a risk analysis, and evaluating the results against risk acceptance criteria.
In addition to planning, preparing, and reporting, risk assessment consists of two main analytical parts: risk analysis and risk evaluation, as shown in Figure 3.1 . First, a risk analysis is carried out to identify and describe relevant accident scenarios and likelihoods, which together, define the risk. The second part, risk evaluation, compares the risk determined by the risk analysis with risk acceptance criteria, as discussed in Chapter 5.
3.1.1 The Role of the Risk Analyst
The main role of the risk analyst is to answer the three questions in the definition of risk in Section 2.2.1 , and to carry out the risk analysis as correctly and accurately as possible. The analyses and the evaluations should be as objective as the available data permit, and they should be neutral, impartial, and dispassionate.
During the risk assessment process, the risk analysts have to make many judgments and interpretations that are subjective, and hence, different analysts seldom reach exactly the same answers to the three questions. Even so, the analysts should make an effort to do the job as objectively as possible.
Remark 3.1 (Terminology)
Some authors do not distinguish between a risk analysis and a risk assessment, but use the two terms as synonyms.
3.2 Risk Assessment Process
The risk assessment process consists of a sequence of steps and substeps. The steps in the process are shown in Figure 3.2 . We divide the process into six steps, and the steps are described in separate sections. The risk evaluation is very briefly described in this chapter and in general, this book does not go into a lot of detail on this process. Chapter 5 explains the principles of risk acceptance criteria, but does not go into the process of deciding whether or not the risk should be accepted because this usually involves wider considerations than just risk.
The activities are not always carried out in the same sequence as shown in Figure 3.2 . Several activities may be performed in parallel, and we may also jump backwards and forwards in the process. The choice of approach may, for example, imply that the competence in the study team must be supplemented with an additional specialist, and delimitation of the study object may require that the project plan be revised. Even if the main steps are always performed, all the substeps defined in the following sections are not relevant for all types of risk assessment.
3.2.1 Step 1: Plan the Risk Assessment
For a risk assessment to provide the required results, the process must be carefully planned and prepared. One can easily be impatient in this part of the study, aching to get on with the “real work” as soon as possible. In most cases, it is beneficial to allocate sufficient time and resources to prepare the study. Experience has shown that this is perhaps the most critical part of the whole risk assessment process. Without a good understanding of what we want to achieve, it is hard to do a good job.
As described later in the book, there are many ways of doing a risk analysis. A reason why so many different methods have been developed is that risk assessment is applied to a wide range of problems and study objects. To be able to select the best methods, we need to have a good understanding of the problem that we are going to analyze. Further, risk assessments are normally performed to provide input to decisions about risk – whether we need to do something with the risk and if so, what should be done. Understanding the decision alternatives is also important before starting the work. The structure of step 1 is shown in Figure 3.3 .
3.2.1.1 Step 1.1: Clarify Decision and Decision Criteria
The reasons for performing risk assessments may vary, but common to all is that they are done to provide input to decisions. It is therefore important that the study team understands the decision alternatives and gives clear and specific answers that can be used in the decision‐making process. If the objective of the risk assessment is not clear from the beginning, it is not likely that the assessment will answer the questions that need to be answered to support the actual decision‐making.
The study team needs to know who the users of the results are and their knowledge about risk assessment. The users may influence both how the risk assessment should be carried out, and particularly, how the results should be presented. If the users are risk assessment experts, the presentation of results and the use of terminology may be different compared to if the users have no or little experience with risk assessment.
If risk acceptance criteria for risk evaluation have been established, these must be known to the study team such that the results from the risk assessment are given in a format that enables comparison with the criteria (see Chapters 5 and 6). A good understanding of these criteria and how they should be interpreted is essential.
Example 3.1 (Decision criteria)
A railway company has established a risk acceptance criterion that states “The frequency of fatal accidents per year in our operation shall be less than 0.1 per year.” In this case, it is necessary to clarify if this applies only to employees or also to passengers and/or people crossing the railway line. The answer to this question affects the scope of the risk assessment.
If several types of assets are considered in the risk assessment, separate decision criteria need to be established for each type of asset. This may apply even if people are the only asset considered. Different criteria may, for example, be applied for employees and neighbors living close to a hazardous plant. Criteria for when the assets are considered to be harmed may also need to be established. Many risk assessments are concerned only with people, and the criteria for harm are then usually whether people are injured or not.
The risk assessment should demonstrate that all the significant accident scenarios related to each decision alternative have been considered. If only one decision alternative is considered, the risk assessment should cover the risk if the decision alternative is chosen and also the risk if the decision alternative is not chosen.
The role of the risk assessment in the decision‐making process is illustrated in Figure 3.4 . From the decisions, the required input can be defined, and this determines
