Page 543 - Marine Structural Design

Chapter 29 Risk Assessment Methodology 519

third to a half of the total effort of a risk assessment into the consequence evaluation,
depending on the number of different accident scenarios and accidental sequence being
considered.
29.1.8 Risk Estimation
A general expression of risk “R” is:

R’C fbC) (29.1)
where p and C denote frequency and consequence of accidents respectively. The risks due to
all possible events shall be summed up for all situations considered in the analysis. The
results of an uncertainty analysis can be presented as a range defined by upper and lower
confidence bounds and the best estimates. It should also be kept in mind that potential severe
accidents usually generate greater concern than smaller accidents, even though the risk
(product of frequency and consequence) may be equivalent.
The estimated frequencies and consequences are integrated into presentation format on an
absolute basis compared to a specific acceptance criterion, or on a relative basis to avoid
arguments regarding the adequacy of the absolute numbers.
When evaluating risk estimates, it is recommended to calculate the importance of various
components, human errors and accident scenarios to the total risk. It may be useful to
calculate the sensitivity of the total risk estimates to changes in assumptions, frequencies or
consequences. Through these exercises, the major risk contributors may be identified, and on
which risk-reducing measures can then be taken.
29.1.9 Risk Reducing Measures
Risk reducing measures include frequency reducing and consequence reducing activities, and
their combinations. The measures may be of technical, operational, andor organizational
nature. The choice of types of measures is normally based on a broad evaluation, where risk
aspects are in focus. Emphasis should be put on an integrated evaluation of the total effects
that risk-reducing measures may have on risk. Possible coupling between risk reducing
measures should be communicated explicitly to the decision-makers, if alternative measures
are proposed. Priority is normally given to the measures that reduce the frequency for a
hazardous situation to initiate and develop into an accident event. In order to reduce
consequence, measures should be taken in the design of load bearing structures and passive
fire protection, etc. Layout arrangements shall be suitable for the operations and minimize the
exposure of personnel to accidental loads.
In selecting risk reducing measures, consideration is given to their reliability and the
possibility of documenting and verifying the estimated extent of risk reduction. Consequence
reducing measures (especially passive measures such as passive fire protection) will often
have a higher reliability than frequency reducing measures, especially for the operating
conditions.
The possibility of implementing certain risk reducing measures is dependent on factors such as
available technology, the current phase in the activity, and the results of cost benefit analysis.
The choice of risk reducing measures shall therefore be explained in relation to such aspects.