Define System
In this task the system or the activity – hereafter denoted system – being analyzed is described
and all assumptions regarding the system representation and idealizations are stated. This
includes a justification of the parts of the system not being considered in the analysis. The
system representation will have consequences for the level of detail in the risk analysis and
this aspect should be addressed in the system description.
Identify Hazard Scenario
As a next step the system is analyzed to learn how it might fail and/or have consequences.
Three steps are usually distinguished in this analysis, namely the:
Decomposition of the system into a number of components and/or subsystems. This
decomposition will form the basis of further assessment of hazards and the logical and
numerical treatment of their risks.
Identification of possible states of failure for the considered system and sub-systems – i.e.
the hazards associated with the system. This step may be performed on the basis of
experiences from similar systems and information from databases containing records of
failures for different kinds of systems and subsystems.
Identification of how the hazards might be realized for the considered system and
subsystems, i.e. the identification of the scenarios of failure events of components and
subsystems which will lead to system failure if they occur. An important aspect in this step
is to consider possible “common cause” failures, which may lead to failure of two or more
of the components or subsystems of the considered system.
Analysis of Consequences
The consequences to be considered in the consequence analysis are the same as those
contained in the specification of the acceptance criteria. Typically economic consequences,
loss of lives and adverse effects on the environment have to be considered. The estimation of
consequences given failure of the system requires a thorough understanding of the system and
its interrelation with its surroundings. Thus, it is best performed in collaboration with experts
who have “hands-on” experience from the considered type of activity.
Analysis of Probability
The assessment of the probabilities of failure for the individual components and subsystems
may be based on two different approaches depending on the type of component/sub-system
and the information available in regard to its performance. For components in electrical
systems or process facilities where significant amounts of information are available the
probabilities of failure may be assessed on the basis of observed failure rates. For structural
components the situation is different in the sense that failure rate information is virtually non-
existent. In these cases methods of structural reliability theory are required for the assessment
of probabilities of failure.