Incident Investigation fault tree analysis

The earliest logic trees were based on engineering fault tree analysis methods. Today, companies use a number of variations or combinations of logic trees and call them by different names, such as Why Tree,< 9) Causal Tree,(20,21) Cause and Effect Logic Diagram (CELD),<22) and Multiple-Cause, Systems-Oriented Incident Investigation (MCS011).<23,24) tools have more similarities than differences. 

Causal factor identification is relatively easy to learn and apply to simple incidents. For more complex incidents with complicated timelines, one or more causal factors can easily be overlooked, however, which inevitably will result in failure to identify their root causes. There are a number of tools, such as Barrier Analysis, Change Analysis, and Fault Tree Analysis, that can assist with bridging gaps in data and the identification of causal factors. Each of these tools has merits that can assist the investigator in understanding what happened and how it happened. 

This investigation/systems analysis/fault tree analysis of this incident provided an excellent opportunity to demonstrate the power of the investigation method and the ease with which it may be adapted to a systems analysis for facilitating improvements and corrective actions. 

Rohm and Haas uses Multiple-Cause, Systems-Oriented Incident Investigation techniques (MCSOII), or mac-soy. It is a direct adaptation of the Fault Tree Analysis logic and the Deming Principles of Systems and Quality. [10] The method was developed to improve the overall quality of investigations, to increase the uniformity of investigation made by various teams, and improve the usefulness of the proposed corrective actions. The quality of the mac-soy or MCSOII investigation is improved because the method [10]... 

The SSHA evaluates hazardous conditions, on the subsystem level, which may affect the safe operation of the entire system. In the performance of the SSHA, it is prudent to examine previous analyses that may have been performed such as the preliminary hazard analysis (PHA) and the failure mode and effect analysis (FMEA). Ideally, the SSHA is conducted during the design phase and/or the production phase, as shown in Chapter 3, Figure 3.4. However, as discussed in the example above, an SSHA can also be done during the operation phase, as required, to assist in the identification of hazardous conditions and the analysis of specific subsystems and/or components. In the event of an actual accident or incident investigation, the completed SSHA can be used to assist in the development of a fault tree analysis by providing data on possible contributing fault factors located at the subsystem or component level. 

Fault tree analysis serves to ascertain logic linkage of component and subsystem failures leading to an undesired incident. The undesired incident must be a given. Then all causes which could lead to the incident are investigated. 

When undesired incidents which can bring about failures are investigated in this manner, then, by means of fault tree analysis or the decision table technique, system behavior in the case of such incidents can be examined. If the required data are available, weak-point analysis can be continued quantitatively. 

The contract may require a wide range or types of system safety analyses to be performed for a variety of reasons during the life of the contract. For example, any time new equipment or hardware is introduced into the work environment, a series of system safety analyses should be performed. Likewise, when existing equipment is modified to the extent that critical functions of the equipment may be affected, a series of analyses should be conducted prior to the first operational use of the modified equipment. In addition, prudent system safety protocol will dictate that certain analyses be conducted under certain circumstances. For example, an accident investigation may utilize fault tree analysis, or the system safety technique known as MORT (Management Oversight and Risk Tree) to determine the exact cause(s) that lead to an accident/incident/mishap. 

Eliminate other distractions from the room if possible. Do not allow the witness to see any documents, such as causal factor charts, fault trees, showing the incident investigation team analysis of the occurrence. This may he appropriate for later interviews when only specific information is needed or a specific time gap is being filled in. 

As a result of the AIC s efforts, we now have a process for investigating accidents in which we construct an event tree for each incident. The tree is quite similar to a fault tree from the quantitative risk analysis discipline, except that in the investigations we often sacrifice some structural rigor to get the most results in a reasonable time. Basically, the process uses a team to reconstruct the chronology of the incident and to construct the event tree. We try to include those who are most familiar with what actually happened, including the injured person(s) if any. We use the same basic method to investigate process failures, spills, injuries, or any other system failures. Emphasizing the system aspects of the failure removes much of... 

While MORT is based on the fault tree method of system safety analysis, its logic diagram does not require statistical entries and computations for event probabilities. MORT is presented as an incident investigation methodology and as a basis for safety program evaluation. 

WJe now have a process for investigating accidents in which we construct an Event Tree for each incident. The Tree is quite similar to a Fault Tree from the Quantitative Risk Analysis discipline, except that in the investigations we often sacrifice some structural rigor to get the most results in a reasonable time. 

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. 

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