Bowtie diagrams are a tool to help you visualise your risk. One of their main purposes is to understand the barriers you have in place to preventor mitigate the consequences of an incident. Bowtie diagrams typically have 3 types of barriers, namely:
Aside from their physical location on the bowtie diagram, what are their differences?
To prevent something, or to stop something from happening. In Major Accident industries, it is best practice to make sure that prevention barriers (applicable to the specific threat) must be able to fully stop the top event from being realised on its own.
Note: this doesn’t mean that your prevention barrier is 100% effective, or fully reliable all the time.
When formulating such barriers, ask yourself:
It is good practice to write down the barriers in the order that they would act.
A mitigation barrier helps in two ways:
Referring to the Swiss Cheese model – every barrier has “holes” i.e. conditions where the barrier is ineffective from preventing / mitigating a hazard from being realised.
The conditions are summarised through “Degradation factors”. Degradation controls are measures that patch the holes in the barrier and support their original function.
Degradation controls can typically be human actions and organisational measures that support the validity of barriers. These controls can be made stronger if they adhere to the best practice requirements for the other barrier types i.e. to be independent, effective and audit-able, as well as having all elements of decide, detect and act.
Did we miss anything? From your experience, do you have any top tips for formulating barriers? Comment below!