Bow Tie analysis is to provide holistic overview of how each Major Accident Hazard (MAH) scenario is managed and also to propose recommendations for identified inadequacies and demonstrate ALARP.
Objective Bow-Tie Analysis
The intention of the Bow Tie exercise is to develop detailed / comprehensive Bow Tie diagrams which are to be dynamic in nature and can be regularly maintained by asset to reflect live status of each barrier condition. A Bow Tie diagram is a powerful tool for communicating how the control of major accident hazards is achieved.
Bow-Tie Analysis Methodology
The Bow Tie analysis will include Bow Tie diagram, for “each MAH scenario” presenting its causes (threats), consequences and potential escalation scenarios, along with the barriers that prevent, control or mitigate the scenarios. The barriers shall be further analysed for their defeat mechanism (called escalation factors) and corresponding measures provided to overcome the barrier defeat mechanism (called escalation factor controls). The MAH scenario shall encompass both process MAHs and other MAHs applicable to the facility. The process of creating a Bow Tie is most effectively accomplished using a Bow Tie workshop. It is important before the workshop to establish the scope and the context under which the bow ties will be developed.
MAH identification will be done by referring HAZID, HAZOP, LOPA / SIL, QRA and SIMOP, where MAH identification was already carried out and include those identified MAHs into the facility MAH risk register.
Overview of Bow-Tie
The general focus of bow tie is managing Major Accident Hazards, as working personnel need to understand how these may occur and the barriers and escalation factor controls deployed to prevent them.
Bow Tie diagrams shall be unit level / equipment specific MAH scenarios. A Bow Tie diagram shall be prepared for each MAH scenarios, presenting its causes, consequences and potential escalation scenarios, along with the barriers that prevent, detect, control or mitigate the scenarios (either preventative barrier or mitigating barrier). The barriers will be further analysed for their defeat mechanism (escalation factors) and measures provided to overcome the barrier defeat mechanism (escalation factor controls).
Key elements consider while developing the bow tie:
A hazard refers to an operation, activity, or material that possesses the potential to cause harm. The hazard plays a crucial role in delineating the extent of the entire bow tie. Hazards that are too general can result in generic bow ties, which is why it is important to identify and address specific hazards. This practice is known to enhance the value as it elevates the level of intricacy in the remaining part of the bow tie.
- Top event
The top event occurs when there is a loss of control over the hazard or its containment, resulting in the release of its harmful potential.
Consequences refer to adverse outcomes that may arise from a given event, potentially resulting in harm or damage.
Threats have the potential to cause a loss of control over a hazard, which can ultimately result in the occurrence of the top event.
- Barrier (Prevention and mitigation)
Barriers are present on the primary routes (posing a risk to the top event or from the top event to the consequence). Barriers must have the capability on their own to prevent or mitigate a Bow Tie sequence and meet all the rule sets/validity requirements for a barrier to be effective, independent, and auditable.
- Escalation Factor
An escalation factor can apply to barriers on either side of the Bow Tie diagram. For clarity of visual appearance, often they flow from the left on the prevention side, and from the right on the consequence side, but they are the same in all other respects.
- Escalation Factor Control
Controls along the escalation factor pathway are called escalation factor controls. The escalation factor is a condition that can reduce the effectiveness of the barrier to which it is attached. An escalation factor does not directly cause a top event or consequence, but since it degrades the main pathway barrier, the likelihood of reaching undesired consequences will be higher.
Bow Tie structure shall be developed before Bow Tie workshop and also assimilate all the relevant details of technical site survey carried out along with the existing barrier condition & barrier effectiveness. Typically, there is repetition of prevention barriers between different bow ties and often almost complete duplication of mitigation barriers.
Bow Tie analysis is conducted within a workshop involving all key stakeholders including appropriate discipline engineers and facilitated by an independent facilitator, along with a scribe. Facilitator & scribe shall be from consultant side.
Steps for execution of development of Bow-Tie
The following approach will be used for execution of development of Bow tie,
- Identify the hazard and a top event
- Identify threats, consequences and barriers for each top event.
- Identify preventive barrier
- Identify mitigative barrier
- Identify escalation factors and escalation factor controls
- Review Bow-Tie for completeness and quality
The report contains Bow-Tie diagram & all the findings of Bow-Tie workshop, including recommendations / safeguards, action tracking including target for completion date etc.
The report will be prepared for each production facilities / stations / operating units/ equipment specific MAH scenarios so that the identified and listed safeguards are not generic but very specific to the MAH scenario under consideration. The identified inadequacies and recommendations must be captured in the report.
For traceability, the outcomes from meetings, workshop sessions, group sessions, and agreements must be recorded in the MOM Which shall be finalized and signed by all parties participating in the Bow-Tie workshop. Prepare excel based live action tracking workbook for all the generated actions that can be periodically updated. And also log any discrepancy noted in the operations, drawings / technical documents.
The Bow Tie Analysis is a graphical illustration of the causes of an event or risk to its potential consequences, presented in a concise and qualitative cause-consequence diagram. It is an essential component of risk treatment planning, particularly in situations where there is a significant level of risk or when control effectiveness is evaluated as inadequate.