Bow Tie Analysis
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.
The intention of 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 vital tool for communicating how control over major accident hazards is attained.
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 analyzed 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 accomplished by organizing Bow Tie workshop. It is important before the workshop to establish the scope and the context under which the bow ties shall 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. Barrier type identifies the operating characteristic of a barrier. While several classifications might be possible, as an sample it is suggested to use five types as below
|Active Human||The barrier consists of human actions, often interacting with technology.|
|Active Hardware||All elements of the barrier are executed by technology.|
|Active Hardware – Human||It is a combination of Human behavior and technological proceedings.|
|Continuous Hardware||The barrier is always operating.|
|Passive Hardware||It works by virtue of its presence|
Each valid barrier should be auditable, independent, effective, and have the capability to completely abstain the threat from migrating to top event or, if a mitigation barrier, significantly decreasing or eliminating the consequence. Each barrier should be independent of other barrier on the pathway and threat. Barrier can itself fully address the Threat or Consequence only if it complies with all the three following rules
Bow Tie Analysis Barriers categorized into nine types
|Safety Critical Equipment||SCE are any part of the installation, whose failure will either cause or contribute to a major accident, or the purpose of which is to avert the effect of a Major accident.|
|Safety Critical Task||SCTs should also be identified as part of defined activities in the MAH management process or human intervention as a step in a procedure that if not executed as defined would directly top event into consequence.|
|Procedural||Apart from operations SOP all other procedures fall under procedural category|
|Design||Items which considered during initial stages of design|
|Asset Integrity||Barriers which prevents the failure/ breakdown of equipment /system.|
|Training||Barriers which involves training/ competency building.|
|Operations||Items directly under the operations purview will be considered as “Operations” category, like Operations’ SOP, Interlocks, sampling, etc.|
|Maintenance||Items directly under the maintenance purview will be considered as “Maintenance” category|
|Others||Items such as PPE, Heavy lifting equipment, etc.|
The following Sample color coding shall be followed, when identifying the barriers to ensure the uniformity of depicting a barrier to assist users to recognize a particular barrier in Bow Tie. Below Sample figure shows barrier color codes.
Qualitative ALARP demonstration will be done in Bow Tie workshop to valid preventive & mitigation barriers are in place for each MAH. All the existing barriers both preventive & mitigation shall be identified even if criteria for the minimum number of barriers has been met.
The ALARP demonstration shall be done for all recommendations identified in Bow Tie workshop. For recommendation, that does not come under broadly acceptable region of the risk tolerability criteria or ALARP (As low as reasonably practicable) region will follow hierarchical approach for identification of additional risk reduction initiative where it is essential as a role of ALARP demonstration:
- Eliminate, to physically remove the hazard
- Alternatives to replace the hazard with a less hazardous substitute
- Closeting from Hazard by distance or isolation
- Prevention by engineering e.g. process detection, ESD depressurization
- Mitigation by engineering e.g. Fire and Gas Detection, and active/passive fire protection.
- Prevention/mitigation by organization e.g. manning, training, procedures
- Mitigation by intervention e.g. emergency response, maintenance
- Personal protective equipment.
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 Analysis 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 to be considered while developing Bow Tie Analysis:
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 practise 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 elements.
Consequence refers 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 lucidity of visual appeal, often they glide from the left on the prevention side, and from the right on the consequence side, but they are the same in all aspects.
- 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 recurrence of preventive barriers amidst different bow ties and very often absolute duplication of mitigation barriers. Bow Tie development to be carried out using the typical steps as shown below.
It’s not likely that the Bow Tie will be absolutely correct as developed in the workshop. The workshop focuses to use time with experts in the team to capture their actual experience of the barriers in use rather than whether the structure of the diagram is exactly the same – this can be rectified in post workshop tasks. The following approach will be used for execution of development of Bow tie, The flowchart below shows a workshop iteration loop.
Steps for execution of development of Bow Tie Analysis
The following approach will be used for execution of development of Bow tie,
- Identify the hazard and a top event
- Prepare Bow Tie diagrams using appropriate software, as indicated above. Consideration will be given to pre-constructing Bow Tie diagrams to improve efficiency in the workshop.
- Identify threats, consequences and barriers for each top event.
- Identify preventive barrier
- Identify mitigative barrier
- Identify escalation factors and escalation factor controls.
- Identify critical tasks that maintain the performance of the barriers or escalation factor controls
- Determine if additional barriers are required to reduce risks to as low as reasonably practicable (ALARP).
- Identify the barriers and escalation factor controls including identification of MAH critical elements (MCEs) and tasks (MCTs).
- All identified barriers and their escalation factor controls (both preventative and mitigation) required to meet qualitative ALARP criteria of minimum numbers will be qualified as MCEs and/or MCTs related to critical procedures.
- The Bow Tie analysis approach should then be repeated for all identified hazard /top event combinations.
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.
Core Team shall consist of:
- Project/Operations in-charge
- Operations Representative
- Maintenance Representative
- Process Engineer (or equivalent)
- Instrumentation Engineer (or equivalent)
- Technical HSE Engineer (or equivalent)
Non-Core Team shall include the following:
- Specialist Engineering Representatives
- Regional/Operational HSE Representative (or equivalent)
- Corporate HSE&Q Representative
The Report contains Bow-Tie Analysis 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. Criticality ranking of all recommendations from a risk based perspective shall be done. The recommendations will be mapped against the relevant CCPS PSM element(s) as shown below.
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.