Consequence analysis event tree

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

HAZOP and What-If reviews are two of the most common petrochemical industry qualitative methods used to conduct process hazard analyses. Up to 80% of a company s process hazard analyses may consist of HAZOP and What-If reviews with the remainder 20% from Checklist, Fault Tree Analysis, Event Tree, Failure Mode and Effects Analysis, etc. An experienced review team can use the analysis to generate possible deviations from design, construction, modification, and operating intent that define potential consequences. These consequences can then be prevented or mitigated by the application of the appropriate safeguards. 

Three hazard analysis techniques are currently used widely Fault Tree Analysis, Event Tree Analysis, and HAZOP. Variants that combine aspects of these three techniques, such as Cause-Consequence Analysis (combining top-down fault trees and forward analysis Event Trees) and Bowtie Analysis (combining forward and backward chaining techniques) are also sometimes used. Safeware and other basic textbooks contain more information about these techniques for those unfamiliar with them. FMEA (Failure Modes and Effects Analysis) is sometimes used as a hazard analysis technique, but it is a bottom-up reliability analysis technique and has very limited applicability for safety analysis. 

Cause-consequence risk evaluation combines event tree and fault tree analysis to relate specific accident consequences to causes. Tlie process of cause-consequence evaluation usually proceeds as follows ... 

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). 

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

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. 

Risk is the product of the probability of a release, thepjpbability of exposure, and the consequences of the exposure. Risk is usually described graphically, as shown in Figure 11-15. All companies decide their levels of acceptable risk and unacceptable risk. The actual risk of a process or plant is usually determined using quantitative risk analysis (QRA) or a layer of protection analysis (LOPA). Other methods are sometimes used however, ORA and LOPA are the methods that are most commonly used. In both methods the frequency of the release is determined using a combination of event trees, fault trees, or an appropriate adaptation. 

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]. 

Fault tree analysis (FTA) and event tree analysis (ETA) are the methods most commonly applied quantitatively. Since they only address the likelihood of undesired events, these methods are often combined with consequence severity calculations in a quantitative risk analysis, as described by CCPS (1999b). Layer of protection analysis (LOPA) uses a semiquantitative, order-of-magnitude approach. It is documented with worked examples in CCPS (2001b). 

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 ... 

The event trees represented in Figures 2.2 and 2.3 are most effectively utilized when they are automated so that the output of one or more models is fed directly into subsequent models. The availability of event trees in consequence analysis tools enables the user to efficiently perform sensitivity and what-if analysis as well as mitigation effectiveness assessment. 

The methodology of PSA is in principle a combination of event tree and fault tree analyses. The target of the analysis decisively determines the selection and the definition of modes in which the technical system is at the end of an event sequence. These modes comprise both the safe ones resulting from successful safety measures and also those unwanted ones characterized by a certain release of hazardous materials. A PSA is extended to a PRA, if the consequences of the different system modes are determined and linked with the event frequencies of those modes. 

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 technique for human error-rate prediction (THERP) [ Swain and Guttmann, 1980] is a widely applied human reliability method (Meister, 1984] used to predict human error rates (i.e., probabilities) and the consequences of human errors. The method relies on conducting a task analysis. Estimates of the likelihood of human errors and the likelihood that errors will be undetected are assigned to tasks from available human performance databases and expert judgments. The consequences of uncorrected errors are estimated from models of the system. An event tree is used to track and assign conditional probabilities of error throughout a sequence of activities. 

Perform event tree analysis for the scenario in Exercise 1. Capture the sequence of events that lead to the consequence by taking into account both the success and failure of every associated safety function considered. 

Making Component Failure Probability Estimates Combinatorial Failure Probability Analysis Using MIL-STD-882 Event Tree Analysis Cause-Consequence Analysis... 

The description of event tree analysis given here is based on the analysis of plant internal sequences. It must be emphasized, however, that event trees are fundamental for describing accident consequences, e.g. events following the release of a flammable substance like fires or explosions. This is dealt with in detail in Chap. 10. 

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