QUALITATIVE AND SEMI-QUANTITATIVE RISK ANALYSIS

Figure 4.1 illustrates the risk assessment process for qualitative and semi-quantitative techniques that can be apphed to transportation safety scenarios. Typically, these will be a scenario and issues developed and escalated for more detailed analysis during the identification and prioritization process described in Chapter 3. 

For those issues or transportation scenarios requiring more detail and insight than a qualitative approach offers, the next level of analysis is a semi-quantitative evaluation. A semi-quantitative risk analysis includes some degree of quantification of consequence, likelihood, and/or risk level. Once evaluated, through the combination of consequence and likelihood, a risk-based decision can be made from the analysis or additional refinement in consequence or likelihood may be necessary. This refinement of the risk analysis may rely on additional information included as part of a semi-quantitative analysis or, as warranted, on some minimal level of quantitative techitiques described in the next chapter. 

Table 4.4 summaries the results of the qualitative risk analysis. Three of the raw materials operations (chlorine, butadiene, and acrylonitrile) and three of the products/wastes (chlorine cylinders, herbicides, and aqueous HCl) were screened out for further risk analysis. Therefore, four chemical movements were escalated for a more detailed semi-quantitative risk analysis... 

While qualitative techniques represent a very basic analysis, with little modification, they can be expanded and evaluated in a semi-quantitative risk analysis. Additionally, events and scenarios elevated for further analysis can be further evolved by considering the range of potential consequences (small to large) and/or likelihood (low to high probability of occurrence). 

Semi- Quantitative Risk Analysis (Chapter 4) Faciiity Levei Qualitative risk analysis resulted in the escalation of four of the nine chemical/ mode transportation operations from the Asian facility Increased detail resulted in screening out two of the chemical movements Bulk chlorine and pesticide recommendations developed Rerouting Repackaging... 

Consequence assessment for the purposes of establishing design basis differs from consequence assessment in the context of a risk analysis study (see Qualitative and Quantitative Methods, below). A qualitative, or semi-quantitative (order of magnitude) consequence severity estimate may suffice for the latter. 

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. 

At this point in the example, the eompany-wide reviews eompleted in Chapters 2 and 3 are replaeed with a first-level qualitative risk analysis foeused on the transportation aetivities of a single XYZ Cheinieal faeihty. To eontinue the example, a single XYZ Chemical facility is selected. This facility will continue to be evaluated as the level of analysis detail increases from qualitative to semi-quantitative in this chapter to quantitative risk analysis (Chapter 5). As the level of detail increases, the analysis will be directed at specific questions that remain following each level of analysis. This facility will continue to be the focus of Chapter 6 where the security vulnerabihty of the hazardous materials in transit is evaluated. Chapter 7 where risk reduction options are evaluated, and Chapter 8 where the ongoing management of risk continues in the future. 

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. 

Based on any unacceptable and unmitigated risk identified during hazard analysis, further risk assessment and risk mitigation techniques need to be applied. LORA and conceptual SIS designs based on Risk Matrix can be employed if a qualitative to semi-quantitative method is preferred. Fault tree and event tree analyses with a robust LOPA can be applied if a quantitative method is essential... 

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. 

Pure quantitative analysis is generally only needed in a small select group of high risk industries, such as nuclear and offshore. The final and probably most widely used evaluation of risk is a combination of the qualitative and quantitative approaches and is referred to as semi-quantitative risk. 

The results of a risk analysis are fed into the risk reduction activities (Chapter 7) where different options for managing risk are evaluated and compared. For all issues that go through qualitative, semi-quantitative, or quantitative analysis, periodic re-evaluation of the level of risk should be conducted as part of the sustainability process described in Chapter 8, with changes implemented as required. 

These types of benchmarking issues can be used to identify other issues that may require further analysis due to the potential for severe consequences or higher perceived likelihood for an accident. The issues identified through a qualitative analysis can be directly used as inputs in a semi-quantitative analysis with a focus on developing a better understanding of these issues and their influence on the level of risk, and need for additional risk mitigation measures. 

A qualitative analysis may result in an escalation of specific issues that require a more detailed analysis before appropriate decisions can be made regarding the level of risk and suitability of potential risk mitigation options. The next level of risk analysis is semi-quantitative. Semi-quantitative analyses differ from qualitative analyses in that some degree of quantification is conducted to refine the estimates of consequence, likelihood, or level of risk. 

Data quality and uncertainty are present at all levels of risk analysis (qualitative, semi-quantitative, and full quantitative). Increasing the level of analysis detail may not result in a more accurate estimate of risk and, in fact, going to a deeper level of analysis may actually increase the uncertainty. When conducting or participating in a QRA, the following questions should be asked about the data ... 

When the problem or risk issue is well understood an appropriate tool for risk analysis has to be chosen. For the hazards identified during the brainstorm the associated risks has to be estimated. This may be a qualitative description eventually using semi-quantitative descriptors, as high , medium or low or it may be a quantitative analysis with scores according to a pre-defined scale. The estimation of risks in Sect. 21.3 occurred following a qualitative method. It enabled the prioritisation of two risks within a rather simple process and no comparison to a fixed value was felt to be necessary. 

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). 

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