Fault tree analysis generally

Identification and quantitative estimation of common-cause failures are general problems in fault tree analysis. Boolean approaches are generally better smted to mathematically handle common-cause failures. 

All team members should be familiar with PrHA objectives, the PrHA method to be used, and their roles in performing the PrHA. A 1- or 2-hour overview at the beginning of the first team review session is generally sufficient for this purpose. However, the more demanding PrHA methods, such as fault tree analysis (FTA), require more training and/or a greater depth of experience than less-rigorous methods, such as what-if and checklist analyses. 

Identification and quantitative estimation of common-cause failures are general problems in fault tree analysis. Boolean approaches are generally better suited to mathematically handle common-cause failures. The basic assumption is that failures are completely independent events, but in reality dependencies will exist and these are categorized as common cause failures (CCFs). Both qualitative and quantitative techniques can be applied to identify and assess CCFs. An excellent overview of CCF is available (AIChE-CCPS, 2000). 

A systems hazards analysis (SHA) is a systematic and comprehensive search for and evaluation of all significant failure modes of facility systems components that can be identified by an experienced team. The hazards assessment often includes failure modes and effects analysis, fault tree analysis, event tree analysis, and hazards and operability studies. Generally, the SHA does not include external factors (e.g., natural disasters) or an integrated assessment of systems interactions. However, the tools of SHA are valuable for examining the causes and the effects of chemical events. They provide the basis for the integrated analysis known as quantitative risk assessment. For an example SHA see the TOCDF Functional Analysis Workbook (U.S. Army, 1993-1995). 

The events and gates each have a Description and a Label associated with them. The Description is intended for use by persons reading the results of the analysis. It provides detail as to the purpose and function of that event or gate. The label is a unique identifier used by the fault tree software. Generally, the label will be composed of a letter and a number. The letter G indicates a gate the letter E an event (any type). 

The main tool of a probabilistic analysis is fault tree analysis. It is based on deriving deductively the failure of a system from the failure of its sub-systems and sub-sub-systems and so forth. The failure of the latter is in turn derived from the failure of its components. The result of this analysis is represented by the so-called fault tree, which shows the logical relationships between the failure of a system and that of its components. In general, only two states of the system and its components are admitted functioning and failure. These states occur with a certain probability. The probabUity of failure of the system then results from a... 

The expected frequencies of the initiating events are in general derived from observation. Either estimates are directly obtained from operating experience (e.g. for the occurrence of pipe leaks) or the initiating event is decomposed into such sub-events for which operational experience is available. The frequency of occurrence is then assessed using fault tree analysis (vid. Sect. 9.1.2.7). Additionally, there are cases where one has to have recourse to expert judgment. 

Fault tree analysis (FTA) is a deductive method, which usually serves for quantification. Just like any method of systems analysis it requires in the first place a qualitative investigation of the system under analysis. After system failure or more generally the undesired or unwanted event (e.g. toxic release) has been defined, logic relationships with the so-called primary or basic events are identified and represented by a fault tree (vid. Fig. 9.8). The primary event may represent the failure of a technical component, an operator error or an impact from outside the plant like flooding or the spreading of a fire from neighbouring installations. 

Instrumentation and Control (I C) systems are very often subject of probabilistic examination either within separate structural reliability analysis or Probabilistic Safety Assessment of a whole technological complex (e.g. Nuclear Power Plant). Use of programmable components in the design of these systems represents a challenge and utilizes the methods, which have been developed for components with a different behaviour. The typical method used for above mentioned examination is Fault Tree Analysis (FTA) (Vesely et al., 1981). The way of software faults modelling within Fault Trees vary a lot between particular models and there is no generally accepted modelling technique. 

Based on the results of the PHA, recommendations made by 30% review boards, and guidance provided in the system safety program plan, detailed hazard analyses are made of specified (critical) subsystems. The techniques for these SSHAs are as outlined in the system safety program plan or as selected by the SSWG. Failure modes and effects analysis (FMEA) and/or fault tree analysis (FTA) are generally the techniques of choice. Software hazard analysis, common cause analysis, and/or sneak circuit analysis may also be appropriate. 

Fault tree analysis is used primarily as a tool for conducting system or subsystem hazard analyses, even though qualitative or top-level (that is, limited number of tiers or detail) analyses may be used in performing preliminary hazard analyses. Generally, FTA is used to analyze failure of critical items (as determined by a failure mode and effects analysis or other hazard analysis) and other undesirable events capable of producing catastrophic (or otherwise unacceptable) losses. 

The symbols used on the MORT chart are basically those used for other analytical trees (Chapter 10) and fault tree analysis (Chapter 15). They include the rectangle as the general event symbol, the circle as the base event symbol, the diamond as an undeveloped terminal event, the and gate, the or gate, and the ellipse as a constraint symbol (Rgs. 18-1 and 18-2). 

The following seven steps are generally used to perform fault tree analysis [1,39] ... 

As conflicts can be top events in fault tree analysis or generally constitute rather high level events, the probabilities and nature of conflicts is regarded an important issue within the literature. One way to assess conflicts in traffic is the so-called traffic conflict technique [35]. A traffic conflict may be characterized by considering approaching object trajectories which, extrapolated in time, would exhibit an increased probability for collision unless one of the participants changes his current state of motion [35]. This definition could be extended on non-observable situations and single vehicle conflicts. 

Chapter 11 of this text discusses the use of fault tree analysis in determining system reliability, failure potential, and even accident cause factors through examination of specific or general fault paths. Additional information on the application and use of probability values in fault tree analysis is also provided in Chapter 11. 

In general, the remainder of this chapter focuses on the explanation of the various MORT event tree symbols and their use and meaning. Since the tree is an analytical model, the information presented in the previous chapter (on fault tree analysis) will be helpful and should be reviewed. 

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. 

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