Risk assessment process quantitative analysis

The approach used in an FHA is to assume ignition of releases. In reality, not all releases result in afire. The likelihood of ignition can be addressed in the quantitative risk assessment process. However, in an FHA it is important to identify if ignition sources are present for the fire scenarios to occur. In some instances, fire scenarios can be eliminated from analysis because of the lack of a credible ignition source. 

Uncertainties inherent to the risk assessment process can be quantitatively described using, for example, statistical distributions, fuzzy numbers, or intervals. Corresponding methods are available for propagating these kinds of uncertainties through the process of risk estimation, including Monte Carlo simulation, fuzzy arithmetic, and interval analysis. Computationally intensive methods (e.g., the bootstrap) that work directly from the data to characterize and propagate uncertainties can also be applied in ERA. Implementation of these methods for incorporating uncertainty can lead to risk estimates that are consistent with a probabilistic definition of risk. 

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. 

We suggest a semi-quantitative approach where the initial part of the risk assessment process is carried out in a workshop by the use of expert elicitation. The information gathered in the workshop is then refined by the risk analysts. The alternative safety measures are categorised by cost-effectiveness to provide support for decision-making. In the proposed method evaluation of cost-effectiveness is based on calculated expected values as in a traditional cost-effectiveness analysis (e.g. expected cost per ejqiected nmnber of lives saved), as well as uncertainties. 

The four steps of the risk-assessment process are hazard identification, analysis of exposure, analysis of effect, and risk characterization. In the hazard identification step, the risk assessor identifies chemicals of concern, environmental pathways of exposure, and populations and subpopulations at risk. The exposure analysis develops exposure scenarios and estimates the chronic daily intake of each chemical of concern. In the analysis of effect, the risk assessor combines the chronic daily intake calculated in the exposure analysis with toxicity data from animal studies (and/or human epidemiological studies, if available) to estimate the risk of toxic effects in exposed populations, whereby risks to public health are divided into two broad categories noncancer health effects and cancer. The final step of the risk-assessment process, risk characterization, is a narrative that marshals all the evidence of risk to public health, including quantitative risk assessments and qualitative evidence of risk. The risk assessor weighs all the evidence and uses professional judgment to draw conclusions about risks. 

Process Hazards Analysis. Analysis of processes for unrecogni2ed or inadequately controUed ha2ards (see Hazard analysis and risk assessment) is required by OSHA (36). The principal methods of analysis, in an approximate ascending order of intensity, are what-if checklist failure modes and effects ha2ard and operabiHty (HAZOP) and fault-tree analysis. Other complementary methods include human error prediction and cost/benefit analysis. The HAZOP method is the most popular as of 1995 because it can be used to identify ha2ards, pinpoint their causes and consequences, and disclose the need for protective systems. Fault-tree analysis is the method to be used if a quantitative evaluation of operational safety is needed to justify the implementation of process improvements. 

In the course of assessing your company s current PSM status, you and your team have almost certainly gained a clear sense of which facilities pose the greatest risk, whether by virtue of inherent process hazards, human factors, management systems, or a combination. As you set priorities for implementation you should closely review information gleaned from the assessment tasks. In addition, you should try to validate or flesh out your impressions through some more quantitative analysis that can help to identify priority facilities. 

A program of research has been supported for several years by the United Kingdom Health Safety Executive (HSE) to address the effects of sociotechnical factors on risk in the CPI. The initial emphasis of this work was to develop a methodology so that chemical process quantitative risk analysis (CPQRA) would take into accotmt the effects of the quality of the management factors of plant being assessed. This work has been described in a series of publications (e.g., Bellamy et al., 1990 Hurst et al., 1991 Geyer et al., 1990 and Hurst et al., 1992). 

The third category of methods addressed in this chapter are error analysis and reduction methodologies. Error analysis techniques can either be applied in a proactive or retrospective mode. In the proactive mode they are used to predict possible errors when tasks are being analyzed during chemical process quantitative risk assessment and design evaluations. When applied retrospectively, they are used to identify the underlying causes of errors giving rise to accidents. Very often the distinction between task analysis and error analysis is blurred, since the process of error analysis always has to proceed from a comprehensive description of a task, usually derived from a task analysis. 

The other main application area for predictive error analysis is in chemical process quantitative risk assessment (CPQRA) as a means of identifying human errors with significant risk consequences. In most cases, the generation of error modes in CPQRA is a somewhat unsystematic process, since it only considers errors that involve the failure to perform some pre-specified function, usually in an emergency (e.g., responding to an alarm within a time interval). The fact that errors of commission can arise as a result of diagnostic failures, or that poor interface design or procedures can also induce errors is rarely considered as part of CPQRA. However, this may be due to the fact that HEA techniques are not widely known in the chemical industry. The application of error analysis in CPQRA will be discussed further in Chapter 5. 

In addition, the chapter will provide an overview of htunan reliability quantification techniques, and the relationship between these techniques and qualitative modeling. The chapter will also describe how human reliability is integrated into chemical process quantitative risk assessment (CPQRA). Both qualitative and quantitative techniques will be integrated within a framework called SPEAR (System for Predictive Error Analysis and Reduction). 

The Chemical Process Industry (CPI) uses various quantitative and qualitative techniques to assess the reliability and risk of process equipment, process systems, and chemical manufacturing operations. These techniques identify the interactions of equipment, systems, and persons that have potentially undesirable consequences. In the case of reliability analyses, the undesirable consequences (e.g., plant shutdown, excessive downtime, or production of off-specification product) are those incidents which reduce system profitability through loss of production and increased maintenance costs. In the case of risk analyses, the primary concerns are human injuries, environmental impacts, and system damage caused by occurrence of fires, explosions, toxic material releases, and related hazards. Quantification of risk in terms of the severity of the consequences and the likelihood of occurrence provides the manager of the system with an important decisionmaking tool. By using the results of a quantitative risk analysis, we are better able to answer such questions as, Which of several candidate systems poses the least risk Are risk reduction modifications necessary and What modifications would be most effective in reducing risk ... 

This chapter provides general information for performing qualitative or quantitative risk assessments on buildings in process plants. For detailed guidance on risk assessment techniques, the user is referred to other CCPS books on this subject, including Reference 3, Guidelines for Hazard Evaluation Procedures, Second Edition, and Reference 4, Guidelines for Chemical Process Quantitative Risk Analysis. 

William R. Rhyne received a B.S. in nuclear engineering from the University of Tennessee and M.S. and D.Sc. degrees in nuclear engineering from the University of Virginia. Dr. Rhyne is currently an independent consultant and earlier cofounded H R Technical Associates, Inc., where he remains a member of the board of directors. He has extensive experience in risk and safety analyses associated with nuclear and chemical processes and with the transport of hazardous nuclear materials and chemicals. From 1984 to 1987, he was the project manager and principal investigator for a probabilistic accident analysis of transporting obsolete chemical munitions. Dr. Rhyne has authored or coauthored numerous publications and reports in nuclear and chemical safety and risk analysis areas and is author of the book Hazardous Materials Transportation Risk Analysis Quantitative Approaches for Truck and Train. He is a former member of the NRC Transportation Research Board Hazardous Materials Committee, the Society for Risk Assessment, the American Nuclear... 

The methodology outlined in this chapter follows that in Cuidelines for Chemical Process Quantitative Risk Analysis (CCPS, 2000). NFPA 550 Cu/de to the Fire Safety Concept Tree provides another example of fire risk assessment. There are three keys to a successful fire risk assessment ... 

Fault tree analysis is based on a graphical, logical description of the failure mechanisms of a system. Before construction of a fault tree can begin, a specific definition of the top event is required for example the release of propylene from a refrigeration system. A detailed understanding of the operation of the system, its component parts, and the role of operators and possible human errors is required. Refer to Guidelines for Hazard Evaluation (CCPS, 1992) and Guidelines for Chemical Process Quantitative Risk Assessment (CCPS, 2000). 

Process Hazard Analysis— An organized effort to identify and evaluate hazards associated with chemical processes and operations to enable their control. This review normally involves the use of qualitative techniques to identify and assess the significance of hazards. Conclusions and appropriate recommendations are developed. Occasionally, quantitative methods are used to help prioritize risk reduction measures. 

HAZAN, on the other hand, is a process to assess the probability of occurrence of such accidents and to evaluate quantitatively the consequences of such happenings, together with value judgments, in order to decide the level of acceptable risk. HAZAN is also sometimes referred to as Probabilistic Risk Assessment (PRA) and its study uses the well-established techniques of Fault Tree Analysis and/or Event Tree Analysis ... 

Burk, A., Principal Safety Consultant. Du Pont, Newark. DE, presentation on Process Hazards Analysis and Quantitative Risk Assessment (July 20, 1989). 

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