Risk assessment qualitative analysis level

When performing human reliability assessment in CPQRA, a qualitative analysis to specify the various ways in which human error can occur in the situation of interest is necessary as the first stage of the procedure. A comprehensive and systematic method is essential for this. If, for example, an error with critical consequences for the system is not identified, then the analysis may produce a spurious impression that the level of risk is acceptably low. Errors with less serious consequences, but with greater likelihood of occurrence, may also not be considered if the modeling approach is inadequate. In the usual approach to human reliability assessment, there is little assistance for the analyst with regard to searching for potential errors. Often, only omissions of actions in proceduralized task steps are considered. 

Once the specific issues and scope of the analysis are defined, a semi-quantitative risk assessment may be conducted using either risk indexing or a risk ranking matrix. The risk indexing and risk matrix techniques should be built on the information from the earlier analyses. Each level of risk analysis should not be considered a separate effoit, but a continued understanding of the transportation issue. Additionally the information gained from these activities can be used to update the qualitative analysis, especially benchmarking comparisons. 

Analytical methodology was developed for accurate quantitative analysis of trichothecenes at low part-per-billion levels in blood. Although this methodology was arduous and lacked the ruggedness normally demanded of an analytical procedure which must nave a low failure rates it proved to be both qualitatively reliable and quantitatively accurate when it was combined with a well planned quality assurance program. An indispensable part of developing the quality assurance plan was a formal risk assessment which specifically took into account the possibility of human error. 

Several methods relating to hazards analysis and risk assessment exist. They are generally divided into qualitative and quantitative (lEC 61508 lEC 61511). The choice of specific method depends on accident scenario being considered and available data. When a risk evaluated for scenario considered is high, it is necessary to reduce it to an acceptable level using protection layers, each of specified reliability, expressed often as the probability of failure on demand (PFD) (LOPA 2001). 

In most cases a three tier approach is adopted, as shown in Fig. II/4.2.3-1. Initially, a qualitative or semiquantitative approach is taken to assess the risk and screen it. When risks are in a high risk zone or there is the possibility of a major accident event, then quantitative risk assessments are carried out to prescribe necessary control measures. It is quite common that in many cases a combined approach is necessary to justify consequence analysis. Mostly, when a quantitative approach is undertaken, prior preliminary analysis is done. From the diagram it is seen that whenever all replies to the queries shown in the diagram after qualitative analysis are NO, then the action stops. If any reply is YES, then the next level of analysis is carried out. A similar approach is applicable for semiquantitative and quantitative analyses (Fig. II/4.2.3-1). 

There are several levels of risk analysis within most methodologies for assessing risk quantitative, semi-quantitative, and qualitative. For PSSR concerns we deal almost exclusively with qualitative assessments, that is, just a determination of high or low risk. Generally any truly quantitative risk analysis (QRA) indicated for a trigger event would be performed to enhance the process hazard analysis. The associated PSSR for such a trigger event would simply follow action item progress related to the quantitative risk assessment s action items. In this case the PSSR helps assure that any action items from a QRA are appropriately followed. 

A quantum of the level of risk (likelihood x severity) will reflect the current controls in place, which in the case of the example above may rate a medium risk if a qualitative analysis is applied (see Chapter 5). The risk assessment record should then identify controls that... 

Fire and explosion risks consider both probability of an undesired event, and severity of its consequences. For risk analysis purposes, the analyst defines the level of detail and system boundary. The common qualitative, quantitative, semi-quan-titative risk assessment approaches are supposed to be known to the reader EC/FDIS 31010 2009). 

Faults are analyzed through a graphical representation of causality known as Fault Tree Analysis (FTA). Faults are used to analyze the effect of failures on the system, subsystem, or operating environment (i.e., to facilities, equipment, or personnel). Failures are associated with a quantitative analysis of the design of the system. Hazards are assessed qualitatively, aud must be analyzed and either eliminated or reduced to an acceptable level of risk through a mitigation process. The relationship between faults, failures, and hazards may best be understood as follows not aU faults are failures and not aU failures present a hazard to the system. 

Many methods have been developed that are suitable for assessing risks associated with the operation of facilities involving chemical reactivity hazards. The more commonly used methods are summarized in Table 4.9. They differ in their applicability, level of effort, and how systematic they are in identifying accident scenarios. All of the methods except layer of protection analysis (LOPA) may be applied qualitatively, and all except checklist reviews may be performed in at least a semiquantitative manner. CCPS (1992a) is a basic source of information on each of these methods. 

The deviation scenarios found in the previous step of the risk analysis must be assessed in terms of risk, which consists of assigning a level of severity and probability of occurrence to each scenario. This assessment is qualitative or semi-quantitative, but rarely quantitative, since a quantitative assessment requires a statistical database on failure frequency, which is difficult to obtain for the fine chemicals industry with such a huge diversity of processes. The severity is clearly linked to the consequences of the scenario or to the extent of possible damage. It may be assessed using different points of view, such as the impact on humans, the environment, property, the business continuity, or the company s reputation. Table 1.4 gives an example of such a set of criteria. In order to allow for a correct assessment, it is essential to describe the scenarios with all their consequences. This is often a demanding task for the team, which must interpret the available data in order to work out the consequences of a scenario, together with its chain of events. 

The outcome of a risk analysis is a combination of qualitative and quantitative results. Risk criteria are applied to support the assessment of the acceptability ofthe risk. High level risk criteria are expressed in form of societal risk, group risk or individual risk goals. In some industries, like the nuclear field, lower level system failure risk criteria are in use. 

The projected frequency of the occurrence of an event, usually based on statistical analysis of past similar events. Used for determining risk levels. May be qualitatively assessed or based on historical data for various types of hazard assessments. May also be sometimes referred to as Likelihood. See also Likelihood Risk. 

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