The smart way to visualise hazards
25 September 2016
John Martin explains what needs to be considered when preparing a bow tie analysis – a diagram that visualises risk in a single, simple graphical format.
Bow tie diagrams are already widely used in the oil and gas industry and are becoming increasingly popular within the chemicals sector too, forming a useful part part of the armoury of tools used in the journey to avoid the realisation of hazardous scenarios.
There is much talk about bow tie analysis. However, the real value of bow ties comes from using the diagram as a visual representation of the analysis which should already have been carried out prior to construction of the diagram. The centre of the bow tie diagram is the ‘top level’ event, the significant event that needs to be avoided – the hazard. Examples might include overfilling of a gasoline storage tank or the occurrence of a runaway reaction. On the left hand side of the top level event are the initiating events which begin the journey towards the hazard. These are often referred to as ‘causes’. In between the causes and the hazard are the preventive barriers which, if successful, will interrupt the journey from cause to hazard and if unsuccessful will allow that journey to proceed. In effect this side of the diagram is a fault tree without numerical data. On the right hand side of the diagram are the ‘consequences’ and in between the hazard and the consequences are the mitigation barriers which, if successful, interrupt or mitigate the journey from the hazard to the consequence. This half of the bow tie diagram is effectively an event tree without numerical information.
Bow tie diagrams are flexible and can be used anywhere in an industrial setting that a visual representation of risk and barriers would be beneficial. It allows important information to be quickly conveyed to people from different backgrounds and disciplines. The bow tie can quickly demonstrate when pathways have very few barriers or, for example, when all the barriers in a pathway rely on human intervention. It is also possible to display the effect of certain barriers becoming impaired or failing.
Producing bow tie diagrams is not without challenges and there will be times when a fault or event tree with quantitative data is required to allow decisions to be taken. It is important to consider how much information is displayed in the bow tie diagram – the benefit of visual impact can be lost if the structure becomes too complex or if too much annotation is added so that the diagram becomes unwieldy and cumbersome.
Bow tie success
Bow ties are best constructed once the input data has been generated using a recognised and structured methodology for the identification and assessment of process hazards. In the process industries HAZOP and PHR are widely used for this purpose. Process Flow Diagrams (PFDs) or Piping and Instrumentation Diagrams (P&IDs) are divided into sections and considered by a suitably qualified team using guidewords under the leadership of a competent chairperson to identify hazards, initiating events, ultimate consequences and barriers.
Failure to use a structured methodology can result in ineffective hazard identification and may lead to key data not being present in the bow tie diagram. Once the hazard identification study is complete the bow tie author selects hazardous events to be considered and must concentrate on presenting the data in a clear, understandable and useful diagram. Proprietary software can be useful for the construction of the diagrams as this provides a consistent and professional appearance.
Bow ties should be used to clearly and broadly communicate the key hazards and controls on site. It might be possible to construct too many bow ties so that either quality suffers and/or the majority of diagrams are never used and key information not communicated.
Before investing time and effort in the construction of the diagrams it is important to consider how the diagram will be of assistance on that journey. One approach could be to only develop bow tie diagrams for the most serious hazards and here it would be beneficial to have used a recognised hazard identification methodology as such a methodology can include risk scoring which will assist in identifying those hazards associated with the most severe consequence or the greatest risk.
John Martin works for ABB Consulting.
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