PT Notes
Human and Organizational Factors in Bow Tie Analysis
PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.
Bow tie analysis is a risk assessment tool that is widely used in various industries to identify potential hazardous events, their consequences, and control measures. Human and organizational factors play a crucial role as human errors are one of the leading causes of accidents and incidents. Thus, the performance of people in roles that impact process safety is of vital importance. People appear in bow tie diagrams in several ways.
People as threats. Humans are fallible and prone to errors. They may cause hazard scenarios through errors of omission, commission, and execution. Thus, human failures appear as threats in bow tie diagrams, together with causes from equipment failures and external events. Human error is often a symptom of underlying causes, such as inadequate training, poor communication, or ineffective procedures and such root causes must be addressed so that targeted solutions can be developed to address them. Barriers that block human failures from leading to the top event are needed.
People as barriers. Some barriers rely on actions by people. For example, “active operator monitoring of reactor temperature with manual shutdown on deviations outside the safe operating limit” may be considered as a human barrier. Other barriers may involve human actions, e.g. operator activation of a fire suppression system. However, barriers must meet qualification criteria, e.g. for effectiveness, independence and auditability. Barriers that rely on people may not meet such criteria and may need to be supported by engineered barriers.
People as degradation factors. Human errors may result in the degradation of barriers, e.g. failure to perform maintenance properly on a pressure relief system. In such situations, the focus should be on ensuring the robustness of the barriers against human degradation factors, e.g. use of a pressure relief system design that minimizes the potential for maintenance errors, and ensuring that needed degradation controls are in place, e.g. refresher training for maintenance personnel.
People as degradation controls Human degradation controls may protect against degradation factors. For example, a competency assurance program helps to protect against human failures that impact barriers, e.g. the failure to perform maintenance properly on a pressure relief system. Other examples of human controls that may act as degradation controls include training programs, monitoring of fitness for duty, and checks on operational discipline.
Most barriers depend on human performance to ensure they are in place and effective, and to maintain their availability, reliability, and/or survivability, e.g. through periodic inspections and preventive maintenance. Of course, barriers may be degraded or defeated by human actions or inactions, e.g. from neglect of required maintenance or forgetting to restore bypasses. The consideration of human and organizational factors in bow tie analysis provides the opportunity to review human performance capabilities, limitations, and behaviors.
Communication is an essential element in process safety management. Bow tie diagrams provide the means to clearly, effectively and concisely communicate the hazards of processes and their controls to affected personnel. Bow tie analysis encourages organizations to establish effective communication channels between all parties involved in risk management, including employees, management, and external stakeholders.
Bow tie analysis with the consideration of human and organizational factors helps organizations to create a safer working environment and reduce the likelihood of costly accidents and incidents.
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