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PT Notes

Human Factors in the Performance of PHA and LOPA Studies

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Process hazard analysis (PHA) and Layers of Protection Analysis (LOPA) address human failures in operating and maintaining processes and the human factors that influence them, amongst other types of failures. PHA is used to identify hazard scenarios and LOPA is used to evaluate their risk. Often, human failures are causes of, or contributors to, hazard scenarios, and various human factors influence the rates of failure. Process safety regulations require that such human failures and human factors be addressed in PHA, and by implication in LOPA. Other failure types such as equipment failures and external events must also be addressed.

People perform PHA and LOPA studies and, therefore, such studies themselves are subject to various possible human failures influenced by various human factors. Much less attention has been paid to the human factors that influence the performance of PHA and LOPA studies than human factors that influence process risks. Human failures in the performance of PHA and LOPA studies should be of significant concern to practitioners as they can have a significant adverse impact on study results. Human factors that influence these failures are not difficult to understand but, unfortunately, often they are not recognized or they are ignored in the performance of studies. Human factors can adversely affect all stages of performing studies including preparation, conduct, recording, documenting, and follow-up.

Some examples of human factors issues are provided by the psychology of decision making by teams of people. Groups of people may make more risky decisions than they would individually owing to the sharing and diffusion of responsibility for the decisions. Also, they may mistakenly believe a consensus has been reached when it has not because no one wants to acknowledge a difference of opinion and it is buried. Moreover, individuals in a group mistakenly may think that they are the only person with a different opinion and, therefore, not express it. The phenomenon of groupthink causes a group of people to think and make decisions in the same way owing to their similar backgrounds. As a result, they may reach false conclusions based on the common assumptions they hold. These problems may be mitigated by allowing anonymous input to meetings from the team members and including a devil's advocate as a PHA team member.

Another example of a human factors issue is the influence of human perception on estimates of event likelihoods in PHA studies. A person's experience is reflected in their estimates. Usually, people underestimate the probability of an event they have not experienced and overestimate the probability of an event they have experienced. In process safety, the greatest concern is with catastrophic, low-probability, high-consequence events. An attempt can be made to reduce subjectivity in the likelihood estimates for such events by requiring the team to start with the highest (most frequent) likelihood level and justify successive reductions to lower levels. However, if there is any doubt in assigning values, they should be assigned conservatively. The possibility of underestimating scenario likelihoods owing to the role of subjective judgment and human perception is one reason why some companies use LOPA to calculate the frequencies of scenarios that have high consequence severities, regardless of their estimated likelihoods.

Many other examples of human factors issues and guidelines to minimize the extent to which they may impair study quality can be found in the paper:

The Role of People and Human Factors in Performing Process Hazard Analysis and Layers of Protection Analysis, Journal of Loss Prevention in the Process Industries, Volume 26, Issue 3, Pages 1352-1365, November 2013

The paper can be accessed for free on ScienceDirect until 26th March, 2014 by clicking on the link below. No sign up or registration is needed.


You may also contact Primatech for further information by clicking here.

Click on the links below for information on related training courses.

Leadership and Facilitation Skills for Managing Process Hazard Analysis (PHA) Teams

Understanding and Applying Human Factors for Process Safety

Process Hazard Analysis (PHA) for Team Leaders

Layers of Protection Analysis (LOPA) for Process Safety Management (PSM)

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Primatech Certification Services

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