Risk Assessment. Marvin Rausand
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A study object may lead to several potential hazardous events. It may therefore be relevant to establish the consequence spectrum for the study object rather than for a single hazardous event. Each hazardous event then has a consequence spectrum, as shown in Figure 2.5 . Combining the consequence spectra for all the relevant hazardous events yields the consequence spectrum for the study object. This consequence spectrum has the same form as for a hazardous event. The consequence spectrum may also be presented in a table, as shown in Table 2.8 .
Table 2.8 Consequence spectrum for a study object (example).
|
Consequence |
Probability |
1 | Operator is killed | 0.001 |
2 | Operator is permanently disabled | 0.004 |
3 | Operator is temporarily injured | 0.008 |
|
|
|
m | Minor material damage | 0.450 |
The probability
In some cases, it may be possible to measure the consequences of a hazardous event
In this case, it may be meaningful to talk about the mean consequence or mean loss if the hazardous event should occur
(2.9)
Observe that (2.9) is the conditional mean loss given that the specified hazardous event has occurred. The minimum and maximum loss and the standard deviation may easily be provided. In cases where the consequences cannot be easily measured with a common unit, it is considered much more meaningful to present the entire consequence spectrum to the decision‐maker, primarily for the whole study object but also for the most critical hazardous events (or end states).
2.5.5 Time of Recording Consequences
Some of the consequences of an accident may occur immediately, whereas others may not materialize until years after the accident. People are, for example, still (claimed to be) dying of cancer in 2019 as a consequence of the Chernobyl accident in 1986. A large quantity of nuclear fallout was released and spread as far as northern Norway. During the accident, only a few persons were harmed physically, but several years after the accident, a number of people developed cancer and died from the fallout. The same applies for other accidents involving hazardous materials, and notably for the Bhopal accident that took place 23 December 1984, in Bhopal, India. When we assess the consequences of an accident, it is therefore important not only to consider the immediate consequences but also to consider the delayed effects.
2.5.6 Severity
In some cases, it is useful to define a limited set of possible consequence classes or categories and use these rather than a continuous spectrum of consequences. The term severity is sometimes used to describe these classes:
Definition 2.29 (Severity)
Seriousness of the consequences of an event expressed either as a financial value or as a category.
The categories may be, for example, catastrophic, severe loss, major damage, damage, or minor damage. Each category has to be described to ensure the categories are understood by all relevant stakeholders. This is discussed further in Chapter 6.
2.6 Additional Terms
This section defines a number of terms that are associated to risk and that are treated in more detail in later chapters of the book.
2.6.1 Barriers
Most well‐designed systems have barriers that can prevent or reduce the probability of hazardous events, or stop or mitigate their consequences.
Definition 2.30 (Barrier)
Physical or engineered system or human action (based on specific procedures or administrative controls) that is implemented to prevent, control, or impede energy released from reaching the assets and causing harm.
Barriers are also called safeguards, protection layers, defenses, controls, or countermeasures. Barriers are discussed in more detail in Chapter