Event tree analysis decisions

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

J. D. Andrews, S.J. Dunnett, Event Tree Analysis Using Binary Decision Diagrams, Lough-... 

Andrews, J.D. Dunnett, S.J. 2000. Event-tree analysis using binary decision diagrams. IEEE Transactions on Reliability 49(2) 230-238. 

A significant development of the study was the use of event trees to link the system fault trees to (lie accident initiators and the core damage states as described in Chapter 3. This was a response to the ditficulties encountered in performing the in-plant analysis by fault trees alone. Nathan Villalva and Winston Little proposed the application of decision trees, which was recognized by Saul Levine a.s providing the structure needed to link accident sequences to equipment failure. 

Inductive methods, such as check lists, Failure Mode and Effect Analysis (FMEA), event trees, decision tables, Analysis of Potential Problems (APP). These methods proceed from an initial cause of the deviation and construct a scenario ending with the final event. They are based on questions of the type What if ... 

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. 

AQS is a procedure that drives the analyst and allows him to identify all the missions that are fimction-ally to be performed in order to mitigate the accident that is described by the PSA sequence. In these missions some are mainly driven by human decisions and actions. We call them Human Factor missions (HF mission). If it is plausible, the failure of one HF mission is an HFE one HFE happens if a HF mission fails. Then headings in PSA events trees are HF missions. Since the HF missions are built from a safety function analysis, they cover all the cases described in the table 1 by providing the analyst with all the required functions (or sub fimctions) to consider. The failure is then define as the deactivation of a safety funtion or sub fimction, that contributes to degradate the situation. 

As the name suggests Fig. 11/3.7.2-1 is like an event tree diagram, which shows various decisions and actions concerned people are expected to do. This operator action tree is mainly used for post accident cases to pinpoint an operator s failure. This is very much used in accident analysis in air crashes. The level of breakdown of the main task is crucial. Too many breakdowns may pose a problem in handling. From visual inspection it is possible to identify the crucial operation that can initiate the event. The figure will give a clearer idea. 

The simulation runs of the first analysis steps were performed without running the EDS code. They were performed just by the combination of Crew Module, Probabilistic Module and the Scheduler of MCDET. That combination ran very fast and provided more than 100 Dynamic Event Trees (DETs) for each of several conditions. Those conditions identified as being decisive to the human actions of the fire fighting process were given by the relevant states of the fire detection and alarm system (none, only one or at least two of the detectors operate as required) as well as by the fire progression (e. g. visibility of smoke in front of the door when shift personnel reaches the fire compartment or production of smoke in the fire compartment). 

Mahboob Straub (2011). This may be a serious limitation for civil engineering applications. In principle event trees can deal with such dependencies however this requires great care during an analysis as observed by Faber Stewart (2003). Moreover both methods suffer from the difficulty in updating based on new information. Petri Nets provide a powerful platform, but the evaluation often takes basis in Monte Carlo simulations requiring considerable computational demands, Nishijima et al. (2009). These drawbacks can be overcome by the use of Bayesian probabilistic networks with discrete nodes supplemented by decision and utility nodes, Nielsen Jensen (2007). 

Cost identification often involves the development of a probability or decision tree of the therapeutic pathway that describes all relevant downstream events related to use of that therapy and its comparator(s). Once the relevant resources are identified and measured (e.g. number of physician visits, treatment of side effects, number and duration of hospital visits, etc.), local costs/prices can be applied to those resources to determine the overall cost of that intervention. The scope of the resources (and costs) included in an analysis is determined by the perspective (or intended audience) of the study. 

Traditional accident models were devised to explain losses caused by failures of physical devices (chain or tree of failure events) in relatively simple systems. They are less useful for explaining accidents in software-intensive systems and for non-technical aspects of safety such as organizational culture and human decision-making. Creation of an infrastructure based on which safety analysis can function efficiently and effectively is needed. A so called safety culture for a development company and processes associated with routine tasks there, in general, is now identified as an area of root cause of accidents and that there is the greatest... 

The top event of the FT directs all of the rest of the analysis. If the top event is incorrectly defined (and this happens a surprising number of times), then the FTA will be incorrect, which can result in wrong decisions being made [NASA Fault Tree Handbook, paragraph 3.4]. 

ABSTRACT In Human Reliability Analysis (HRA) the assessment of dependence between human failure events refers to evaluating the influence of the failure on one task on the performance of the subsequent task. In Probabilistic Safety Assessments (PSAs), human action dependencies are commonly evaluated with the THERP method, often extended with Decision Tree (DT) models, to reduce the expert judgment element. This paper compares different DT models used in the HRA practice. The comparison addresses the factors entering the models and the underlying relationships. The comparison shows that, depending on the features of the task under analysis, the results may vary substantially if different DTs are used. Also, often there is limited guidance for the analyst in the assessment of the DT factors this prejudices the repeatability of the assessments because different analysts may very well decide for different assessments. 

When presenting an overall risk picture (step 4) the analyst s should present a nuanced and balanced risk picture to the decision-makers based on the cause analysis and consequence analysis. The analysis should establish a risk picture covering ah the dimensions (A, C, C, U, P, S, K). The probabhity quantification of for example a top event in a fault tree is presented along with uncertainty assessments, and a sensitiv-... 

Are there documents that provide comprehensive analysis of all potential safety and health hazards of the worksite Are there documents that provide both the analysis of potential safety and health hazards for each new facility, equipment, material, or process and the means for eliminating or controlling snch hazards Does documentation exist outlining the step-by-step analysis of hazards in each part of each job, so that yon can clearly discern the evolution of decisions on safe work procedures If complicated processes exist, with a potential for catastrophic impact from an accident but low probability of such accident (as in nnclear power or chemical production), are there documents analyzing the potential hazards in each part of the process and the means to prevent or control them If there are processes with a potential for catastrophic impact from an accident but low probability of an accident, have analyses such as fault tree or what if been documented to ensure sufficient backup systems for worker protection in the event of multiple control failures ... 

System Structure Analysis. After the identification of subsystems to be examined and the definition of undcsired events within the context of preliminary hazard analysis, events which lead to incidents are investigated. These event sequences can be represented as logic structure in a block diagram, a flow diagram, a fault tree, or a decision table. In the presentation which follows (Table 4.9.). a decision table was used. It contains, column by column, the combinations of system states which lead to the undesired event. The presentation permits qualitative identification of weak points in the system. In general, for example, the probability of a system state will decline with the growing number of failed components. The logic structure presentation could form the basis for further quantitative analyses. 

A notional example of a barrier tree for a loss-of-power Fukushima-type event is given in Fig. 16.2, which has some 10 levels of physical, procedural, operational, and managerial barriers. Data and analysis of major events indicate that such barriers are penetrated and bypassed by human actions, decisions, and behaviors with an overall probability that is greater than Order (10 ), consistent with human learning and decision-making errors (Duffey, 2015). 

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