Fault tree analysis initiating event

Under certain circumstances, it may be appropriate to examine the sequence of events that may lead to the initiating event. Techniques such as fault tree analysis or event trees may be used to estimate the frequency of these events. 

There exist different methods like Fault Tree Analysis (FTA), Event Tree Analysis (ETA) and Monte Carlo Simulation (MCS) that can be applied and combined for the purpose of evaluating the frequency and probability of initiating events. However, the MCS can be handled much easier in order to accoimt for bormdary conditions like stochastic dependence, time dependence and physical impact (Hauschild Meyna 2007). The MCS has been apphed successfully for PSA in order to assess the safety of nuclear power plants (Devooght Smidts 1996, Woltereck2001) and especially for taking into account uncertain input data (BfS 2005). 

Layer of protection analysis (LOPA) is a simplified form of event tree analysis. Instead of analyzing all accident scenarios, LOPA selects a few specific scenarios as representative, or boundary, cases. LOPA uses order-of-magnitLide estimates, rather than specific data, for the frequency of initiating events and for the probability the various layers of protection will fail on demand. In many cases, the simplified results of a LOPA provide sufficient input for deciding whether additional protection is necessary to reduce the likelihood of a given accident type. LOPAs typically require only a small fraction of the effort required for detailed event tree or fault tree analysis. 

Bums and Hazzan demonstrated tlie use of event tree and fault tree analysis in tlie study of a potential accident sequence leading to a toxic vapor release at an industrial chemical process plant. The initiator of tlie accident sequence studied is event P, the failure of a plant programmable automatic controller. Tliis event, in conjunction willi the success or failure of a process water system (a glycol cooling system) mid an operator-manual shutdown of tlie distillation system produced minor, moderate, or major release of toxic material as indicated in Fig. 21.4.1. The symbols W, G, O represent tlie events listed ... 

In Section 21.4 tlie effects of the release of toxic vapors were considered in connection witli an accident sequence initiated by the failure of a plant programmable automatic controller. In tliis study, event tree analysis and fault tree analysis led to identification of tlie glycol cooling system circulation pumps as components meriting high priority for inspection. 

Event trees are used to perform postrelease frequency analysis. Event trees are pictorial representations of logic models or truth tables. Their foundation is based on logic theory. The frequency of n outcomes is defined as the product of the initiating event frequency and all succeeding conditional event probabilities leading to that outcome. The process is similar to fault tree analysis, but in reverse. 

Tlie use of event trees is soiiietinies limiting for liazard analysis because it may lack tlie capability of quantifying tlie potential of tlie event occurring. The analysis may also be incomplete if all initial occurrences are not identified. Its use is beneficial in examining, rather than evaluating, tlie possibilities and consequences of a feilure. For this reason, a fault tree analysis (FTA) should supplement tliis, to establish tlie probabilities of tlie event tree branches. Tliis topic was introduced in a subsection of Cliapter 16. 

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. 

Bi represents basic initiating events in the fault tree analysis, leading to an undesired event, and Cy represents different possible end events resulting from the event tree analysis. C-z are the aggregation of the consequences of all end events into a common risk measure. 

A more detailed application of LOPA requires sufficient rather than absolute independence between protection layers or between a protection layer and an initiating event. The principles within BS EN 61511 -1 and 61511 -2 (eg clauses 9.4, 9.5 and 11.2) present the requirements on the BPCS when used as a protection layer. For example a detailed evaluation would need to be performed of the possible failure modes of each element of the protection layer - typically involving techniques such as Failure Modes and Effects /Analysis, Human Reliability /Assessment and Fault Tree /Analysis. Great care needs to be taken in using this approach to ensure that consistent assumptions about the condition of equipment or people are made throughout the analysis. 

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). 

This method is approximately the reverse of fault tree analysis. It begins with an "initiating event" and tracks its consequences. The initiating event may be a human error or a system failure. 

Similar to fault tree analysis, this works from a selected initiating evenF, such as a pressure control failure. It is, basically, a systematic representation of all the possible states of the processing system conditional to the specific initiating event and relevant for a certain type of outcome, e.g. a pollution incident or a major fire. 

Fault-tree analysis (FTA) focuses on the identification of multiple point failures by using a deductive top-down method to analyze effects of initiating faults and events occurring in complex systems. FTA works very well, showing how complex systems can overcome single or... 

In this figure, the seleeted initial event can be selected from the design environment, or ean be extracted by applying automated HAZOP, which highlight list of top events that may eause hazard. In this case, product flow rate (F ) is low has been selected as the initial event. The result of the fault tree analysis as generated by CARA shows total of six cut sets up to order four. The results of the fault tree are fed back to HE Results database as associated with each plant physical object. The relationships among proeess variable, fault, component, and failure are used in the automation of generating the fault tree results. The minimum eut sets represents the different seenarios for the occurrence of the top event (output product flow rate is low). 

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