Fault tree analysis risk-assessment technique

Hazard and risk analysis is a vast subject by itself and is extensively covered in the literature [22]. In order to plan to avoid accidental hazards, the hazard potential must be evaluated. Many new methods and techniques have been developed to assess and evaluate potential hazards, employing chemical technology and reliability engineering. These can be deduced from Fault Tree Analysis or Failure Mode Analysis [23], In these techniques, the plant and process hazard potentials are foreseen and rectified as far as possible. Some techniques such as Hazards and operability (HAZOP) studies and Hazard Analysis (HAZAN) have recently been developed to deal with the assessment of hazard potentials [24]. It must be borne in mind that HAZOP and HAZAN studies should be properly viewed not as ends in themselves but as valuable contributors to the overall task of risk management... 

HAZAN, on the other hand, is a process to assess the probability of occurrence of such accidents and to evaluate quantitatively the consequences of such happenings, together with value judgments, in order to decide the level of acceptable risk. HAZAN is also sometimes referred to as Probabilistic Risk Assessment (PRA) and its study uses the well-established techniques of Fault Tree Analysis and/or Event Tree Analysis ... 

Allowing time in the early stages of design for critical reviews and evaluation of alternatives would involve studies such as an early hazard and operability (HAZOP) study, using flowsheets, before final design begins,4 Fault tree analysis, quantitative risk assessment (QRA), checklists, audits, and other review and checking techniques can also be very helpful. These techniques are extensively discussed in the technical literature and will not be discussed in detail here. 

Toward the end of the Second World War, systems techniques such as fault tree analysis were introduced in order to predict the reliability and performance of military airplanes and missiles. The use of such techniques led to the formalization of the concept of probabilistic risk assessment (PRA). The publication of the Reactor Safety Study (NRC, 1975)—often referred to as the Rasmussen Report after the name of principal author, or by its subtitle WASH 1400—demonstrated the use of such techniques in the fledgling nuclear power business. Although WASH 1400 has since been supplanted by more advanced analysis techniques, the report was groundbreaking in its approach to system safety. 

As a project is developed and more detailed design data are available, a system hazard analysis (SHA) and subsystem hazard analyses (SSHAs) may be conducted to provide more detailed, in-depth risk assessment information. Two of the more widely used techniques for performing SHAs and SSHAs are the failure modes and effects analysis (FMEA) and fault tree analysis (FTA). 

Although intrinsically contained, the complexity could lead to operator error and automation of the valve sequencing could be beneficial. Jefferis and Schlager applied a quantitative risk assessment technique based on a fault tree analysis to a similar bioreactor sampling valve assembly. Their analysis illustrated the benefits of automation. 

Fault tree analysis is an analytical technique that is used to trace the chronological progression of factors (events) contributing to the accident situation, and is useful in accident investigation and as a predictive, quantitative model in risk assessment. Again,the principle of multicausality is utilised in this type of analysis. (A fuller treatment on fault tree analysis is given at section 10.6). 

Literature on the many techniques for making risk assessments is abundant. For example, in ANSI/ASSE Z690.3. Risk Assessment Techniques—reviews are included of 31 techniques. Examples are such as Primary Hazard Analysis, Fault Tree Analysis, Hazard and Operably Studies, Bow Tie Analysis, Markov Analysis, and Bayesian Statistics. Uncomplicated systems that could be introduced to supervisors and front-line employees are not as prevalent. Such a system is contained in an extension of the previously cited European Community bulletin. It follows. 

In ANSI/ASSE Z590.3—2011, the Prevention through Design standard, Addendum G comments on only eight hazard analysis and risk assessment techniques, intentionally. They are Preliminary Hazard Analysis, What-If Analysis, Checklist Analysis, What-If Checklist Analysis, Hazard and Operability Analysis, Failure Mode and Effects Analysis, Fault Tree Analysis, and Management Oversight and Risk Tree (MORT). It was also said in Z590.3 that ... 

Although this guidance focuses on the LOPA technique, other techniques such as fault tree analysis or detailed quantitative risk assessment, used separately, may be a more appropriate alternative under some circumstances. Quantified methods can also be used in support of data used in a LOPA study. It is common practice with many dutyholders to use detailed quantified risk assessment where multiple outcomes need to be evaluated to characterise the risk sufficiently, where there may be serious off-site consequences, where the Societal Risk of the site is to be evaluated, or where high levels of risk reduction are required. 

Having identified a number of initiating events, the demand tree can be used as an input to other analysis techniques to carry out a more detailed risk assessment. This further stage would typically use either a fault-tree analysis or a layer of protection analysis (so long as the LOPA methodology used has sufficient flexibility to treat each cause separately and then combine them when assessing the frequency of the hazardous event). 

Quantitative risk assessment is a complex and hotly debated subject. Practitioners use techniques such as Event Tree Analysis or Fault Tree Analysis to give estimated failure rates to key actions in the sequence of events. An example from everyday life might be the probability of stopping... 

The assembly process (Figure 10-1) brings together all of the assessment tasks to provide the risk, its significance, how it was found, its sensitivity to uncertainties, confidence limits, and how it may be reduced by system improvements. Not all PSAs use fault trees and event trees. This is especially true of chemical PSAs that may rely on HAZOP or FMEA/FMECAs. Nevertheless the objectives are the same accident identification, analysis and evaluation. Figure 10-1 assumes fault tree and event tree techniques which should be replaced by the equivalent methods that are used. 

Based on any unacceptable and unmitigated risk identified during hazard analysis, further risk assessment and risk mitigation techniques need to be applied. LORA and conceptual SIS designs based on Risk Matrix can be employed if a qualitative to semi-quantitative method is preferred. Fault tree and event tree analyses with a robust LOPA can be applied if a quantitative method is essential... 

The objectives of this standardized observational technique are risk assessment as well as effectiveness evaluation of traffic facilities, not estimations regarding the quantity of accidents [35]. Thereby, conflicts have a probability to become accidents, which does not mean that accidents can be predicted with the method [35]. The transition probabilities between conflicts and accidents, as needed, for example, in the above-mentioned fault tree analyses, can be assessed [42]. Compared to accident analysis, investigating conflicts has the following advantages [35] ... 

The SESAR safety assessment approach typically uses static risk modelling techniques safety criteria and objectives are identified based on accident incident models and further safety requirements are derived using Fault Trees, Failures Modes and Effects Analysis or similar techniques. This section presents criteria to identify specific cases where DRM application is required. 

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