Qualitative/quantitative methods, risk assessment

Although the main emphasis of this chapter will be on qualitative human reliability methods in risk assessment, this section will illustrate the importance of both qualitative and quantitative methods in CPQRA. An example of a typical assessment, described by Ozog (1985) will be considered. The stages of the risk assessment are as follows ... 

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. 

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. 

In the absence of definitive human data, risk assessment may have to depend on the results of cancer bioassays in laboratory animals, short-term tests, or other experimental methods. Hence the following issues must be addressed under such circumstances the ability of the test system to predict risks for man (quantitatively as well as qualitatively) the reproducibility of test results the influence of species differences in pharmacokinetics, metabolism, homeostasis, repair rates, life span, organ sensitivity, and baseline cancer rates extrapolation across dose and dose rates, and routes of exposure the significance of benign tumors fitting models to the data in order to characterize dose-incidence relationships and the significance of negative results. 

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. 

A hazard is anything that will produce an adverse effect on human health and the environment. In environmental risk assessment, the hazard component generally refers to toxicity. Exposure is the quantitative or qualitative assessment of contact to the skin or orifices of the body by a chemical. Traditional pollution prevention techniques focus on reducing waste as much as possible however, risk assessment methods used in pollution prevention can help quantify the degree of environmental impact for individual chemicals. This approach provides a powerful tool that enables engineers to better design processes and products by focusing on the most beneficial methods to minimize all aspects of risk. 

Buelke-Sara. J., and Maetiuus, C. K (1990). Workshop on the qualitative and quantitative comparability of human and animal developmental neurotoxiciiy. Work Group II repon Testing methods in developmental neurotoxicity for use in human risk assessment. Neurotoxicoi TeraU>i 12(3), 269-274. 

Both qualitative and quantitative methods of analysis can be applied to the assessment of risk. 

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... 

In line with the two elements, which make up risk, the methods of qualitative and quantitative analysis of plant engineered systems are treated in Chap. 9. Their results are the expected frequencies for the occurrence of the categories of Fig. 8. 1. This is followed by the description of the methods for assessing the extent of damage in Chap. 10. Chapters 11 and 12 are dedicated to two important applications, functional safety and the determination of appropriate distances between industry and residential areas. 

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). 

AS/NZS 4360 discusses different types of analysis, ranging from purely qualitative to purely quantitative ones. According to the Standard, the selection of the type of analysis should, amongst other, depend on the availability of numerical data and the proportionality of the time and effort needed for a fuller assessment. Unlike the AS/NZA 4360, Basel II is explicit about its preference for quantitative methods (for computing VaR). This difference can be attributed to the characteristics of the (financial) risks covered by Basel II while Basel II only deals with the financial sector, the AS/NZA 4360 Standard is supposed to fit a wide range of firms and industries. 

HSE assessments have a long tradition within the oil-and gas industry. These assessments use a wide range of methodologies, from the strict quantitative methods such as QRA (Quantitative Risk Analysis) and FMECA (Failure Mode Effect and Criticality Analysis) to the more qualitative methods such as HAZOP (HAZard OPerability analysis). Most methods combine qualitative and quantitative data and approaches. For example, an FMECA basically uses generic failure data, expert judgments are likewise important. 

This also results in mixture of heterogeneous methods and procedures used for solving partial tasks while solving complex risk tasks. Moreover, combination of detailed and descriptive calculations is used along with accurate and inaccurate data and quantities. Some cases are known, where quantitative values are input for qualitative procedures, which results in reversal transformation of qualitative evaluation into quantitative results for risk calculation. Result of this status is that there is not sufficient confidence in results of calculations and risk assessment. 

Risk assessment is a central part of the process of providing input for decision making, and this can be performed using different types of methods - from qualitative to quantitative ones. 

A mixture of qualitative and quantitative methods was used in this work. These include reliability analysis, eapital investment costs, safety, feasibility and environmental impaet assessment (ElA). Tools used include failure mode and effect analysis (FMEA), environmental impaet assessment, and risk analysis. FMEA was used to conduct a reliability analysis, EIA was used to calculate the concentration of the flared gas eomposition partieles affeeting the local environment, and risk assessment was employed to find out the hazards and exposure in the case study in order to assure safety. Cost estimation was used to ealculate the capital investment cost for each option considered. Information and expert opinions were eaptured through interviews and questionnaires. 

The method of risk assessment and representation used by the program should be selected and tailored to fit practical program needs. For some programs a quantitative risk assessment matrix may be appropriate while others may require a qualitative (subjective) matrix. 

Nevertheless, the influence that engineers have had in developing risk assessment methods is obvious. Their passion for numerical precision encourages the use of quantitative methods. As an example, in at least two industries quantitative Failure Modes and Effects Analyses (FMEAs), rather than qualitative FMEAs, are now required to meet quality assurance requirements. Mostly, engineers make those FMEAs. 

Conceptual Design Risk Assessment This review takes place at the concept and drawing level. Formal risk assessment methods, qualitative or quantitative, are used as required. The risk assessments are documented and approved by a multifunction team, of which the environmental, health, and safety personnel are a part. An independent reviewer, not a member of the project team, must also sign off on the risk assessments. Several people at this location have been dained to do Failure Mode and Effects Analyses. 

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