Introduction to bowtie : A barrier-based approach to risk

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Introduction to Bowtie
A Bowtie is a simple diagram used to capture and illustrate threats, controls, and consequences
of a major accident or loss event.
There can be multiple threats and typically multiple preventive controls (barriers) between an
initiating threat and the loss event and multiple mitigative controls (corrective barriers)
between a loss event and a given consequence.

Barrier based management:
The core ideas behind barrier based management are captured in Reason’s famous ‘Swiss
cheese’ model of accident causation:
1. Organizations aim to avoid serious unwanted events by having a number of layers of
protection (barriers) in place between hazards and undesirable consequences or losses.
2. Layers of protection are recognized as being imperfect: which is why they are visualized as
being analogous to slices of Swiss cheese with holes in them (we will talk about the different
types of barriers later).
3. The holes in each layer represent weaknesses in the protection afforded by that layer (we can
talk later about the types of those weaknesses).
4. The size and position of the holes within any layer can continually change.
5. Accidents happen when the holes in all of the layers are in alignment, allowing the release
of a hazard to the point where undesirable consequences occur.

What can really be considered as a barrier?
A barrier (control) can be:
1- An engineered object.
2- A Human act.
3- A system (combination of human act and object).
that prevents or mitigates the effect of a loss event.
But we must know the difference between elements that support barriers and the barriers
themselves. For example:
1- If a control/barrier is that a worker must recognize an alarm and turn off a piece of
equipment, then training, supervision or a procedure document may play a vital role in
supporting that human act or system. But these can not be considered as a barrier.
2- A management plan may identify hazards and controls, but it does not act as a barrier.

3- A program to stop and do a risk assessment is considered as good practice and may lead to
better application of required barriers, but it can not be a barrier itself.
Barriers vs. Safeguards:
The term ‘barriers’ means controls that are assessed as being sufficiently robust and reliable
that they are relied on as primary control measures against incidents.
The term ‘safeguards’ means controls that support and underpin the availability and
performance of barriers but that can't meet the standards of robustness to be relied on as primary
measure.
For instance, safeguards intended to mitigate the risk of human error can range from local
warnings and signs, the design and implementation of alarms and the human machine-interface
to control systems, through job design, operating procedures and cross-checking practices, to
stop the work if the operators have any concerns over safety. The role of these organizational
safeguards is to ensure that the barriers that are expected to be in place are not escalated by any
other factor.
Safeguards do not need to provide the same level of risk reduction as barriers, but their role
should be recognized because of the fact that ineffective safeguards can lead to barrier
failure.(Swiss cheese holes).

Now, you all have a basic idea about Bowtie and how it looks like. Moreover, you have
connected between the Swiss cheese model, barrier based management, and Bowtie.
But we all need to keep in mind that Bowtie is not only a HAZOP visualizing method but it is
also a tool to perform LOPA studies and SMS audits. Furthermore, the role of a certain type of
barriers varies according to its location in Bowtie (LHS prevention branch or RHS mitigation
branch).
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Introduction to bowtie : A barrier-based approach to risk

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Introduction to bowtie : A barrier-based approach to risk