Fault tree analysis procedure

Most hazard identification procedures have the capabiUty of providing information related to the scenario. This includes the safety review, what-if analysis, hazard and operabiUty studies (HAZOP), failure modes and effects analysis (FMEA), and fault tree analysis. Using these procedures is the best approach to identifying these scenarios. 

Failure Mode and Ejfect Analysis (FMEA) This is a systematic study of the causes of failures and their effects. All causes or modes of failure are considered for each element of a system, and then all possible outcomes or effects are recorded. This method is usually used in combination with fault tree analysis, a quantitative technique. FMEA is a comphcated procedure, usually carried out by experienced risk analysts. 

The FMEA approach is a bottom-up approach, looking at component failures and establishing their effect on the system. An alternative approach is to use a top-down approach such as Fault Tree Analysis to postulate system failure modes and establish which processes, procedures, or activities are likely to cause such failures. 

Three hazard evaluation procedures using logic diagrams are (1) fault-tree analysis (FTA), (2) event-tree analysis (ETA), and (3) cause-consequence analysis (CCA). Appropriate references are [2,3,251,261]. 

Several qualitative approaches can be used to identify hazardous reaction scenarios, including process hazard analysis, checklists, chemical interaction matrices, and an experience-based review. CCPS (1995a p. 176) describes nine hazard evaluation procedures that can be used to identify hazardous reaction scenarios-checklists, Dow fire and explosion indices, preliminary hazard analysis, what-if analysis, failure modes and effects analysis (FMEA), HAZOP study, fault tree analysis, human error analysis, and quantitative risk analysis. 

The PHA procedure can be conducted using various methodologies. For example, the checklist analysis discussed earlier is an effective methodology. In addition, Pareto analysis, relative ranking, pre-removal risk assessment (PRRA), change analysis, failure mode and effects analysis (FMEA), fault tree analysis, event tree analysis, event and CF charting, PrHA, what-if analysis, and HAZOP can be used in conducting the PHA. 

Fault tree analysis, Guidelines for Hazard Evaluation Procedures," Section 2.8 p. 2-18, AlChe, 1985. 

Failure rates for both equipment and peoples responses are assigned and the frequency and severity of a TOP Event can be calculated. Should the risk be found to be unacceptable, additional process safety hardware or additional procedures can be recommended. Then, calculations can be made to determine the benefits of the additional hardware or procedures. The Fault Tree Analysis method of evaluation is very sophisticated and a detailed explanation is beyond the scope of this book. 

Earlier method of identifying hazards involved a procedure consisting of asking questions such as what if This approach consists of questioning the proper function at every stage of the process, along with consequences or the remedial features. A checklist for the simplified process hazard analysis by the what if method is shown in Table 3.3. Although this method is an old method of hazard analysis compared with other methods such as hazop or fault tree analysis it has proven to be quite useful. 

The Process Hazards Analysis team takes a systematic approach to identify potential process hazards and to document them [51]. The Hazardous-Operation Analysis (Haz-Op) is a method by which the process procedures, process and instrument diagrams, and process flow diagrams are evaluated for operability and safety. Fault-Tree Analysis (FTA) is also a method, which investigates the assessment of what-if scenarios and failure conditions. The outcomes of this analysis are recommendations for the col-... 

Fault tree analysis is a complete procedure. If consistently applied it generates all event combinations leading to failure. Limitations do not derive from the procedure but from the knowledge and scrupulousness of the analyst. It goes without saying that phenomena not known at the time of analysis cannot be identified. 

A methodical examination of a process, plant and procedure which identifies hazards, assesses risks and proposes measures which will reduce risks to an acceptable level. (May use inter alia Hazops. Fault Tree Analysis, Check-lists, Event Tree Analysis. FMECA, etc). 

In the context of interest here, namely that of fault tree analysis, Lindley Singpurwalla (1986) present a formal probabilistic (Bayesian) procedure for the use of expert opinions, assuming expert input in the form of mean and standard deviation of lognormally distributed failure rates. In Tanaka et al. (1983), Liang Wang (1991) and Huang et al. (2001), basic event probabilities (chances) are treated as trapezoidal fuzzy numbers and the extension principle is applied to compute the probability (chance) of occurrence of the top event. In order to deal with repeated basic events in fault tree analysis. Soman Misra (1993) provide a simple method for fuzzy fault tree analysis based on the a-cut method, also known as resolution identity. Another approach to fuzzy fault tree analysis based on the treatment of the system state as a fuzzy variable has been proposed by Huang et al. (2004). 

In this paper, we have investigated the use of different frameworks for uncertainty representation and propagation in fault tree analysis. The frameworks considered are the probabihstic (Bayesian) and possi-bilistic frameworks, as well as an integratedprobabihs-tic/possibilistic computational framework, referred to as a hybrid approach. The tailoring of the integrated computational framework to the fault tree setting is the main contribution of the paper. Interpretations for the results obtained within the different approaches are provided, as well as a discussion of the approaches in relation to a specific case. However, a direct comparison of the actual results obtained for the different approaches has not been made, as no efforts have been made to make the probabihty and possibility distributions used as input coherent. In future work, we intend to make this comparison based on coherent probability-possibility transforms presented in the literature and to extend the computational procedures in the hybrid approach to produce uncertainty statements about the top event of the fault tree. 

The procedure relies on building a tree structure as shown in Figure 36-3. At the top is the principal or top undesired event. The tree continues by breaking the event into contributing factors and further subdividing them into event causes. Fault tree analysis is a deductive process that moves from the general to the specific. A tree chart considers the causal chain of factors leading to the top event. Interactions between events and elements of the system are a vital part of this method. 

NFPA has developed a Fire Safety Concepts Tree At the top of the tree are fire safety objectives, followed by actions to achieve the objectives. Elements of the tree connect using AND and OR gates, similar to fault tree analysis (Figure 36-10). A Fire Safety Concepts Tree can help analyze buildings and designs using qualitative and quantitative procedures. 

Techniques used in systems safety frequently have specific goals and areas that they can address. For example, some techniques are used to analyze the hardware and equipment aspects of the system while other techniques are used to assess the human aspect. From a safety metrics standpoint, systems safety techniques can be used to identify areas for improvement in the organization. While there are hundreds of system safety techniques available, some of the more commonly used are Fault Tree Analysis (FTA), Procedure Analysis, Failure Modes and Effects Analysis, and Root Cause Analysis. 

Fault Tree Analysis was one of the earliest systems safety techniques developed for examining equipment failures. Fault Tree Analysis is a top down procedure that identifies undesirable events and their contributing factors. Once a tree has been developed, probabilities of failures can be determined for individual components in the tree. With the individual probabilities, overall probabilities of failures and can be calculated for event paths using Boolean algebra. 

Describe fault tree analysis and how the procedure can be used to assess safety performance. 

Fault Tree Analysis - working backwards from a failure or potential failure, this logical procedure identifies all the possible causes, and hence, the origins of that failure. The fault tree consists of branches connected by AND and OR nodes - all the branches below an AND node need to coincide for the event above the node to occur, but only one of the branches below an OR node is required to condition the same. In these terms, a cause-effect map of failure is constructed. The advantage of such analysis is that it codifies a common understanding of the intrinsic logic of failure possibility. 

The engineering methods and techniques used for demonstrating the satisfaction of equipment safety requirements (e.g Fault Tree Analysis, Event Tree Analysis, Zonal Hazard Analysis etc.) are relatively well understood by the wider safety engineering community compared with those for people and procedures and will therefore not be discussed further here. The remainder of this paper will discuss how the above approach to safety requirements specification and realisation can be developed in the case of human-based subsystems, using Human Factors methods and techniques. 

System behavior analysis and prognosis can be executed by means of various procedures. The procedures generally described in the literature can be traced back to three standard types, namely failure effect analysis [4-9], fault tree analysis [4-10], and incident progression analysis [4-11], The three procedures will be discussed, as well as the concept of the decision table technique, which is also a good tool but has rarely been discussed in the literature in connection with this application. To begin, the customary analysis techniques [4-9], [4-10], [4-11] will be discussed in alphabetical order. This will serve to delineate and distinguish the procedures. 

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