Quantitative risk assessments

In any discussion of risk management and risk assessment, the question of quantified acceptability parameters must be considered. Richard E. Olson (undated) provides the following discussion pertaining to quantitative risk assessment. 

In the design of many high-risk systems such as nuclear power facilities or weapon systems, there is often a strong tendency to rely solely upon statistical analysis for hazard evaluations. Management finds such an approach somewhat easier to accept since it provides a convenient (if not entirely realistic) medium to express safety in terms to which they can relate. However, the unwary can be easily trapped in their failure to establish reasonable limits on the acceptability of a probability of risk occurrence. 

Occasionally the qualitative and intuitive methods of risk assessment fail, and something better is required. For example, a designer may have taken steps to address some hazard, but be unsure whether these are sufficient. There is also the possibility of failure of equipment, control systems or operating procedures which may reduce margins of safety. In these circumstances, quantitative risk assessment may be considered. This is an attempt to put numbers to the risks so that we can judge them objectively. 

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CCPS G-51. 1998. Understanding Quantitative Risk Assessment. American Institute of Chemical Engineers, Center for Chemical Process Safety, New York. 

H. j. Pasman, Quantitative Risk Assessment in Europe, Process Safety Progress, 14, No. 4 (October), 229-31, 1995. 

TNO Institute of Environmental Sciences, Guidelines for Quantitative Risk Assessment, Amsterdam Netherlands Organization for Applied Scientific Research, 1999. 

Hazard analysis (HAZAN) is a quantitative way of assessing the likelihood of failure. Other names associated with this technique are risk analysis, quantitative risk assessment (QRA), and probability risk assessment (PRA). Keltz [44] expressed the view that HAZAN is a selective technique while HAZOP can be readily applied to new design and major modification. Some limitations of HAZOP are its inability to detect every weakness in design such as in plant layout, or miss hazards due to leaks on lines that pass through or close to a unit but cany material that is not used on that unit. In any case, hazards should... 

Transportation should be considered when assessing risks associated with planned or existing plants. The design of new chemical processing units should include at the earliest opportunity a qualitative or quantitative risk assessment of the whole system including production, use, and transportation in order to minimize overall risk. A brief discussion of the inherent safety aspects of transportation is included in Chapter 5. 

There was much to do. What should we do first My colleagues and I used what we now call quantitative risk assessment (QRA) to help us decide priorities—which risks to reduce first which to leave alone for the time being. Our calculations were crude but within a couple of decades the technique had advanced so much that sophisticated computerized programs for estimating risk were available. 

A facility risk review (FRR) is intermediate between a qualitative HAZOP and a quantitative risk assessment (QRA) achieved by broad probability and consequence classifications. Although not a risk assessment, an FRR uses PSA to get optimum risk cost-benefit. 

QRA - Quantitative Risk Assessment Qualitative risk - Risk expressed without numbers. 

The last three decades have seen the development of a new science to help us better understand the risk of events about which there is often very little information. The reason there is interest in such a science is that there are a great many societal benefits from activities that involve risk risk that if properly managed through better understanding can greatly benefit the quality of all life on the planet earth, both plant and animal. That science is quantitative risk assessment, also known by such names as probabilistic risk assessment and probabilistic salety assessment, the latter being the preferred name for this text. Probabilistic safety assessment divides the risk question into three questions "What can go wrong " "How likely is it " and "What are the consequences "... 

Risk characterization should preferentially include qualitative and, if possible, also quantitative risk assessment based on steps 1-3. 

Exposure assessment, step three, allows a risk assessor to estimate the significance of the effects induced by high doses of a chemical in experimental animals in a human situation. Exposure assessment is, in fact, a prerequisite for quantitative risk assessment because it allows a comparison between effects induced by high dose with those induced by low doses, and also allows... 

S. Crump, K. S. (1996). The linearized multistage model and the future of quantitative risk assessment. Human Exper. Toxicol, 15, 787-798. 

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). 

Further details about fault tree and event tree applications in quantitative risk assessment (QRA) are given in CCPS (1989b). 

Quantitative human reliability data collection systems for generating human error probabilities for use in quantitative risk assessment. 

There is considerable interest in developing a database on human error probabilities for use in chemical process quantitative risk assessment (CPQRA). Nevertheless, there have been very few attempts to develop such a database for the CPI compared, for example, with the nuclear industry. Some of the reasons for this are obvious. The nuclear industry is much more highly integrated than the CPI, with a much greater similarity of plant equipment... 

This application is similar to evaluation except that it may be performed as part of an overall qualitative or quantitative risk assessment. In the latter case, quantitative assessment techniques such as those described in Chapter 5 may be applied. 

CPQRA Chemical Process Quantitative Risk Assessment... 

Risk measurement. Few companies have undertaken the quantitative risk assessments necessary to indicate the level of risk they face for... 

INDEX OF CHEMICAL PROCESS QUANTITATIVE RISK ASSESSMENTS... 

Kazarians, M. and R. F. Boykin. Quantitative Risk Assessment for Chemical Operations. Proceedings of the International Symposium on Preventing Major Chemical Accidents, February 3-5, 1987, Washington, D.C. AIChE-CCPS, New York. 

Altliough the technical conununity has come a long way in understanding how to do a better job in luizard identification, dose-response assessment, and exposure assessment portions of risk assessment, it lias only begun to understand how to best cluiractcrize hcaltli risks and how to present tliese risks most appropriately to both the public and decision makers. Tlie next tliree sections specifically address tlicse issues. Tliis section deals witli qualitative risk assessment while tlie next two sections deal witli quantitative risk assessment. 

Point to Consider People in tl>e community are often more concerned about such issues as trust, credibility, competence, control, voluntariness. fairness, caring, and compassion tlian about mortality statistics and tl>e details of quantitative risk assessment. 

D. Hendershot, "A Simple Example Problem Illustrating tlie Methodology of Chemical Process Quantitative Risk Assessment," paper presented at AICliE Mid-Atlantic Region "Day in Industry" for Chemical Engineering Faculty, Apr. 15, 1988. 

Quantitative risk assessment is now used extensively for determination of chemical and microbial risks in food. This concept helps to systematically and scientifically judge whether certain hazardous compounds may reach unacceptable risk levels when ingested. Quantitative risk assessment can support both quality design and quality assurance but, we discuss it from the assurance perspective. In the past decade, much attention has been paid to assessment of microbial risks due to then-typical differences as compared to chemical risks ... 

Although qualitative and quantitative risk assessment techniques differ in approach, they have some features in common. Regardless of the technique... 

Evaluating risk to process plant building occupants can be accomplished through detailed qualitative and/or quantitative risk assessment. However, because of the large numbers of buildings and varying plant situations involved, these types of studies could be costly and time-consuming if applied in all cases, and should be reserved for those situations for which cost-effective solutions cannot otherwise be identified. 

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. 

For companies to make risk-based decisions from risk screening or quantitative risk assessments, company-specific risk tolerance criteria or methodologies should be developed for both individual risk and aggregate risk. Chapter 4 provides a discussion of risk criteria and methodologies, and offers guidance for companies to consider in developing their own approaches to risk tolerability. 

The methodologies offered in this book allow some degree of building evaluation without the development of explicit risk tolerance criteria. However, additional guidance in the development and selection of risk tolerance criteria will benefit those companies that wish to fully utilize risk screening and quantitative risk assessment as evaluation tools. 

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