A typical representation of acceptable risks is given in the Farmer diagram shown in Figure
13.1 and in the risk matrix illustrated in Figure 13.2. In the diagram two lines have been
specified of which the upper defines the activities (region of combinations of consequences
and probabilities which under no circumstance are acceptable) and the lower line defines the
activities, which in all cases are acceptable. The region between, the so-called grey area
defines the activities for which risk reductions are desired. In the BUWAL (1991) farmer
diagrams are provided for different indicators of consequences e.g. loss of lives, release of
toxic substance, etc. The acceptable and non-acceptable areas are different for the different
indicators of consequences.
Farmer diagrams are broadly used when defining and documenting acceptable risks. For
activities found to lie in the area between acceptable and non-acceptable the generally applied
philosophy is to implement risk reduction measures on the basis of cost efficiency
considerations. A commonly used principle for this is the As Low As Reasonably Practically
(ALARP). It simply implies that risk reduction in this area should be performed as long as the
costs of risk reduction are not disproportional large in comparison to their risk reducing
effects.
The Farmer diagrams, even though simple in use, also have an unfortunate property when
applied for the validation of the risk acceptance for a specific activity. Their unfortunate
property is that they are not consistent in regard to the total risk. To illustrate this, three step-
curves have been plotted in Figure 13.1 representing the so-called risk profiles for three
specific activities, e.g. engineering projects. It is seen that two of the step-curves are lying
below the acceptance line and thus are both immediately acceptable. The third step-curve,
however, is in principle not immediately acceptable as it crosses out into the grey zone. When
the total risk associated with each of the three activities is evaluated it is seen that the total
risk associated with the immediately acceptable activities is lower than the risk associated
with the activity which cannot immediately be accepted. For this reason it is thus questionable
whether Farmer diagrams are applied appropriately in praxis.
Within the offshore industry another format for acceptance criteria related to life safety is
applied, namely the Fatal Accident Rate (FAR).
The Fatal Accident Rate (FAR) is defined as:
108
P PPLL
FAR
NH
(13.1)
where NP is the number of persons on a given offshore facility and HP is the yearly number
of exposure hours. I.e. if all persons on the facility are working and living there, it is
hours/year. The PLL (Potential Loss of Lives) is the expected number of
fatalities per year. If the event of total loss of the facility is considered with an annual
probability of occurrence , and it is assumed that all persons will be lost, the PLL is given
as:
HP 365 24 8760
PF
PLLP NFP (13.2)
Typical ranges of acceptable FAR lie in the interval 10-15.