Risk Assessment. Marvin Rausand

Risk Assessment

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 images per year.

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, creating domino effects or escalating situations, as illustrated in Example 2.2 .

Example 2.2 (Accident scenario involving more than one hazard)

A possible accident scenario may comprise the following events:

1 (a) A car driver loses control over her car on a road.

2 (b) The car moves over in the opposite lane.

3 (c) A truck coming in the opposite direction tries to avoid collision and swerves off the road.

4 (d) The truck topples over.

5 (e) The truck carries petrol that is spilled.

6 (f) The petrol ignites, causing a large fire.

In this case, several hazards can be identified: The speed (kinetic energy) of the first car, the speed of the truck, and the petrol in the truck.

The term “hazard” is widely used in many different ways in connection with risk assessment. The systematic methods that we use to answer the question “what can go wrong?” are often called “hazard identification methods,” even if it is initiating events or hazardous events that are identified.

Many hazards are related to energy defined in a very wide sense. Examples are mechanical, thermal, or electrical energy. Table 2.3 lists some hazards together with brief comments, to further illustrate the definition of hazard.

Table 2.3 Examples of hazards.

Hazard	Comment
A car	A stationary car can be a hazard if it is located on top of a hill (in a specific state) and can start rolling (releasing potential energy). It is also a hazard because it contains fuel that represents thermal energy (a property of the material) that can ignite or explode. There is also electrical energy stored in the battery. A moving car represents an additional hazard because it has kinetic energy.
Propane gas under pressure	In this case, both the state (under pressure) and a property (flammable) represent hazards associated with the propane.
Water in a hydroelectric power dam	It is the water that is stored that is the main hazard in this case. The water contains massive amounts of potential energy and if the water is released this may cause severe damage. The dam itself can also be regarded as a hazard, but then just locally, because it can collapse and kill people nearby or damage equipment.
Ammunition for a gun	According to the definition, it is the gunpowder that is the hazard. It is the thermal energy in the gunpowder that makes it expand rapidly and fire the bullet. Pulling the trigger on a gun that is loaded is the initiating event that releases the energy.
A large crowd in a confined space	A full football stadium can be seen as a hazard. Some unexpected event can lead to panic and trigger rapid evacuation. This is a situation that can cause injuries and even fatalities.
Tension between tectonic plates	It may seem strange to define this and not earthquake as a hazard, but the earthquake is the event that follows from release of the energy that is otherwise under control (at least temporarily).
Pressure differences in the atmosphere	This is also an unusual definition and in most cases storm would be seen as the hazard. Similarly to earthquake, storm is the event that results from the pressure difference.
Tension in an offshore structure	The initiating event in this case could be “structural collapse,” caused by the tension in the structure exceeding the load‐bearing capacity.

Some of these hazards may seem counterintuitive and needlessly formal. In practice, the term “hazard” is used in many different ways and often without any clear definition. We come back to this issue in later chapters.