Using the bow tie method to develop critical risk controls.

Critical risks are the risks identified in a business where there is the most threat and likelihood of harm. They are where a business’ efforts should be focused to prevent major hazards causing harm, and minimise harm if the inevitable accident happens. However, it is often difficult to easily document the preventative and rescue controls. A practical example is shown here to illustrate a particularly useful method, the bow tie diagram.

Bow tie diagrams were first used in industry to gain an effective understanding of the risk controls in place on oil rigs, led by the Royal Dutch/ Shell group. After the 1988 Piper Alpha platform oil rig disaster, they required “assurance that appropriate risk controls are consistently in place throughout all worldwide operations” and found that bow tie diagrams were useful as a visual tool to keep overview of risk management practices. The following is an example exercise undertaken by a BVT engineer for his role as a Surf Lifesaver.



I used to work as a lifeguard for Surf Life Saving (SLS) ­ Northern region. After becoming an engineer and formally learning about risk and hazards, I realised that the same methods we use to control risks in a machine shop are just as useful to identify risks and controls at my old workplace, the beach.

There are many risks involved at the beach, but unlike a machine shop, it is difficult to guard against swimming and still have fun. It is not uncommon for people to put themselves in dangerous situations at the beach. Jumping into fast moving currents with little or no experience with the water, operating marine vehicles both powered and unpowered, in large swells and with little regard for safety, are commonplace in New Zealand.

However, there are many actions that can be taken to prevent people from getting into dangerous situations, and then more in the event someone finds themselves in trouble.


Introducing the bow tie diagram

As I’ve progressed my career as an engineer, I’ve learnt more about structured methods to control risks. A particularly useful method I’ve found to keep track of risks is a bow tie diagram, which keeps all of the potential hazards associated with a major risk on the left and then controls which are used to stop that hazard turning into something worse. If that control doesn’t work, it can then turn into a bad situation, which flows through to the right. To stop the risk becoming a bad situation, a reactive control can be put in place.

A basic example is shown below, where the proactive measures taken to control hazards leading to the critical risk are coloured green, whereas the reactive measures are identified in orange on the right.


Looking at the figure below, it can be easily seen that preventative actions are important in ensuring the safety of people at the beach when you look at the amount of green controls on the left, and these are just a few key examples. SLS focus most of their energy in this area, but have a couple of key rescue techniques or orange controls to help prevent a dangerous situation getting worse.