Logic trees described

The overall frequency of the top event is calculated by combining together the constituent probabilities and frequencies of the various events in the fault tree using the appropriate logical relationships described by the AND and OR gates (the detailed calculation is given in CCPS, 1989b). 

Figure 9-1, the two flowcharts describing root cause determinations using Methods A and B, presents general frameworks for root cause determination. Method A focuses on the logic tree method using a simplified fault tree approach. Method B focuses on the predefined tree method. 

At this point, the logic tree structure is examined to ensure the tree is logically consistent and compatible with the known facts. In some instances, there may be inconsistencies and application of the fact/hypoth-esis matrix will be appropriate. This powerful tool is described in more detail later in this chapter. Inconsistencies found at this point require further tree development or rearrangement. 

The initial tasks are similar to those of the logic trees previously described ... 

The following case study describes the investigation work process for a hypothetical occurrence using a logic tree based multiple root-cause systems approach. An example incident investigation report follows the work process description. The example is intended for instructive purposes only descriptions of process equipment and conditions are not intended to reflect actual operating conditions. 

The team developed logic trees to describe the events. To reduce the complexity of the trees, the team chose to treat the operator fatality, the contractor injury and the injuries to the fire brigade as a separate tree. Since explosion at the catalyst storage tanks resulted from the spread of the fire... 

Risk analysis is required to evaluate the accident frequency and consequences. In railway industry. Safety Risk Model (SRM) is used to estimate system risk, SRM consists of Fault Tree Analysis (FTA) and Event Tree Analysis (ETA). Fault tree estimates accident frequency considering system failure logic (Muttram 2002). It calculates top event frequency or probability using minimal cut sets. Basic events in fault tree describe the component failures they can model revealed repairable failure, revealed unrepairable failure and unrevealed repairable failure with periodic inspection (Andrews Moss 2002). The above failure models for basic event are not enough to consider the effects of maintenances on risk as these models cannot describe multi-level repairsor inspections in details. 

The logical structure of a fault tree can be described in terms of boolean algebraic equations. Some specific prerequisites to the application of this methodology are as follows. 

The logic of event trees is usually built into the structure of detailed consequence and risk models (CCPS, 1989a Chemical Manufacturers Association, 1994). Typically the user specifies a list of scenarios where one or more of the release events described by 1 through 4 above are triggered. 

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