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APPENDIX C
SUMMARY OF IMO 13F
GUIDELINES
[The following are excerpts from a report to the American Bureau of
Shipping entitled Review and Improvement of the IMO Probabilistic Meth-
odology for Evaluating Alternative Tanker Designs, by K. K. Tikka, Webb
Institute, April 1998.]
Regulations 13F and 13G of the 1992 Amendments to Annex I of MARPOL
73/78 mandate the phasing out of single hull tankers and their replacement
by double-hull tankers or by tankers whose environmental performance
is "equivalent" to double-hull tankers. In order to assess the equivalency
of a design concept to a double-hull tanker, the International Maritime
Organization (IMO) developed the Interim Guidelines for the Approval of
Alternative Tanker Designs under Regulation 13F of Annex I of MARPOL
73/78 (IMO 13F Guidelines) (IMO 1996). These guidelines employ a prob-
abilistic methodology to evaluate "equivalency" between designs (the
methodology is referred to in the text as "IMO methodology").
The IMO 13F Guidelines use a "pollution prevention index" to
assess the equivalency of designs. The pollution prevention index com-
bines three oil outflow parameters: the "probability of zero outflow" (P0),
the "mean outflow" (OM) and the "extreme outflow"(OE). The "probability
of zero outflow" indicates the likelihood of no outflow. In other words, it
measures the tanker design in terms of its ability to avoid spills. The "mean
outflow" is the mean value of outflows from all casualties and it measures
the overall outflow characteristics of a design. The "extreme outflow" is the
mean of the upper 1/10th of the accidents which measures the perfor-
mance of a design in severe accidents.
The calculation is based on the assumption that an accident has
taken place and that the outer hull is breached. Therefore, no probabilities
associated with the accident occurrence are included. The damage sce-
narios are described by probability density functions provided for damage
locations and damage extents. Probability densities are provided for:
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SUMMARY OF IMO 13F GUIDELINES
125
Longitudinal and vertical locations, longitudinal and vertical extent
and transverse penetration of side damage due to collision
Longitudinal and transverse location, longitudinal and transverse
extent, and vertical penetration of bottom damage due to grounding.
The probability densities are based on limited historical data (50
to 60 incidents) collected by classification societies for IMO (Lloyd's Reg-
ister 1991). The data is for single-hull tankers above 30,000 DWT. All the
variables describing damage scenarios are assumed to be independent.
The variables are non-dimensional: Longitudinal location and longitudinal
extent are divided by the ship's length between perpendiculars. Trans-
verse penetration in side damage, as well as transverse extent and location
in bottom damage are divided by the ship's breadth. Vertical penetration
in bottom damage, as well as vertical extent and location in side damage
are divided by the ship's depth.
The outflow parameters are calculated separately for collisions
and groundings, and combined in the ratio of 40 percent of collisions and
60 percent of groundings. For the calculation of the outflow parameters,
the ship is loaded to the maximum load line with zero trim and heel. The
cargo tanks are assumed to be 98 percent full and the cargo density is
based on this assumption.
IMO Guidelines include two alternative calculation methods: the
conceptual method and the survivability method. The conceptual method
is intended for evaluating the environmental performance of a new design
concept relative to a reference double-hull tanker. The survivability analysis
is intended for the approval of a final shipyard design.
The conceptual method assumes that the vessel survives the
damage in each casualty. No damage stability calculations are required.
In the side damage cases, the oil outflow is equal to the total amount of
oil carried in the damaged compartments. In the bottom damage cases,
the vessel is assumed to rest at its initial drafts, with zero trim and heel.
Oil outflow from the damaged compartments is calculated based on hydro-
static balance principles, i.e., oil outflows from a compartment until the
hydrostatic pressure of the fluid in the tank is equal to the hydrostatic
pressure of sea water at the bottom of the compartment.
The survivability method requires damage stability calculations.
Survivability is defined in terms of the requirements of the IMO regu-
lation 25(3) of Annex I of MARPOL 73/78. If the vessel does not survive,
i.e., it fails to satisfy the requirements, all oil onboard is assumed lost both
in side damage and bottom damage cases. If the vessel survives, the oil

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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
126
outflow in side damage cases is equal to the total amount of oil carried
in the damaged compartments. In bottom damage cases, the oil outflow
is calculated based on hydrostatic balance principles at the equilibrium
waterline.
The outflow calculations in bottom damage cases, both in con-
ceptual and survivability analyses, are done for 0 meter, 2 meter and
6 meter tides. However, the maximum tide to be analyzed is 50 percent
of the ship's maximum draft. The vessel is assumed stranded at a water
depth equal to its draft in the 0-meter tide condition. In the 2-meter and
6-meter tides, the assumed water depth is reduced 2 and 6 meters respec-
tively. The equilibrium condition of the vessel in the damaged condition
is found through an iterative calculation in which the oil outflow is cal-
culated based on the hydrostatic balance between oil and surrounding
water at the lowest point of the damaged tank. The outflow corresponding
to a damage case is a weighted average of the outflows in the three tidal
conditions. The relative weights for the tidal conditions are:
0.4 for 0 meter tide
0.5 for 2 meter tide
0.1 for 6 meter tide
An inert tank pressure of 0.05 Bar Gauge is assumed for the hydrostatic
balance calculations. The location of pressure balance calculations is the
lowest point in the damaged tank.
A minimum outflow of one percent of the total tank volume is
assumed for cargo tanks adjacent to the bottom shell to account for initial
outflow and dynamic effects due to current and waves.
If the bottom of a damaged cargo tank is adjacent to a ballast tank,
the pressure balance calculation is carried out at the lowest point of the
damaged cargo tank. The ballast tank is assumed to contain 50 percent of
sea water and 50 percent of oil by volume, i.e., 50 percent of the volume
is captured oil in a ballast tank that is directly below a cargo tank.
Many of the above assumptions were included in the regulation
somewhat arbitrarily (SNAME 1998). The selection of tidal heights was
arbitrary, the one percent minimum oil outflow was partially supported
by model tests and the 50 percent capture of oil by ballast tanks below
cargo tanks was investigated by model tests, but not conclusively.
After the oil outflow calculations are performed, the outflow pa-
rameters and the pollution prevention index are determined. The pol-
lution prevention index E is calculated with the following formula:

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SUMMARY OF IMO 13F GUIDELINES
127
E = 0.5 P0 + 0.4 0.01 + OMR + 0.1 0.025 + OER
P0R 0.01 + OM 0.025 + OE
P0, OM, OE are the probability of zero outflow, mean oil outflow and ex-
treme outflow, respectively. These values are determined from the oil
outflow calculations where the likelihood of each damage scenario is de-
scribed by the independent probability densities. P0R, OMR, OER are the
corresponding parameters for the reference double-hull tankers of the
same cargo capacity. The reference tankers are defined in the IMO 13F
Guidelines. The reference tankers were selected as representative of de-
signs with favorable outflow performance in order to require alternative
designs to be equal to "good" double-hull tankers (Sirkar et al. 1997). If
an alternative concept has a pollution prevention index greater or equal
to one, the concept is considered equivalent or better than the reference
double-hull tanker.
The pollution prevention index equation contains several factors,
which according to those involved in the development have no rigorous
basis (SNAME 1998). The factors 0.5, 0.4, and 0.1, which weigh the con-
tribution of the probability of zero outflow, mean oil outflow and extreme
outflow to the index, were chosen rather arbitrarily as a compromise that
would assure the equivalency of the double-hull and mid-deck concepts
(Sirkar et al. 1997). A heavy weight on the probability of zero outflow favors
double-hull designs, whereas a heavy weight on mean outflow favors mid-
deck tanker designs.
REFERENCES
ABBREVIATIONS
IMO International Maritime Organization
SNAME Society of Naval Architects and Marine Engineers
IMO. 1996. Interim Guidelines for the Approval of Alternative Methods of Design and
Construction of Oil Tankers Under Regulation 13F of Annex I of MARPOL
73/78. MARPOL 73/78 1994 and 1995 Amendments. London.
Lloyd's Register. 1991. Statistical Analysis of Classification Society Records for Oil
Tanker Collisions and Groundings. STD Report 2078-3-0 (draft). London,
Nov.
Sirkar, J., P. Ameer, A. Brown, P. Goss, K. Michel, F. Nicastro, and W. Willis. 1997. A
Framework for Assessing the Environmental Performance of Tankers in
Accidental Groundings and Collisions. SNAME Transactions, New York.
SNAME. 1998. Presentations in the Workshop on Accidental Oil Outflow from
Tankers. Sponsored by SNAME Ad Hoc Panel on the Environmental Per-
formance of Tankers, Jan. 15 16, Washington, D.C.