Bow Tie Diagram Calibration

Bow Tie Diagram Calibration

The Bow Tie methodology is used for risk assessment, management and communication. The Bow Tie Methodology facilitates the process of understanding the risk and mitigating measures required to be in place to alleviate them. The resulting bow tie can then be used as a source of information for risk management and aid in the demonstration of ALARP. The bow tie is a pictorial representation of how a hazard can hypothetically be realised, the interaction between risk generating factors, and in what manner they give rise to a number of consequences. The advantage of adopting the Bow Tie Methodology is that it is an extremely powerful representation of a hazard analysis and can be easily created and understood by all levels of people.

This diagram illustrates all plausible accident scenarios (i.e. threats and consequences) that could exist for a certain hazard (i.e. top event) with possible preventative control measures (i.e. barriers), in addition to identification of relevant mitigating functions. The centre of the diagram is the top event which is the outcome of the risk. The left hand side of the diagram identifies the threats and concentrates on those preventative measures or barriers that prevent the threats. The right hand side of the diagram indicates the controls needed to mitigate or recover once a consequence has been realised. For example the top event can be ‘loss of containment’, left hand side could be ‘excessive pressure’, and right hand side may be ‘blowdown’. Thus pictorially the centre is the top event incident, and on its left hand side are the threats due to failure of preventative measures and on its right hand side are the mitigating measures which could alleviate the risk.

By visualising the interaction between Controls (i.e. preventative and mitigating factors) and their escalations, one can see how the overall system responses to a possible threat and the risk consequence. The diagram can be a rigorous management tool to identify and control the risks, and also can also be used effectively to assure that hazards are managed to an acceptable level (ALARP).

It is evident that bow tie method is entirely qualitative and descriptive, where no risk level is quantified. The purpose of this paper is to propose a calibration methodology for bow tie diagrams so the different control measures and risk outcomes can be quantified. The bow tie is principally used for risk assessment and its management, so it will be a more effective tool if it can evaluate the ‘top event’ risk quantitatively. A quantitative estimation can be used for better understanding of the risk levels and more informed decision making process.

For this purpose the concept of risk matrix is introduced for determination of the risk level associated with top event hazardous incident. In this case the bow tie diagram uses two risk matrices as follows;

1. The Risk Ranking Matrix for quantification of the risk level for ‘threat’ and 'consequence'.
2. The Risk Ranking Matrix for quantification of ‘top event’ risk level based of ‘threat and consequence’ estimated risk levels.

The first risk matrix is used to describe what could be the outcome of the particular hazardous event in terms of severity for safety, environmental and asset loss, and then how likely it is. This matrix is applied to both left hand side of the bow tie diagram, i.e. threats and associated preventative/barrier control measures; and right hand side of the diagram, i.e. the consequences and the mitigating control measures. The outcome of this analysis is quantified risk levels for relevant ‘threats’ and ‘consequences’. There are potentially many threats and consequences which could result into top event incident scenario, so the risk levels for each case are averaged to obtain one generalised index for threats and another for consequences.

In the next step, the second risk matrix axes are calibrated on the bases of threat and consequence risk indices in order to obtain top event risk level. The matrix quantification indices quantify what could be the outcome of the previously obtained risk levels for top event from threats and consequences hazardous levels. In order to evaluate top event risk level, the matrix indices are grouped together in three different distinct zones, as follows;

• Short term temporary risk reduction is required to manage risk, and long term risk reduction plan must be implemented.
• Risk is tolerable.
• Risk is managed and no further action is required.

Thus the risk level index associated with particular threat and consequence level will fall within any of above zones which indicate the top event risk quantification. This proposed methodology can be used in HAZID group workshop or individually in order to calibrate the top event risk level.