Risk assessment consequence analysis

Compatibility. See incompatible materials Compliance, audits, 114—115 Concept stage, life cycle issues, 20-23 Consequence analysis, risk assessment, 91 Corporate memory, documentation,... 

Risk assessment - Identification of initiating events Cause analysis and consequence analysis - Risk picture... 

In this paper, risk is defined as usual, i.e., it is a combination of the frequency F of an (undesired) event and its consequence C. This definition follows the ISO standards in risk management terminology in general (ISO 2002) as well as specifically in the Code (ISO 2005). The Code references to the management standard in all terms related to risk, i.e., risk, risk analysis, risk assessment, risk evaluation, andriskmanagement. 

Risk associated with very harmful consequences need risk assessment and analysis. 

The purpose of hazard analysis and risk assessment ia the chemical process industry is to (/) characterize the hazards associated with a chemical facihty (2) determine how these hazards can result in an accident, and (J) determine the risk, ie, the probabiUty and the consequence of these hazards. The complete procedure is shown in Figure 1 (see also Industrial hygiene Plant safety). 

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. 

Consider the consequences of possible errors for risk assessment or for cost-benefit analysis when considering alternative ERS. 

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. 

The objective of consequence analysis is to evaluate the safety (or quality) consequences to the system of any human errors that may occur. Consequence Analysis obviously impacts on the overall risk assessment within which the human reliability analysis is embedded. In order to address this issue, it is necessary to consider the nature of the consequences of human error in more detail. 

Generally, risk assessment has focused on the first type of error, since the main interest in human reliability was in the context of human actions that were required as part of an emergency response. However, a comprehensive Consequence Analysis has to also consider other types, since both of these outcomes could constitute sources of risk to the individual or the plant. 

Because most research effort in the human reliability domain has focused on the quantification of error probabilities, a large number of techniques exist. However, a relatively small number of these techniques have actually been applied in practical risk assessments, and even fewer have been used in the CPI. For this reason, in this section only three techniques will be described in detail. More extensive reviews are available from other sources (e.g., Kirwan et al., 1988 Kirwan, 1990 Meister, 1984). Following a brief description of each technique, a case study will be provided to illustrate the application of the technique in practice. As emphasized in the early part of this chapter, quantification has to be preceded by a rigorous qualitative analysis in order to ensure that all errors with significant consequences are identified. If the qualitative analysis is incomplete, then quanhfication will be inaccurate. It is also important to be aware of the limitations of the accuracy of the data generally available... 

This cliapter serves to introduce tlie general subject of hazard risk assessment and analysis, including cause-consequence risk evaluation. The cause-consequence aspect of this topic is perliaps tlie key to understanding hazard risk. As such, it is treated in a separate section later in diis cliapter. 

Tliis part of tlie book reviews and develops quantitative metliods for tlie analysis of liazard conditions in terms of the frequency of occurrence of unfavorable consequences. Uncertainty characterizes not only Uie transformation of a liazard into an accident, disaster, or catastrophe, but also tlie effects of such a transformation. Measurement of uncertainty falls witliin tlie purview of matliematical probability. Accordingly, Chapter 19 presents fundamental concepts and Uieorems of probability used in risk assessment. Chapter 20 discusses special probability distributions and teclmiques pertinent to risk assessment, and Chapter 21 presents actual case studies illustrating teclmiques in liazard risk assessment tliat use probability concepts, tlieorems, and special distributions. 

Professor Martel s book addresses specifically some of the more technical eispects of the risk assessment process, mainly in the areas of hazard identification, and of the consequence/effect analysis elements, of the overall analysis whilst where appropriate setting these aspects in the wider context. The book brings together a substantial corpus of information, drawn from a number of sources, about the toxic, flammable and explosive properties and effect (ie harm) characteristics of a wide range of chemical substances likely to be found in industry eind in the laboratory, and also addresses a spectrum of dangerous reactions of, or between, such substances which may be encountered. This approach follows the classical methodology and procedures of hazard identification, analysing material properties eind... 

Hazard assessment is a consequence analysis for a range of potential hazardous chemical releases, including the history of such releases at the facility. The releases must include the worst-case scenario and the more likely but significant accident release scenarios. A risk matrix can be used to characterize the worst-case and more likely scenarios. 

The terminology used varies considerably. Hazard identification and risk assessment are sometimes combined into a general category called hazard evaluation. Risk assessment is sometimes called hazard analysis. A risk assessment procedure that determines probabilities is frequently called probabilistic risk assessment (PRA), whereas a procedure that determines probability and consequences is called quantitative risk analysis (QRA). 

The hazards identification procedures presented in chapter 10 include some aspects of risk assessment. The Dow F EI includes a calculation of the maximum probable property damage (MPPD) and the maximum probable days outage (MPDO). This is a form of consequences analysis. However, these numbers are obtained by some rather simple calculations involving published correlations. Hazard and operability (HAZOP) studies provide information on how a particular accident occurs. This is a form of incident identification. No probabilities or numbers are used with the typical HAZOP study, although the experience of the review committee is used to decide on an appropriate course of action. 

Figure 23-1 shows the hazards identification and risk assessment procedure. The procedure begins with a complete description of the process. This includes detailed PFD and P I diagrams, complete specifications on all equipment, maintenance records, operating procedures, and so forth. A hazard identification procedure is then selected (see Haz-ard Analysis subsection) to identify the hazards and their nature. This is followed by identification of all potential event sequences and potential incidents (scenarios) that can result in loss of control of energy or material. Next is an evaluation of both the consequences and the probability. The consequences are estimated by using source models (to describe the... 

Decision Analysis. An alternative to making assumptions that select single estimates and suppress uncertainties is to use decision analysis methods, which make the uncertainties explicit in risk assessment and risk evaluation. Judgmental probabilities can be used to characterize uncertainties in the dose response relationship, the extent of human exposure, and the economic costs associated with control policies. Decision analysis provides a conceptual framework to separate the questions of information, what will happen as a consequence of control policy choice, from value judgments on how much conservatism is appropriate in decisions involving human health. 

By contrast, the nature of certain accident scenarios could prove to be quite sensitive to some design parameters. It should not be ruled out during the risk assessment phase, especially during detailed design, that discoveries during consequence analysis could lead to the revision of the design basis of the facility or some equipment or components. 

From those techniques given in Table 1 my personal preference is for failure mode, effects, and criticality analysis (FMECA). This technique can be applied to both equipment and facilities and can be used to methodically break down the analysis of a complex process into a series of manageable steps. It is a powerful tool for summarizing the important modes of failure, the factors that may cause these failures, and their likely effects. It also incorporates the degree of severity of the consequences, their respective probabilities of occurrence, and their detectability. It must be stressed, however, that the outcome of the risk assessment process should be independent of the tool used and must be able to address all of the risks associated with the instrument that is being assessed. 

Figure 5-1 shows how the FHA is integrated into an overall risk assessment. A process hazard analysis is required to identify likely fire scenarios that are carried forward to the FHA. An FHA provides the tools to characterize the hazards and evaluate consequences. The results are incorporated into an overall risk assessment. See Chapter 6 for more information on fire risk assessment. 

In some cases, after completing the consequence portion of the analysis, the impact of the consequences is deemed so severe that the company may decide to provide fire protection that will provide mitigation without completing the likelihood analysis. It is important to take the time to analyze the consequences (conduct an FHA) and determine if reasonable mitigation measures can be applied before continuing with the fire risk assessment. Credit for additional mitigation measures can be taken in the fire risk assessment. 

Risk assessment—The process by which the results of a risk analysis are used to make decisions either through a relative ranking of risk reduction strategies or through comparison with risk targets. Risk assessment is often defined as the qualitative estimation of probability and consequence of an incident or incidents. 

Another important reason for using multiple scenarios is to represent major sources of variability, or what-if scenarios to examine alternative assumptions about major uncertainties. This can be less unwieldy than including them in the model. Also, the distribution of outputs for each separate scenario will be narrower than when they are combined, which may aid interpretation and credibility. A special case of this occurs when it is desired to model the consequences of extreme or rare events or situations, for example, earthquakes. An example relevant to pesticides might be exposure of endangered species on migration. This use of multiple scenarios in ecological risk assessment has been termed scenario analysis, and is described in more detail in Ferenc and Foran (2000). 

The type of decision that needs to be made will influence the choice of uncertainty analysis method. Consequently, the process must include a dialogue between the risk assessor and decision maker throughout the risk assessment. The uncertainty associated with the risk assessment must be clearly communicated so that all parties involved in the risk assessment process understand it. 

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 ... 

SuperChems Expert 161 is a code developed by Arthur D Little Inc. for risk assessment consequence analysis, which also has a relief system sizing option. The code has a physical properties package that can handle highly non-ideal properties. It can also consider the effect of chemical reaction in the relief system piping. The code uses the DIERS drift flux methods for level swell and has the option of a rigorous two-phase slip model for the. relief system capacity. 

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