Quantitative risk assessment, system safety

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. 

Merz, H. A., "Methods of Quantitative Risk Assessment The Case of the Propellant Supply System," Minutes of the Twenty-First Explosives Safety Seminar, Houston, TX, Aug. 1984, pp. 1485-1506. 

The Phase 1 quantitative risk assessment for Pueblo and several other stockpile sites with assembled chemical munitions completed several years ago showed that the stockpile at Pueblo presents risk to public health several orders of magnitude lower than any other site. This is because it contains only mustard agent, which is less volatile than other agents, and therefore would not be carried very far in the event of a fire or explosion. Nevertheless, the Army has undertaken several risk and safety assessments to meet the legislative requirement that the technology chosen for Pueblo be as safe as or safer than the baseline system. The committee believes that the incineration technologies under consideration will have very low risk and will meet reasonable interpretations of safety criteria, even if the actual risk numbers marginally exceed the baseline criteria. 

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. 

Food Safety Council, Scientific Committee, Quantitative risk assessment, in, Proposed System for Food. Safety, Assesament 1978, Food and Cosmetics Toxicology.16. Supplement 2, 109-136. 

A large number of references to FTA applications to safety systems are given in Chapter 3 of the CCPS book on Quantitative Risk Assessment (CCPS 2000). 

In order to produce believable RACs or any other quantitative risk assessment, reliable, valid data are required. Even though considerable data exist, they are not necessarily available or in the correct format. Improvements can be made in the sharing of lessons learned, mishap information, reliability data, and the other information needed to support the system safety effort. A well-organized effort to identify and catalog existing databases and to develop plans for the systematic collection and dissemination of new data would benefit the entire safety community. 

Sound quantitative data on incident probabilities are seldom available. My associates skilled in system safety, a field in which quantitative risk assessments are routine, are not overly pleased when I say that most quantitative risk assessments are really qualitative risk assessments because so many judgments have to be made in the process to decide on the probability levels to be selected. 

Risk assessment has been a legal requirement within the EU for some time, but in all its guises it is merely the means to an end. The end is the timely implementation of workable control measures designed to either eliminate or reduce risks to an acceptable or insignificant level. There are many established risk assessment systems. The HSE have developed their Five Steps to Risk Assessment which is largely qualitative in nature. The BSl have included a quantitative risk assessment model in their guide to occupational safety and health management systems. Chapter 2.4 provides an alternative risk assessment model. 

Many argue that safety is too expensive for developing countries and that they just cannot afford it. However, the same reasons developed countries should use system safety and risk assessment methods apply to developing countries. Pe Benito Claudio (1988) states that even though quantitative risk assessment is not practiced in developing countries as much as in the developed world, qualitative risk assessments are used. 

Approval of a system is usually based on safety first principle, i.e., a quantitative risk assessment (QRA) and also the cost effectiveness (cost-benefit analysis)... 

Apportionment of Safety targets and quantification of requirements. To demonstrate that the risks resulting from the changes to the railway are tolerable, ALARP and no greater than the level of risk in the current Railway System, quantitative risk assessments are carried out, and a set of requirements related to the new hardware system constituents have been quantified. 

Aven, T., 2003. Foundations of risk analysis a knowledge and decision-oriented perspective. Chichester Wiley. Aven, T., 2011. Quantitative risk assessment the scientific platform. Cambridge Cambridge University Press. Aven, T., 2013. Probabilities and background knowledge as a tool to reflect uncertainties in relation to intentional acts. Reliability Engineering System Safety, 119(0), 229-234. doi http //dx.doi.0rg/lO.lOl6/j. ress.2013.06.044. 

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. 

LOPA is a semi-quantitative tool for analyzing and assessing risk. This method includes simplified methods to characterize the consequences and estimate the frequencies. Various layers of protection are added to a process, for example, to lower the frequency of the undesired consequences. The protection layers may include inherently safer concepts the basic process control system safety instrumented functions passive devices, such as dikes or blast walls active devices, such as relief valves and human intervention. This concept of layers of protection is illustrated in Figure 11-16. The combined effects of the protection layers and the consequences are then compared against some risk tolerance criteria. 

Furthermore, if safety measures for self-rescue can not be assessed within a quantitative risk analysis, the risk judgement system does not motivate companies or local authorities to take measures, because their effects are not visible in a state-of-the-art risk assessment. 

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