Marvin Rausand - Risk Assessment

Risk (as used in this book) is always related to what can happen in the future. In contrast to our ancestors, who believed that the future was determined solely by the acts of God (e.g. see Bernstein 1998), or by destiny, we have the conviction that we can influence the future by analyzing and managing risk in a rational way. Our tool is risk assessment , and our goal is to inform decision‐making concerning the future.

The possibility that events with unwanted effects may happen is an inherent part of life. Such events can be caused by natural forces, such as flooding, earthquake, or lightning; technical failures; or human actions. Some events can be foreseen and readily addressed, whereas others come unexpectedly because they appear unforeseeable or have only a very remote likelihood of occurrence. In many systems, various safeguards are put in place to prevent harmful events or to mitigate the consequences should such events occur. Risk assessment is used to identify what harmful events can occur, the causes of these events, to determine the possible consequences of harmful events, to identify and prioritize means to reduce risk, and to form a basis for deciding whether or not the risk related to a system is tolerable . 1

For the purpose of this book, we follow Kaplan and Garrick (1981) and define risk as:

The combined answer to the three questions: (1) What can go wrong? (2) What is the likelihood of that happening? and (3) What are the consequences?

The three questions may be explained briefly as follows 2:

1 (1) What can go wrong?To answer this question, we must identify the possible accident scenarios that may harm some assets that we want to keep and protect. An accident scenario is a sequence of events, starting with an initiating event and ending with an end state that affects and causes harm to the assets. The assets may include people, animals, the environment, buildings, technical installations, infrastructure, cultural heritage, reputation, information, data, and many more.

2 (2) What is the likelihood of that happening?The answer to this question can be given as a qualitative statement or quantitatively as probabilities or frequencies. We consider each accident scenario that was identified in Question 1, one‐by‐one. To determine the likelihood, it is often necessary to carry out a causal analysis to identify the basic causes (hazards and threats) that may give rise to the accident scenario.

3 (3) What are the consequences?For each accident scenario, we must identify the potential harm or adverse consequences to the assets mentioned in Question 1. Most systems have safeguards or barriers that may prevent or mitigate harm. The harm to the assets depends on whether or not these barriers function as required when the accident scenario occurs.

A risk analysis is carried out to provide answers to the three questions in the definition of risk. Risk analysis and risk assessment are further defined and discussed in Chapter 3. The first question “what can go wrong ?” clearly shows that we focus on scenarios that give “negative” consequences – even if risk refers to both gains and losses in economic theory.

Observe that classifying a consequence as positive or negative represents social judgments and cannot be derived from the nature of the accident scenario (Klinke and Renn 2002). This implies that consequences can be regarded as positive by some people and negative by others. Examples include terrorist attacks or other cases where someone wants to cause harm. The terrorists aim to cause as much harm as possible to get attention. For them, it is therefore a positive consequence. For most others, it is seen as negative.

The word danger is used in our daily language, both as a noun and as the associated adjective dangerous . Standards for risk assessment very seldom use the noun danger, but the adjective dangerous is commonly used in expressions such as dangerous chemicals, dangerous behavior, and dangerous activity. We follow the standards and refrain from using the noun danger in this book.

The answer to the first question in Definition 2.1gives a set of accident scenarios . The answer to the second question gives the likelihood (i.e. usually the frequency) of accident scenario for each . If accident scenario occurs, it may harm several assets in different ways and with different probabilities. The answer to the third question is therefore a set of possible harms with associated probabilities. This set is called the consequence spectrum associated with accident scenario , for . The consequence spectrum may be expressed as a multidimensional vector that includes various types of harm or damage to all relevant assets (e.g. people, property, and the environment). may sometimes be time dependent if the magnitude of damage varies with time. This means that the answers to the three questions in Definition 2.1can be answered by the triplet for each , where

provides a name, a precise definition, and a description of potential accident scenario

is an estimate of the likelihood (e.g. frequency) of accident scenario

is a multidimensional vector of the potential types of harm/damage to all relevant assets caused by accident scenario , with associated probabilities, that is, the consequence spectrum for

The risk related to the study object can now, according to Kaplan and Garrick (1981), be expressed by the set of triplets

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