Human Reliability Assessment HRA

HRA analyses the relationship between human behavioural tendencies and the work context to provide a better understanding in anticipating human errors, violations and severe system outcomes. This analysis requires a fundamental understanding of  

The way humans process information, including their capabilities and limitations at such processing (Wickens (1992)). 

Human factors and ergonomics design consideration (Sanders and McCormick (1987)). 

rule and knowledge based framework, which describes distinct levels of information processing at which workers perform (Rasmussen (1982,1986)). 

Psychosocial considerations that increase the likelihood of performing violations (CCPS (1994)). 

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Since PRA by that time had become established as the industry standard for how to deal with the safety and reliability of technical systems, it was also the natural starting point when the human factor needed to be addressed. Extending PRA to include human factors concerns led to the development of a number of methods for Human Reliability Assessment (HRA). At first, the... 

The idea of contrasfing two approaches to safety was itself inspired by a similar debate that took place within the field of Human Reliability Assessment (HRA). In 1990 the Human Reliability Analysis (HRA) community was seriously shaken by a concise exposure of the lack of substance in the commonly used HRA approaches (Dougherty, E.M. Jr. (1990), Human Reliability Analysis - where shouldst thou turn Reliability Engineering and System Safety, 29, 283-99). The article made it clear that HRA needed a change, and emphasised that by making a distinction between the current approach, called first-generation HRA, and the needed replacement, called second-generation HRA. 

Techniques for the identification and evaluation of human error are typically labeled Human Reliability Assessments (HRA). A complete HRA starts with a definition of the problem and development of a task analysis to support the HRA (Kirwan, 2005). The core of HRA is the Human Error Identification (HEI) and Human Error Quantification (HEQ) stages, and several methods have been developed to specifically focus on these two areas. From these, control measures can be identified to reduce the overall system risk. 

Event trees or networks show how a sequence of events can lead from primary events to one or more outcomes. Human reliability analysis (HRA) event trees are a classic example of this approach (Figure 6). If probabilities are attached to the primary events, it becomes possible to calculate the probability of outcomes, as illustrated in Section 3.2.4. This approach has been used in the field of risk assessment to estimate the reliability of human operators and other elements of complex systems (Gertman and Blackman 1994). Chapter 32 provides additional information on human reliability analysis and other methods of risk assessment. 

ABSTRACT In Human Reliability Analysis (HRA) the assessment of dependence between human failure events refers to evaluating the influence of the failure on one task on the performance of the subsequent task. In Probabilistic Safety Assessments (PSAs), human action dependencies are commonly evaluated with the THERP method, often extended with Decision Tree (DT) models, to reduce the expert judgment element. This paper compares different DT models used in the HRA practice. The comparison addresses the factors entering the models and the underlying relationships. The comparison shows that, depending on the features of the task under analysis, the results may vary substantially if different DTs are used. Also, often there is limited guidance for the analyst in the assessment of the DT factors this prejudices the repeatability of the assessments because different analysts may very well decide for different assessments. 

Human Reliability Analysis (HRA) is the part of Probabilistic Safety Assessment (PSA) that deals with human performance and its impact on risk. The main activities in an HRA are the identification of the human failure events (HFEs) to model, their analysis, and the quantification of their probability. 

Human reliability models (HRA-trees) They depict the human actions considered in the event- and fault-trees and finally provide probabilistic assessments for the failure / success of the actions. 

The increasing use of Probabilistic Safety Assessment (PSA) to support regulatory and operational decisions requires that methods be developed to the extent possible on an empirically sound basis. For Human Reliability Analysis (HRA), this means that methods should be based on data from operational experience, studies in simulator environments, as well as theoretical cognitive models. A significant step in this direction has been achieved with the International and US HRA Empirical Studies, which aimed at an empirically-based understanding of the strengths and weaknesses of a number of HRA methods (Forester et al. 2013). 

The fundamental idea behind these approaches to calculating risk is that risk is equal to the frequency (of events) multiplied by the seriousness of their consequences. This fundamental equation still forms the basis for risk assessment methods (System-Risk Assessment or SRA) and by extension for methods used to assess human reliability Human-risk assessment (HRA) [7]. 

Failures of human actions are embedded either as basic events in the fault-tree or as function events in the event-tree models of a PSA. Corresponding probabilistic assessments are obtained from the application of HRA (Human Reliability Analysis)-trees. 

Similar to the event-tree, the HRA-tree model describes cause-effect sequences rather than a time-dependent process of human actions. The model can neither account for human performance as a dynamic process which evolves in interaction with the system and process dynamics, nor can it account for the mutual dependences between human actions and stochastic influences in the course of time. Therefore, the question arises whether the static HRA-model is able to account for all relevant situations of the process of human actions which contribute to an adequate probabilistic assessment of human reliability. 

There have been several methods used to assess human reliability. Out of various methods, the technique for human error rate prediction (THERE) is in use since the beginning and still quite popular. Many of the HRA methods have been developed for specifically for various plants, for example, nuclear action reliability assessment (NARA). Short-working methods of important HRA methods, used as general purpose, in majority plants are shown in Fig. V/6.0-1B. 

The primary goals of HRA are to assess the risks attributable to human error and determine the ways of reducing system vulnerability due to human error impact. These goals are achieved by its three principal functions of identifying what errors can occur (human error identification), deciding how likely the errors are to occur (human error quantification), and, if appropriate, enhancing human reliability by reducing this error likelihood (human error reduction). The HRA process can be broken down into several steps as seen below ... 

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