2.1 There are three basic steps involved when
applying this regulation:
-
.1 determine the probability of penetrating each
oil tank within the cargo block length, for both side damage (collisions)
and bottom damage (strandings);
-
.2 assess the expected oil outflow from each damaged
oil tank; and
-
.3 compute the mean outflow parameter and compare
to the specified maximum permissible value.
2.2 This approach differs from the Revised Interim
Guidelines (2)
footnote, which
calls for calculation of three separate outflow parameters: the probability
of zero oil outflow, the mean outflow, and the extreme oil outflow.
-
.1
the probability of zero outflow,
P0, represents the likelihood that no oil will be released
into the environment, given a collision or grounding casualty which
breaches the outer hull. P0 equals the cumulative probability
of all damage cases with no outflow;
-
.2
the mean outflow parameter, OM,
is the non-dimensionalized mean or expected outflow, and provides
an indication of a design's overall effectiveness in limiting oil
outflow. The mean outflow equals the sum of the products of each damage
case probability and the associated outflow. OM equals
the mean outflow divided by the total quantity of oil onboard the
vessel; and
-
.3
the extreme outflow parameter,
OE, is the non-dimensionalized extreme outflow, and provides
an indication of the expected oil outflow from particularly severe
casualties. The extreme outflow is the weighted average of the upper
10% of all casualties (i.e. all damage cases within the cumulative
probability range from 0.9 to 1.0).
2.3 In accordance with the Revised Interim Guidelines,
the parameters are combined using the following formula, in order
to provide an overall assessment of a design's outflow performance
in the event of a collision or grounding. P0, OM,
and OE are the oil outflow parameters for the alternative
design, and P0R, OMR, and OER are
the oil outflow parameters for the reference ship of equivalent size.
The pollution prevention index "E" must be greater than or equal to
1.0, for a design to be considered equivalent to the reference ship.
2.4 Application of the Revised Interim Guidelines
requires determination of the probability of occurrence and oil outflow
for each unique damage case. For a typical tanker, this involves assessment
of thousands of damage conditions. These data are then applied when
computing the three outflow parameters.
2.5 A significant difference between regulation 23 and the Revised Interim
Guidelines is in the assessment of damage cases. Rather than determining
each unique damage case and its associated probability, the probability
of damaging each oil tank within the cargo block length is calculated.
This equals the probability that an oil tank will be breached, either
alone or in combination with other tanks, and equals the sum of the
probabilities for all of the unique damage cases which involve that
particular oil tank.
2.6 The simplified probabilistic calculation method
as applied in this regulation is based on the following principle:
where:
|
pi
|
= |
probability
of occurrence of damage scenario i (where one cargo tank or a group
of adjacent tanks may be involved) |
|
vi
|
= |
volume
of oil outflow from cargo tanks involved in damage scenario i under
consideration |
|
i |
= |
subscript denoting
the damage scenario under consideration |
|
pj
|
= |
probability
of occurrence that cargo tank j is damaged (irrespective of the damage
scenarios involved) |
|
vj
|
= |
volume
of oil outflow from cargo tank j |
|
j |
= |
subscript denoting
the cargo tank under consideration |
|
Σ |
= |
symbol for
the summation to be carried out over all possible damage scenarios
i or cargo tanks j respectively resulting in a non-zero contribution
to the mean oil outflow |
2.7 The mean outflow parameter, Om, equals the
mean outflow divided by the total oil onboard, C. For regulation 23 as well as the Revised
Interim Guidelines, C is defined at the total cargo oil capacity at
98% tank filling.
2.8 Because the unique damage cases are not determined,
calculation of the probability of zero outflow and extreme outflow
are not practical with this simplified approach. In regulation 23, the mean outflow
parameter alone is used to assess the outflow performance. Of the
three parameters, mean outflow performance is considered to be the
best indicator of overall outflow performance.
2.9 This is considered a reasonable simplification,
as each design must also meet the provisions of regulation 19. It is assumed that
the double hull provisions of regulation
19 and the more rigorous analytical approach contained in the
Revised Interim Guidelines assures that the design provides adequate
protection against the likelihood of spills, as is measured by the
probability of zero outflow parameter. The extreme oil outflow parameter
provides an indication of the expected oil outflow from particularly
severe casualties. To a large extent, the impact of large spills is
reflected in the mean outflow parameter, as it represents the weighted
average of all spills.