Event sequence

Mitigation Reducing the risk of an accident event sequence by taking protective measures to reduce the likelihood of occurrence of the event, and/or reduce the magnitude of the event and/or minimize the exposure of people or property to the event. 

The expression for the likelihood of occurrence of an event or an event sequence during an interval of time or the likelihood of the success or failure of an event on test or on demand. By definition, probability must be expressed as a number ranging from 0 to 1... 

Identify initiating events and event sequences that might contribute significantly to risk,... 

RISKMAN embodies PLG s scenario-based, engineering-approach displays and calculates event sequence diagrams, fault trees, and event trees. It allows for system-specific data updates and accoums for dependencies between systems. 

Event Sequence Diagrams (ESDs) and Event Trees (ETs) were used to logically di plant re.spon.se to various initiator.s in a two step process ... 

Use methods such as STEP (see Chapter 6) to evaluate the event sequence. 

Error analysis techniques can be used in accident analysis to identify the events and contributory factors that led to an accident, to represent this information in a clear and simple manner and to suggest suitable error reduction strategies. This is achieved in practice by identification of the causal event sequence that led to the accident and the analysis of this sequence to identify the root causes of the system malfunction. A discussion of accident analysis techniques is included in Chapter 6. 

The STEP procedure, described by Hendrick and Benner (1987), was developed from a research program on incident investigation methods. STEP is based on the mulHple events sequence method and is an investigative process which structures data collection, representation, and analysis. 

The method is well-structured and provides clear, standardized procedures on how to conduct an investigation and represent the incident process. Also it is relatively easy to learn and does not require the analyst to have a detailed knowledge of the system under investigation. However, the method alone does not aid the analyst in identifying root causes of the incident, but rather emphasizes the identification of the propagation of event sequences. This is an important aspect of developing a preventive strategy. 

The first case study describes the application of the sequentially timed event plotting (STEP) technique to the incident investigation of a hydrocarbon leak accident. Following the analysis of the event sequence using STEP, the critical event causes are then analyzed using the root cause tree. 

The Limerick analysis accounted for a revised list of incident Initiators based on the Limerick plant design and a more detailed analytical modeling of event sequences following each incident initiator. Plant-design-specific and site-specific data were also included in the analysis of the Limerick Mark II containment and in the meterology and demography imput to the evaluation of incident consequences. 

An event of an accident event sequence diat helps to propagate die accident or helps to prevent die accident or midgate die consequences. 

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. 

Seventeen event sequences resulted in a runaway reaction. These are listed in Table 21.5.2 (using Table 21.5.1 notation) along with tlie mean annual frequency of release produced by each sequence G stands for failure of the EIS on demand. The average aiuiual frequency of failure on demand is estimated as 0.592. 

TABLE 21.5.2 Mean Annual Frequency of Event Sequences Terminating in Runaway Reaction ... 

A given event sequence can proceed to various incident outcomes, depending on the sequence of intermediate events, as shown by Table 2.1. For example, a release of flammable vapor could result in a vapor cloud-explosion, flash fire, jet fire, or harmless dispersion. Other incident outcomes that this book addresses are briefly described below. 

A widely used method to study event sequences that can lead to incidents involves logic diagrams, an example of which is shown in Figure 4.1. Through the method illustrated in the figure, it is determined that a runaway reaction will happen only if both the cooling system fails and the reactor contents... 

Figure 23-1 shows the hazards identification and risk assessment procedure. The procedure begins with a complete description of the process. This includes detailed PFD and P I diagrams, complete specifications on all equipment, maintenance records, operating procedures, and so forth. A hazard identification procedure is then selected (see Haz-ard Analysis subsection) to identify the hazards and their nature. This is followed by identification of all potential event sequences and potential incidents (scenarios) that can result in loss of control of energy or material. Next is an evaluation of both the consequences and the probability. The consequences are estimated by using source models (to describe the... 

Event sequence A specific, unplanned sequence of events composed of initiating events and intermediate events that may lead to an incident. 

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