Risk Assessment. Marvin Rausand

      If all relevant accident scenarios are included, the set of triplets is considered to be complete and hence to represent the risk.

The risk – defined by answers to the three questions – may be presented as in Table 2.2 , where the first column lists the accident scenarios that may give harm. The second column lists the frequency of , and the last column lists the consequence spectrum associated with . The frequency and the consequence spectrum depend on the capability and reliability of the barriers that are available in the study object. A more thorough discussion of accident scenarios can be found in Section 2.3 .

Table 2.2 Risk related to a system (example).

	Accident scenario	Frequency	Consequence
1	Gas leak in area 1	0.01	Consequence spectrum 1
2	Falling load from crane 2	0.03	Consequence spectrum 2

      2.2.2 Alternative Definitions of Risk

      Many alternative definitions of risk have been suggested in the literature. Among these are the following:

      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: