Bowties are great for communicating risks by visualizing everything in one simple diagram. Sometimes, however, an operation is so big and complex that, if we were to put the entire operation in one bowtie diagram, we’d lose its strength: the ability to provide a clear overview. In this case, we’d cautiously advise to start chaining bowties. This allows you to break up the entire operation into smaller, more detailed, yet understandable chunks. In this blog post, we’ll give you some theoretical background on chaining bowties and show you how to chain bowties in the software.
The theory behind it
A bowtie diagram analyses a specific section of a longer causal chain. The diagram focuses on the crucial phase that leads to an accident. Although the diagram is usually enough to capture all important factors that need to be considered, sometimes an issue is so complex that we need to look further back. In BowTieXP, we’ve built the ability to chain bowtie diagrams together, so the reach of the bowtie can be extended when the need arises.
NOTE: Chaining bowties is a solution if you need more detail, but should not be the default. Mainly because there’s a trade-off between gaining complexity while giving in on the easiness of communicating the diagram.
Figure 1. An example bowtie from our test file that is linked to two other bowties. On the left side, ‘Driver loss of attention’, and on the right, ‘Impact on person’.
Guidelines for chaining bowties
This article provides some guidance on how best to chain bowties. First, start with building a bowtie. Once you’ve identified threats or consequences that require additional investigation, create a new diagram of which the top event is exactly that threat or consequence.
- Any threat that you want to create a separate bowtie for, should only contain previous threats (essentially creating a left-sided bowtie).
- A bowtie that elaborates on a consequence should only have further consequences (creating a right-sided bowtie).
The above will most likely be sufficient for most purposes.
How to chain bowties in BowTieXP
In the example shown above (Figure 1), our original bowtie is ‘Losing control over the vehicle’. The threat for this top event is: ‘Driver loss of attention’. As you can see, this threat has then been linked to another bowtie group that has ‘Driver loss of attention’ as its top event.
Follow the steps below to do this in the software:
- First, make sure you have created two bowtie groups. One with ‘Losing control over the vehicle’ as its top event, and another with ‘Driver loss of attention’.
- Then, while working in the bowtie group ‘Losing control over the vehicle’, right-click on the threat, ‘Driver loss of attention’ and select Edit threat. The screen as shown in figure 2 should show up.
- Click on the drop-down menu of ‘Equivalent Top Event’, and select the option: ‘Driver loss of attention’.
- Press OK and answer ‘Yes’ to the pop-up question: Show Equivalent Top Event on your diagram?
- You can now hover over the chained bowtie element, and click on the small green arrow button on the top event. This will take you to your other bowtie.
Figure 2: The Edit Threat window.
But what do I do with my barriers?
In the original bowtie (losing control over the vehicle) we might have had two preventive barriers for the threat ‘Driver loss of attention’: Regular driving breaks; Lane departure warning system. The latter only becomes effective after the driver already lost his or her attention.
However, ‘Regular driving breaks’ (as in taking a break, getting a coffee) actually eliminates the threat ‘Driver loss of attention’. So logically, it takes place before the threat. Therefore, when chaining bowties, we remove the eliminating barriers out of the original bowtie and put them in the additional or chained bowtie (figure 3).
Figure 3: Eliminating barriers from the original bowtie ‘losing control over the vehicle’ have been moved to the chained bowtie ‘Driver loss of attention’.
The same is true for recovery barriers on the right side. Some barriers focus on mitigating the consequences, which means they logically take their effect after the consequence has occurred to minimize further consequences. If we’re creating an extension of the bowtie, we can place these mitigation barriers in the next diagram.
How to keep the overview of chained bowties
The software provides a relationship model of your bowties so you can have a better overview. Right-click on a top event, select ‘Draw relationship model’, and then select ‘Bowtie chaining’. This provides you with Figure 4.
In addition, you can hide most threats and consequences by clicking on the ‘Show only connected threats and consequences’ button. Furthermore, we also have a report function of this. Go to reports and look for ‘Hazards > Bowtie chaining’. This will give you an excel sheet that also shows the relationships between the chained bowties.
Figure 4: A relationship model of the chained bowties. In the middle, we have our original bowtie ‘Losing control over the car’.
Should you start chaining bowties?
In summary, chaining bowties can greatly increase the level of detail, but always at a cost of more complexity, so you need to make sure that the situation you’re trying to model is sufficiently complex to warrant this kind of diagram. By making use of the relationship model functionality, you can keep a clear overview of this complexity, hopefully allowing for better insights in the safety of your organization.
Now that you’re aware of this functionality, we’re curious to hear if you would use this functionality. Leave your comments in the section below.