Event Tree Analysis

An event tree provides a diagrammatic representation of event sequences tliat begin with a so-called initiating event and terminate in one or more undesirable consequences. In contrast to a fault tree, which works backward from an undesirable consequence to possible causes, an event tree works forward from the initiating event to possible undesirable consequences. The initiating event may be equipment failure, human error, power failure, or some other event that has the potential for adversely affecting an ongoing process. 

Therefore tlie probability of emergency power supply failure in any given year is 2.098 X 10 , tlie sum of tliese two probabilities. 

Additional details on tliese calculations are provided by Theodore et al.  

Refer to figure 20.2.1. If component A, B, C, and D liave for tlieir respective reliabilities 0.80, 0.80, 0.80, and 0.50, calculate tlie reliability of the system. 

Tlie reliability of the parallel subsystem is obtained by applying Eq. (20.2.2), which yields 

Event tree analysis (ETA) is a logical representation of the various events that maybe triggered by an initiating event (e.g., a component failure). It uses branches to show the various possibilities that may arise at each step and it is often used to relate a failure event to various consequence models. It may also be used to quantify system-failure probabilities, where several contributory causes can only arise sequentially in time. 

Like FTA, event tree analysis is also a logic-based methodology for identifying accident scenarios, but unlike FTA it is a forward thinking method. The analysis begins with a given initiating failure 

Event trees are valuable for examining the consequences of failure. However, they are less effective for the analysis of the causes of system failure and the short timescale over which events are considered may mask longer term consequences such as the gradual deterioration of equipment due to faults elsewhere. 

Construction starts with the initiating event and works through each branch in turn. A branch is defined by a question (e.g., Protective device fails ). The answers are usually binary (e.g., yes or no ), but there can also be multiple outcomes (e.g., 100,20, or 0 percent in the operation of a control valve). Each branch is conditional on the appropriate answers to the previous ones in the tree. 

23 leaks in piping located in the fractionator s overhead and the wet gas compression sections of the FCCU. Follow-up with intensive ultrasonic (UT) shear wave inspection located an additional 73 carbonate cracking like indications in the gas recovery section of the FCCU. As with many forms of SCC, radiography was not considered a suitable inspection technique for carbonate cracking [15]. 

In simple words, an event tree analysis begins at the point at which the fault tree ends. Its primary application is the determination of priorities with respect to consequence modelling. 

The requirements and estimations on efforts necessary are comparable to those for fault tree analyses. 

In the case of standby systems and in particular, safety and mission-oriented systems, the Event Tree Analysis (ETA) is used to identify the various possible outcomes of die system following a 

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Methods for performing hazard analysis and risk assessment include safety review, checkhsts, Dow Fire and Explosion Index, what-if analysis, hazard and operabihty analysis (HAZOP), failure modes and effects analysis (FMEA), fault tree analysis, and event tree analysis. Other methods are also available, but those given are used most often. 

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. 

A simplified form of event tree analysis using selected accident scenarios and order-of-magnitude estimates to determine whether additional protection is needed... 

Recognized systematic approaches include hazard operability study (HAZOP) event tree analysis fault tree analysis. 

Chapter 12 discusses the distribution software BETA for preparing event tree analysis from a work processor table. BETA allows the use of binary conditionals so the nodal probabilities in a vertical line are not necessarily equal but depend on preceding events. 

Step I - Select the combinations of systems that enter the analysis. (This is equivalent to finding accident sequences in event tree analysis.)... 

RISKMAN is an integrated Microsoft Windows , personal computer software system for [H. i forming quantitative risk analysis. Used for PSAs for aerospace, nuclear power, and chemical [iroccsses, it has five main modules Data Analysis, Systems Analysis, External Events Analysis, Event Tree Analysis, and Important Sequences. There are also modules for software system maintenance, backup, restoration, software updates, printer font, and page control. PEG has also integrated the fault tree programs CAFTA, SETS, NRCCUT, and IRRAS into RISKMAN. 

Historical Analysis, Fault Tree Analysis, Event Tree Analysis, Decision Chteria... 

Wyss, G. D. et al., 1990, Accident Progression Event Tree Analysis for Postulated Severe Accidents at N-Reactor, SNL, SAND89-2100. 

Event Tree Analysis (ETA) A method for illustrating the intermediate and final outcomes that may arise after the occurrence of a selected initial event. 

Perform an event tree analysis to find tire path(s) an accident may follow... 

Perhaps the key to detcrnuiiiiig die consequences of an accident is die study of accident mininiization/prcvendon. This topic receives extensive treatment in Section 17.2. The estimation (not calculadon) of consequences is treated in Section 17.3, which is followed by evacuation procedures (Section 17.4). The next section e.xaniiiies failure modes, effects and critical analysis (FMECA). The cluipter concludes with vulnerability analysis (Section 17.6) and event tree analysis (Section 17.7). 

Figures 17.7.1 and 17.7.2 present a sample event tree analysis and an example of an event tree for a dmiii rupture.

Describe staffing requirements for an event tree analysis. 

A single analyst can perform an event tree analysis, but nonnally a team of 2 to 4 people is preferred. The team approach promotes "brainstonning" tliat results in a well defined event tree structure. The team should include at least one member witli knowledge of event tree analysis, witli tlie remaining members having e.xperience in tlie operations of tlie systems and knowledge of the chemical processes that are to be of interest in tlie analysis. 

Event Tree An event tree analysis begins widi a specific initiating event and works forward to evaluate potential accident outcomes. 

Event tree analysis is a teclmique for evaluating potential accident outcomes resulting from a specific initiating event. Tlie results of the event tree analysis are clironological sets of failures or errors that may define an accident. 

Failure sequence modeling techniques such as fault tree analysis or event tree analysis are used to estimate tlie likelihood of incidents in facilities where historical data is unai ailable, or is inadequate to accurately estimate tlie likelihood of the liazardous incidents of concern. Otlier modeling tecluiiques may be required to consider tlie impact of external events (eartliquakes, floods, etc.), common cause failures, and human factors and hmnan reliability. 

In a more quantitative sense, cause-consequence analysis may be viewed as a blend of fault tree end event tree analysis (discussed in tlie two preceding cliapters) for evaluating potential accidents. A major strengtli of cause-consequence analysis is its use as a communication tool. For example, a cause-consequence diagram displays the interrelationships between tlie accident outcomes (consequences) and Uieir basic causes. The method can be used to quantify the expected frequency of occurrence of the consequences if the appropriate chita are available. 

Tliis cliapter is concerned willi special probability distributions and tecliniques used in calculations of reliability and risk. Tlieorems and basic concepts of probability presented in Cliapter 19 are applied to llie determination of llie reliability of complex systems in terms of tlie reliabilities of their components. Tlie relationship between reliability and failure rate is explored in detail. Special probability distributions for failure time are discussed. Tlie chapter concludes with a consideration of fault tree analysis and event tree analysis, two special teclmiques lliat figure prominently in hazard analysis and llie evaluation of risk. 

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. 

Logic Diagram Methods (Fault Tree Analysis, Event Tree Analysis, Cause-Consequence Analysis, Human Reliability Analysis, Success and Failure Trees, etc,)... 

Event tree analysis adapted to accidental hydrogen releases. (From Rigas, F. and Sklavounos, S., lnt. ]. Hydrogen Energ., 30,1501,2005. With permission from International Association of Hydrogen Energy.)... 

Event Tree Analysis (ETA) is suggested by the DOE-STD-1027-92, but not included in the PSM Rule. However, the PSM Rule does allow the use of "an appropriate equivalent methodology." Hence, if ETA is to be used as the PrHA, the PrHA report must justify that the ETA method is appropriate and equivalent to the methods listed in the rule. 

The typical steps in an event tree analysis are3... 

This event tree analysis shows that a dangerous runaway reaction will occur on average 0.025 time per year, or once every 40 years. This is considered too high for this installation. A possible solution is the inclusion of a high-temperature reactor shutdown system. This control... 

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