MARINE SPILLAGE—SOURCES AND HAZARDS 675
tanks are normally assigned to fuel oil or ballast service. The
largest general cargo vessel operating under the U.S. flag has
a maximum fuel oil capacity, including all double bottoms,
deep tanks and settling tanks, of about 3700 tons. About 83
per cent of this tankage, or about 3000 tons, is in the double
bottoms. In general, the operator will tend to carry a mini-
mum weight of fuel oil in order to maximize cargo dead-
weight, hence somewhat less than the 3700 tons of fuel oil is
likely to be aboard.
The largest double bottom fuel tank in this particular
vessel holds about 280 tons. A one-compartment damage
collision, assuming damage from the side to the centerline,
could expose 465 tons of fuel oil to the sea. Two-compartment
damage, again from the side of the centerline, could expose
about 900 tons of fuel capacity to the sea. A grounding inci-
dent, in which the bottom shell is opened to the sea for a con-
siderable portion of the ship’s length, could expose as much
as 2/3 the double bottom fuel capacity, or about 2000 tons, to
the sea. These values represent the maximum quantities likely
to be exposed following a casualty. A considerable portion of
the fuel would be released to the sea in any of these incidents.
Magnitudes of such spills are significant but small relative to
the catastrophic effects of a comparable tank vessel incident.Unitized CargoContainer, roll-on/roll-off and unitized cargo combination
vessels in liner service are larger and higher powered than
the break-bulk vessels, hence carry greater quantities of
fuel oil. Arrangements of fuel, ballast and cargo oil tanks
are varied and fuel oil may be located in bottom, wing or
deep tanks. In general, the mode of release of fuel oil to
the sea would be as discussed for break bulk vessels, with
the quantities somewhat greater. A considerable portion
of the wing and bottom tankage of unitized cargo vessels
is piped only for ballast, thus lessening the probability that
only fuel tanks would be breached in the event of a colli-
sion or grounding.Tank VesselsFuel tanks are of relatively minor importance in the case of
tank vessels involved in casualties. Fuel is generally con-
fined to two or three deep tanks and settling tanks and rep-
resents a small portion of total tankage exposed to the sea
following a casualty.
The evolution of tanker design since the early 1950’s,
with respect to pollution from collision and grounding, was
considered briefly in earlier discussions.Collision and Grounding Protection The cargo section of a
modern tank vessel is arranged with the minimum number of
tank divisions to meet loading, trim and safety requirements.
Crude oil tankers may have as few as five tanks along the
cargo length, divided into port, center and starboard tanks
by a pair of longitudinal oiltight bulkheads, resulting in a
3 5 matrix of cargo tanks. A sixth pair of wing tanks, des-
ignated “slop tanks”, may be located immediately forwardof the machinery spaces, to accommodate cargo oil or cargo
tank washings. A typical cargo tank arrangement is shown
in the outline arrangement, Figure 4, illustrating the tank
arrangement of the EXXON VALDEZ, Table 1. This design
meets the MARPOL requirements for segregated ballast and
limitations on cargo tank volume prevailing at the time of the
construction contract in 1984. Wing tanks numbers 2 and 4
are designated as segregated ballast tanks within the cargo
spaces.
A great variety of segregated ballast tank arrangements
have been adopted to meet the mandated protection of 30%
to 40% of the shell in way of the cargo tanks. The most
common arrangement consists of two pairs of wing tanks, as
in the EXXON VALDEZ, Figure 4. A less common alterna-
tive is to provide a continuous double bottom in way of the
cargo tanks to carry most of the ballast, while providing sig-
nificant grounding protection. This arrangement, typical of
tankers carrying refined products or chemicals, is illustrated
in Figure 5.
By the time of this writing in the spring of 1990, a series
of major casualties had occurred during the 1989 1990
period. These events were followed by a period of intense
investigation and legislative activity directed to development
of improved means of minimizing the consequences of col-
lisions and groundings. The widely publicized grounding of
the EXXON VALDEZ in Prince William Sound, with an esti-
mated outflow of 11 million gallons of crude oil, resulted in
extensive environmental damage and massive cleanup efforts
by EXXON and state and federal agencies. Figure 6 shows,
diagrammatically, the extent of damage, involving eight of
the 11 cargo tanks. It is estimated that 60% or more of the
cargo outflow would have been retained had the EXXON
VALDEZ been designed and built with a continuous double
bottom. It is ironic to note that the EXXON VALDEZ design
was based in part on the earlier design of the 188,700 DWT
B. T. SAN DIEGO class of tankers which were built with
double bottoms.
The recently enacted Oil Pollution Act of 1990, dis-
cussed further in Section 5, establishes requirements for
double hulls for tank vessels operating in U.S. waters.
Requirements include specific minimum values for depth
and breadth of double bottoms and wing tanks, respectively.
In anticipation of these requirements, designers and build-
ers have developed designs of “environmental” tankers and
a significant number of building contracts have been let for
construction of these vessels.
A variety of cargo tank configurations have been devel-
oped for double hull tank vessel designs. The most widely
proposed is a variation of the conventional arrangement,
Figure 4, wherein the longitudinal bulkheads are located
well outboard to form relatively narrow segregated ballast
wing tanks, in association with a continuous double bottom.
Two innovative concepts recently developed are the Japanese
EPOCH design, Figure 7, and the Danish product tanker
design, Figure 8. The latter evolved from a successful series
of bulk carrier designs.
It should be noted that double hull design to meet antici-
pated regulatory safety requirements does not require newC013_003_r03.indd 675C013_003_r03.indd 675 11/18/2005 10:40:59 AM11/18/2005 10:40:59 AM