Risk Assessment - Marvin Rausand

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in Chapter 8.

      Example 2.8 (Helicopter accidents)

      In the SINTEF helicopter studies (Herrera et al. 2010), helicopter accidents are classified into eight categories:

      1 (1)Accident during takeoff or landing on a heliport

      2 (2)Accident during takeoff or landing on a helideck (i.e. on an offshore platform)

      3 (3)Accident caused by critical aircraft failure during flight

      4 (4)Midair collision with another aircraft

      5 (5)Collision with terrain, sea, or a building structure

      6 (6)Personnel accident inside a helicopter (i.e. caused by toxic gases due to fire or cargo)

      7 (7)Personnel accident outside a helicopter (i.e. hit by tail rotor)

      8 (8)Other accidents.

      The studies are limited to accidents involving helicopter crew and passengers. Accidents involving other persons and other assets are not included.

      2.3.11 Incident

      The term “incident” may be defined as:

      Definition 2.18 (Incident)

      A sudden, unwanted, and unplanned event or event sequence that could reasonably have been expected to result in harm to one or more assets, but actually did not.

      This definition is identical to the definition of accident, with the important distinction that incidents do not cause any significant harm. As for accident, the term incident is mainly used about events that have occurred in the past. From an initiating event, the event sequence can develop until it ends either with an accident or with an incident. Other terms that are used more or less with the same meaning as incident are near accident, mishap, and near miss.

      Observe that some authors use the term incident to include both incidents and accidents as they are defined in this book.

      2.3.12 Precursors

      Phimister et al. (2004) define precursors as:

      Definition 2.19 (Precursors)

      Conditions, events, and sequences that precede and lead up to accidents.

      It is sometimes easy to spot precursors after an accident has occurred. More difficult is to identify precursors upfront an accident. Precursors are often technical failures, human errors, or operating conditions that individually or in combination with other precursors, may lead to an accident. Often, precursors can be identified when they lead to incidents and near accidents that were stopped by functioning safety controls. An incident (with no significant consequences) is not a precursor, but may help to reveal precursors (e.g. see U.S. DOE 1996).

      2.3.13 Special Types of Accidents

      Accident causation and accident models are discussed in more detail in Chapter 8. The development of accident theory has been strongly influenced by the views of Charles Perrow and James Reason, who have introduced new notions for major accidents.

      Reason (1997) introduces the concept of organizational accident, defined as follows:

      Definition 2.20 (Organizational accident)

      A comparatively rare, but often catastrophic, event that occurs within complex modern technologies (e.g. nuclear power plants, commercial aviation, the petrochemical industry, chemical process plants, marine and rail transport, banks, and stadiums) and has multiple causes involving many people operating at different levels of their respective companies. Organizational accidents often have devastating effects on uninvolved populations, assets, and the environment.

      Reason (1997) calls accidents that cannot be classified as organizational accidents, individual accidents:

      

      Definition 2.21 (Individual accident)

      An accident in which a specific person or group is often both the agent and the victim of the accident. The consequences to the people concerned may be great, but their spread is limited.

      

      Definition 2.22 (System accident)

      An accident that arises in the interactions among components (electromechanical, digital, and human) rather than in the failure of individual components (Perrow 1984).

      In analogy with the terminology of Reason (1997), an accident that cannot be classified as a system accident is sometimes called a component failure accident:

      Definition 2.23 (Component failure accident)

      An accident arising from component failures, including the possibility of multiple and cascading failures (e.g. see Leveson 2004).

      

      To answer the second question in the triplet definition of risk,“What is the likelihood of that happening?” we need to use concepts from probability theory. A brief introduction to probability theory is given in Appendix A. Essentially, the probability of an event images is a number between 0 and 1 (i.e. between 0% and 100%) that expresses the likelihood that the event occurs in a specific situation, and is written as images . If images , we know with certainty that event images occurs, whereas for images , we are certain that event images will not occur.

      2.4.1 Probability

      Probability is a complex concept about whose meaning many books and scientific articles have been written. There are three main approaches to probability: (i) the classical approach, (ii) the frequentist approach, and (iii) the Bayesian or subjective approach.

      People have argued about the meaning of the word “probability” for at least hundreds of years, maybe thousands. So bitter, and fervent, have the battles been between the contending schools of thought, that they've often been likened to religious wars. And this situation continues to the present time (Kaplan 1997 , p. 407).

      2.4.1.1 Classical Approach

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