A risk analysis is a systematic use of available information to identify hazards and to estimate the risk to individuals or populations, property or the environment. In a plant, it can be further separated in at least three levels [71], depending on how detailed the analysis is to be and the labour resources available, i.e. qualitative methods, semi-quantitative methods, and quantitative methods. During risk analysis, all three levels can be used in sequence. The first methods are used to determine which scenarios are relevant and to be analyzed further in the quantitative risk analysis, to identify the most hazardous events. It may be used as screening methods in the preliminary risk analysis. The methods include HAZOP, What-if, different check-lists, etc. Semi-quantitative methods are used to determine the relative hazards associated with undesired events. The methods are normally called index methods, point scheme methods, numerical grading, etc., where the hazards are ranked according to a scoring system. Both frequency and consequences can be considered, and different design strategies can be compared by comparing the resulting scores. The final level of analysis (i.e. quantitative methods) is the most extensive in terms of quantifying the risk. It is also the most labour intense.

At this level, a distinction can be made between a deterministic analysis and a probabilistic analysis [107]. The deterministic analysis focuses on describing the hazards in terms of the consequences. No consideration is taken of the frequency of the occurrence. A typical example is the determination of the worst case scenario expressed as a risk distance. The probabilistic approach determines the quantified risk based on both frequencies and consequences. The last approach was used in the study.

A quantitative risk analysis is focused on the combined effect of frequencies and consequences of a possible accident, as illustrated in Fig. 4.2. The first step, before starting to quantify the risk, is related to defining and describing the system. Detailed information of the system (such as process flow diagram, operating condition, etc) may be required. The next step is hazard identification. The step seeks an answer to the question: what can go wrong? This is the most important step because hazards that are not identified will not quantified, leading to an underestimation of risk [107]. The third step involves another question: how likely is the accident? Answering the question involves quantification of the probability of each accident scenario. Fault tree analysis may be used for this purpose. The next step is consequence analysis. It aims to quantify the negative impacts of the scenarios. The consequences can be measured in terms of the number of fatalities (that is used in the study), number of injuries, or value of the property lost. The last step of a QRA is to estimate the risk. The risk can be expressed as individual risk or as societal risk. These are the two most frequently used risk measures.

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