Risk Assessment. Marvin Rausand
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 14. Some categories of barriers are listed in Table 2.9 .
Table 2.9 Categories of barriers.
| Physical barriers– Equipment and engineering design– Personal protective equipment (e.g. clothes, hard hats, and glasses)– Fire walls, shields– Safety devices (e.g. relief valves, emergency shutdown systems, and fire extinguishers)– Warning devices (e.g. fire and gas alarms) | Organizational barriers– Hazard identification and analyses– Line management oversight– Supervision– Inspection and testing– Work planning– Work procedures– Training– Knowledge and skills– Rules and regulations |
2.6.2 Safety
Safety is a problematic concept that is used with many different meanings. Many standards and guidelines related to risk assessment use the word safety