants affecting people and the environment.
Recognising and understanding the major accident hazards associated with the operation in question is the first step in preventing incidents and mitigating their consequences. Bowtie analysis is a structured method that assists with identifying such hazards, their initiating events and the controls that could be put in place to prevent or mitigate their consequences.
Refer to our article Additional Uses of Bowtie Diagrams for further uses of this visual risk assessment technique.
Bowtie Diagrams can be far easier to understand than a tabular risk assessment and are a fantastic tool in the toolbox for understanding, communicating and managing risks. That being said, there are drawbacks to using bowtie diagrams that are important to bear in mind, refer to Bowtie Diagrams Advantages & Disadvantages – A Trouble Shooting Guide for more information on their limitations.
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