For bottom damage, oil loss is calculated based on the pressure balance principle.
In accordance with paragraph 7 of regulation 23, for a given tidal condition the mean
outflow from bottom damage is calculated as follows:
As explained below, the factor CDB(i)
accounts for oil entrapped within non-cargo tanks
located immediately below a cargo tank.
Independent calculations are carried out for zero and minus 2.5 m tide conditions and the
outflow values are then combined as follows:
7.5 Tidal Effects7.5.1
When an oil tanker experiences bottom damage as a result of a stranding and remains
aground, the occurrence of a fall of tide may result in an outflow of oil because of the hydrostatic
balance principle. For this regulation, oil loss is calculated assuming tide reductions of 0 and
The random nature of the fall of tide may be described by the following two probability
probability density function of relative fall of tide assuming that the tidal motion
may be represented with sufficient accuracy by a long-periodical harmonic motion
and that the time dependent probability of occurrence of a grounding accident is
uniformly distributed over the tidal period. The relative fall of tide is defined as
the ratio of the actual fall of tide and the double amplitude of the tidal motion.
probability density function of the double amplitude of tidal motion at the time of
the accident. From the statistics, which are restricted to data available from the
OTD study , an approximate analytical description of the probability density
function can be estimated.
From these two probability density functions the probability density function of the actual
fall of tide may be derived. Although extreme tides of 6 m or more occur in certain areas of the
world, such large tides are relatively rare. The probability density function for the fall of tide
shows a significant effect up to about 3 m. That is, the probability of an actual fall in tide in
excess of 3 m is less than 5%.
There is also a reduced probability that vessels will ground at high tide, as under keel
clearances are typically increased.
It was determined that the tidal effect could be reasonably represented by performing
calculations at two tides, 0 m and –2.5 m and then combining the results by 70%:30% ratio.
7.6 Cargo tanks bound by the bottom shell7.6.1
Even if they are in hydrostatic balance, some cargo oil outflow can be expected from
cargo tanks bounding the bottom shell which are penetrated due to bottom damage. These losses
are attributable to initial exchange losses occurring on impact, and dynamic effects introduced
from current and waves.
For the OTD study (1)*, model tests were carried out for the purpose of assessing the
magnitude of these dynamic losses. For the purposes of that study, it was decided that oil
outflow equal to at least 1% of the cargo tank volume should be assumed. This same assumption
is applied in the Revised Interim Guidelines as well as regulation 23.
7.7 Oil retained in non-oil tanks located below the cargo tank7.7.1
When a double hull tanker experiences bottom damage through the double bottom tanks
and into the cargo tanks, a certain portion of the oil outflow from the cargo tanks may be
entrapped in the double bottom tanks. Where the pressure differential between the cargo in the
tank and the outside sea is small (e.g. during a falling tide), it is reasonable to assume that the double hull space will be very effective in retaining lost oil. However, when the pressure
differential is relatively large and the penetration small, model tests conducted during the OTD
study (1)* demonstrated that only about 1/7 of the oil flowing out was retained in the double hull
As a consequence of these studies, it was surmised that –if both outer and inner bottoms
are breached simultaneously and the extent of rupture at both bottoms is the same, it is probable
that the amount of seawater and oil flowing into the double hull space would be the same.– On
this basis, the Revised Interim Guidelines specify that for breached non-cargo spaces located
wholly or in part below breached cargo oil tanks, the flooded volume of these spaces at
equilibrium should be assumed to contain 50% oil and 50% seawater by volume, unless proven
With the simplified approach applied in regulation 23, the combination of tanks involved
in each damage scenario is not determined and therefore oil retention in non-cargo spaces cannot
be directly computed. To account for oil retention in this regulation, the oil outflow from a cargo
tank located above a non-cargo space as determined from the hydrostatic balance calculation is
multiplied by an outflow reduction factor CDB(i)
To determine the outflow factor CDB(i)
, bottom damage outflows for ten actual double
tankers as well as the parametric series of designs discussed in paragraph 8 were calculated with
and without double bottom retention. The outflow reduction factor fell between 0.50 and 0.70
for all of the actual tankers, and 83% of the designs in the parametric series. On this basis, an
outflow reduction factor CDB(i)
of 0.60 was selected. That is, (1 – 0.60) or 40% of the outflow is
assumed to be entrapped by the non-oil tanks below.
* Refers to reference (1) on page 43.