2002).3 (c) “Situation or event where something of human value (including humans themselves) has been put at stake and where the outcome is uncertain” (Rosa 1998).
4 (d) “Uncertainty about and severity of the consequences (or outcomes) of an activity with respect to something that humans value” (Aven and Renn 2009).
5 (e) “The probability that a particular adverse event occurs during a stated period of time, or results from a particular challenge” (Royal Society 1992 , p. 22).
6 (f) “Risk refers to the uncertainty that surrounds future events and outcomes. It is the expression of the likelihood and impact of an event with the potential to influence the achievement of an organization's objectives” (Treasury Board 2001).
One aspect that distinguishes several of these definitions from our definition is the use of uncertainty instead of likelihood or probability.
Thorough discussions of the various definitions and aspects of risk are given, for example, by Lupton (1999) and Johansen (2010). We do not go into further details on this here, just repeat our cautionary comment that risk is not always defined as in this book.
Remark 2.3 (Risk: singular or plural?)
Many standards, books, and articles use the word “risk” in both singular and plural. The plural form – risks – is most often used when assets are exposed to several sources of risk. In this book, we refrain from using the plural form, except when quoting other authors. Instead, we use the term “sources of risk” when it is important to point out that there are several “sources” that may give rise to harm.
2.3 What Can Go Wrong?
To be able to answer the first question in the definition of risk, we need to specify what we mean by “What can go wrong?” So far the term accident scenario has been used to describe this, but we now elaborate more on this question.
2.3.1 Accident Scenario
An accident can usually be described as a sequence of events that harms one or more assets. The term accident scenario is used to describe a possible, future accident and is defined as follows:
Definition 2.2 (Accident scenario)
A potential sequence of events from an initiating event to an undesired end state that will harm one or more assets.
Accident scenarios may vary significantly, both with respect to the number of events and the time interval from the initiating event to the end event or state. The “path” of an accident scenario is diverted by various conditions and when barriers are activated. In cases where no barriers are available, the sequence may be reduced to a single event. The concept of accident scenario is discussed further by Khan and Abbasi (2002) and is a central element in the ARAMIS methodology (ARAMIS 2004).
Example 2.1 (Accident scenario in a process plant)
A possible accident scenario starting with a gas leak in a process plant may proceed as follows:
1 (1) A gas leak from flange A occurs (i.e. the initiating event).
2 (2) The gas is detected.
3 (3) An alarm is triggered.
4 (4) The process shutdown system fails to shut off the gas flow to the flange.
5 (5) The gas is ignited and a fire occurs.
6 (6) The firefighting system is activated.
7 (7) The fire is extinguished within approximately one hour.
8 (8) One person is injured by the fire.
Accident scenarios are identified and described as part of a risk analysis, but this does not mean that they will indeed occur. For events that have actually occurred, the term accident or accident course is more appropriate.
2.3.1.1 Categories of Accident Scenarios
In most risk analyses, it requires too much time and too many resources to study all the possible accident scenarios. A set of representative scenarios is therefore selected for detailed analysis. These are often called reference scenarios.
Definition 2.3 (Reference accident scenario)
An accident scenario that is considered to be representative of a set of accident scenarios that are relevant to include in a risk analysis.
In some applications, it may be relevant to consider the worst possible scenarios:
Definition 2.4 (Worst‐case accident scenario)
The accident scenario with the highest consequence that is physically possible regardless of likelihood (Kim et al. 2006).
A worst‐case release scenario may, for example, involve the release of the maximum quantity of some hazardous material during “worst‐case” weather conditions. Worst‐case scenarios are often used in establishing emergency plans, but should not be used in, for example, land use planning (see Chapter 5).
Because a worst‐case accident scenario often has a remote probability of occurrence, a more credible accident scenario may be more relevant.
Definition 2.5 (Worst credible accident scenario)
The highest‐consequence accident scenario identified that is considered plausible or reasonably believable (Kim et al. 2006).
The terms “plausible or reasonably believable” are not defined, but Khan and Abbasi (2002) suggest that credible accidents are those that have a probability of occurring greater than
2.3.2 Hazard
Hazard is a commonly used term in connection with risk analysis.
Definition 2.6 (Hazard)
A source or condition that alone or in combination with other factors can cause harm.
We are always surrounded by hazards, but a hazard is not critical until it comes out of control or is triggered one way or another. The event where a hazard comes out of control is an initiating event in the way that we define it below. Hazards can therefore be seen as a primary cause of an initiating event. A hazard can be related to a property of a system or a material (e.g. flammability or toxicity), it can be a state (e.g. gas under pressure, potential energy in an object that has been lifted) or it can be a situation (e.g. a train traveling at high speed). Accident scenarios may involve several hazards,
