Bowtie Diagrams typically have 3 types of barriers possible, namely:
- Prevention Barriers, that sit between the threat and the top event.
- Mitigation Barriers, those that sit between the top event and the consequence.
- Degradation controls, located between the Degradation Factor and the particular barrier.
Aside from their physical location on the bowtie diagram, what are their differences?
Using our experience, and the helpful guidance from the Energy Institute and CCPS we developed some “boiled down” basics of the barrier 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:
- “How can we prevent the threat?”
- “How can we stop the top event from occurring once the threat has taken place?”
It is good practice to write down the barriers in the order that they would act.
A mitigation barrier helps in two ways:
- Prevent the top event from realising the consequences.
- Reduce losses by affecting the severity or substantially decreasing the likelihood of the consequence occurring.
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!
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