ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - MARINE SPILLAGE—SOURCES AND HAZARDS docx

668
MARINE SPILLAGE—SOURCES AND HAZARDS
INTRODUCTION
Scope
Pollution of navigable waterways resulting from operation
of commercial and naval vessels may be a consequence of
normal service or from casualties such as collisions and
groundings. Propulsion system fuel oil and liquid cargoes
may be involved in any case and will be considered in this
chapter. Waste disposal from shipping is of comparatively
small magnitude compared to waterway pollution from
shoreside sources and will not be considered. Emphasis will
be on description of the pollution problem and on means for
prevention. The subject of spill collection and disposal is
considered elsewhere in this text.
Problem Definition
Normal operations A decade ago, the primary source of pol-
lution of the world’s waterways was the intentional discharge
of oily ballast water during routine operations. At that time,
for virtually all seagoing operations ballast water was taken
aboard for a portion of the voyage in order to obtain sufficient
draft and trim for propeller immersion, adequate steering abil-
ity, acceptable conditions of seaworthiness; and to satisfy man-
dated operational and regulatory requirements for intact and
damaged stability. If assigned ballast tank capacity was inad-
equate to meet these requirements, it was the general practice
to ballast empty fuel oil tanks, or empty liquid cargo tanks in
the case of tank vessels. This procedure resulted in the neces-
sity for pumping overboard large quantities of contaminated
ballast water before taking on fuel oil or liquid cargoes. These
procedures have been largely outlawed by international agree-

ments developed by the International Maritime Organization
(IMO) of the United Nations, and enforced by the national
regulatory agencies of the member countries.
Casualties Spills resulting from casualties generally
receive more attention in the world press than incidents
involving operational discharges. Spills may occur from
operational mishaps in the pumping of fuel oil bunkers and
liquid cargoes. Incidents of spills that occur from collisions
and groundings are accompanied by associated dangers to
personnel and the environment and are likely to involve the
largest quantities of pollutant discharged in a single incident.
The magnitude of such spills is clearly far greater in the case
of a loaded tank vessel than the grounding and rupture of the
double bottom fuel tanks of a dry cargo vessel.
SHIP CLASSIFICATION AND DESCRIPTION
The arrangements and general characteristics of the various
merchant ship types are well described in such standard texts
as Reference (1) and in the technical literature, including the
comprehensive discussion in Reference (2) covering U.S.
shipbuilding during the 1936Ϫ1976 period. Accordingly,
the following discussion will be restricted to characteristics
pertinent to the pollution problem, for example, arrangement
of tank spaces.
The following standard abbreviations have been used
throughout for convenience:
DWT ϭ deadweight ϭ total displacement Ϫ light weight
ϭ cargo ϩ consumables
mld. ϭ molded
fbd. ϭ freeboard
B.P. ϭ between perpendiculars

Break Bulk Vessels
The greatest variety of seagoing vessels are in this category
which includes the ordinary general cargo vessels carrying
a great variety of dry products in raw material as well as
finished and packaged form. An outline sketch of the cross-
section through a typical cargo hold, showing hatchway,
tween decks and fuel oil or ballast spaces, is shown in
Figure 1. Fuel oil is commonly carried in the double bot-
toms, as indicated, but may also be carried in deep tanks,
particularly outboard of shafting and in the vicinity of the
machinery spaces. Except for settling and daily service
tanks, all bunker spaces are normally piped for fuel oil
or ballast. The availability of cubic capacity for tankage
assigned only to ballast service is limited in such vessels
and frequent use of fuel tanks for ballasting is likely in most
operations.
Unitized Cargo Carriers
Ships in this category are usually designed for the exclu-
sive transport of standard containers or wheeled trailer vans,
and, to a lesser extent as hybrid carriers to handle container,
wheeled vehicles and general break-bulk cargo. In the case
of container ships, as illustrated in the typical hold section,
Figure 2. the cellular nature of the cargo stowage requires
some “squaring off” of the hold spaces, with the result that
considerable wing space is available for ballast tanks. As a
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MARINE SPILLAGE—SOURCES AND HAZARDS 669
FUEL OIL
OR BALLAST

FUEL OIL
OR BALLAST
FUEL OIL
OR BALLAST
INNER BOTTOM
THIRD DECK
SECOND DECK
MAIN DECK
SHIP
16,000 DWT PARIA LIMPA, built in the 1940’s alongside the 326,000 DWT UNIVERSE PORTUGAL
Courtesy, The Motor Ship.
FIGURE 1 Outline midship section through cargo hold, typical break bulk dry cargo vessel.
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670 MARINE SPILLAGE—SOURCES AND HAZARDS
result such vessels are able to operate with clean ballast only
and no ballasting of fuel tankage is normally required.
Roll-on/roll-off or trailer ships are similarly “squared
off” internally and, in addition, may have extensive deep
tank spaces available below the lowest vehicle deck. As with
the container ships, tankage is likely to be available in suf-
ficient quantity to permit full clean ballast operations.
Dry Bulk Carriers
Dry bulk carriers are engaged primarily in the transport of
such commodities as coal, grain and ores. Two general con-
figurations exist, as shown in Figure 3. For light weight,
high cubic cargoes, such as coal and grains, the hold con-
figuration is such that water ballast capacity, in the amount
of 35 per cent to 40 per cent of cargo deadweight, is avail-
able for clean ballast service, as shown in Figure 3a and 3b.

Fuel oil bunkers are generally confined to deep tanks within
the machinery spaces or to portions of the wing and double
bottom tanks adjacent to the machinery spaces. Clean ballast
operation is generally feasible.
Vessels designed specifically for heavier cargoes such as
ores are generally arranged with comparatively small cargo
holds and large surrounding tank spaces as shown in Figure 3c.
Clean ballast operation is readily accomplished under all loading
conditions.
Liquid Bulk Tank Vessels
The modern tank vessel has evolved from the standard 16,000
deadweight ton (DWT) “T2” tanker of World War II to
modern tank vessels exceeding 500,000 DWT capacity. The
transport of liquid cargoes, predominantly petroleum crudes
and refined petroleum products is the single largest category
of waterborne commerce and represents the greatest potential
pollution hazard with respect to normal operations as well
as casualties. Accordingly, characteristics of vessels in the
liquid bulk trades will be considered in somewhat greater
detail than other ship types.
Petroleum Crude and Products Carriers With the exception
of the steam turbo-electric main propulsion machinery and
electric drive cargo pumps, the World War II T2 tanker is,
in general arrangement, a parent of the tanker designs devel-
oped during the early post-war years. Typical characteristics
of these vessels include:
1) Cargo section divided by a pair of longitudinal
bulkheads into port, center and starboard tanks.
2) Relatively short cargo tanks independent of ship
size.

3) Poop, bridge and forecastle superstructures with
navigating bridge located amidships.
4) Forward and after fuel bunkers.
5) Forward and after pump rooms.
6) Relatively long, single screw, main propulsion
machinery, with separate boiler and engine rooms.
From the late 1950s until the present, tanker design evolved
through the following changes, all directly related to reduced
cost of construction and operation:
1) Increase in size to over 500,000 DWT capacity,
with corresponding increases in dimensions and
operating drafts.
MAIN DECK
INNER BOTTOM
BALLAST
BALLAST
FUEL OIL OR BALLAST
FUEL OIL OR BALLAST
FIGURE 2 Outline midship section through container hold, typical container ship.
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MARINE SPILLAGE—SOURCES AND HAZARDS 671
2) Simplification of arrangement, particularly by
reduction in number and increased size of cargo
tanks. Typical modern crude oil tankers are
arranged with as few as five center tanks and ten
wing tanks. Secondary arrangement changes have
included elimination of superstructure and houses
amidships, location of all accommodation and
navigation spaces aft and elimination of forward

pump room and fuel bunkers.
3) Speed has remained within the 14 to 16 knot range.
4) Crew size has been reduced substantially, averag-
ing as low as 19 men on U.S. flag as well as for-
eign tankers.
5) Propulsion system power levels have increased with
size, approaching 40,000 SHP on a single screw.
Centralized pilot house control of all propulsion
machinery is a state-of-the-art development avail-
able to operators of diesel and steam turbine
machinery.
6) Cargo pumping systems are generally similar to
those of the post World War II period, except for
increase in pumping rate with ship size. Elimination
of pump rooms and fitting of deep well pumps in
each cargo tank is a recent trend, following the
arrangements of special products carriers.
Regulatory effects on tanker design, imposed since the 1973
MARPOL Convention for the Prevention of Pollution from
Ships, have been significant. These regulations, imposed
progressively from 1973 through 1985, include the follow-
ing requirements and constraints:
1) Limitation on maximum cargo tank size to 30,000
cubic meters.
2) Segregated ballast tanks (SBT) on all new tankers
larger than 20,000 DWT capacity. SBT capac-
ity must be sufficient to obtain the following
conditions:
• Capacity to obtain minimum mean ballast
draft of 0.02L ϩ 2 meters, where L ϭ length

between perpendiculars
• Trim by the stern no greater than 0.015L
• Draft at the stern sufficient to submerge the
propeller
• SBT located to so as to protect 30% to 40%
of the side shell in way of the cargo tanks.
(Actual requirements vary with ship size and
geometry.)
Since the segregated ballast tanks are restricted to clean bal-
last service only, the net effect of these requirements has
been to increase the ship dimensions to accommodate the
required SBT volume. As a result, modern tankers that meet
the SBT requirements are volume rather than weight limited,
and will only load to the assigned draft marks when carrying
very dense cargoes.
The SBT capacity requirements are considerable,
amounting to about 25% to 40% of the deadweight. The
optimum SBT and cargo tank arrangements, for minimum
ship acquisition cost, vary with ship size and proportions.
A common arrangement is to assign two pairs of wing tanks
within the cargo tank section to SBT service. In some cases
the preferred arrangement is the concentration of segregated
ballast in double bottom tanks extending under the entire
length of the cargo tank section of the ship.
Table 1 include a summary of principal characteristics of
U.S. flag tank vessels built since 1977. All meet the MARPOL
SBT requirements. The SBT arrangements are reviewed later
in connection with protection from collision and grounding.
Special Products A great variety of liquid products are
carried in specially constructed tankers. While the quanti-

ties carried are small compared to the volume of petroleum
FIGURE 3 Outline midship sections through cargo
holds, typical dry bulk carriers.
CARGO HOLD
CARGO HOLD
CARGO HOLD
BALLAST
(c) ORE CARRIER
BALLAST
BALLAST
BALLAST
BALLAST
BALLAST
BALLAST BALLAST
BALLAST
(b) DRY BULK CARRIER, DOUBLE SKIN SIDE SHELL
(a) DRY BULK CARRIER, SINGLE SKIN SIDE SHELL
GRAIN OR
BALLAST
GRAIN OR
BALLAST
GRAIN OR
BALLAST
GRAIN OR
BALLAST
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672 MARINE SPILLAGE—SOURCES AND HAZARDS
crudes and refined products, transport of these commodities
may involve unique containment problems and associated

hazards.
Special products carriers may be classified in the following
manner, according to nature of cargo:
1) Liquefied natural gasses (LNG) and liquefied petro-
leum gasses (LPG).
(a) Low temperature—ambient pressure contain-
ment—The most exacting containment require-
ments are in this category, with cargo carried at
about −260ЊF for liquefied natural gas (LNG). The
largest LNG carriers at this time have capacities of
about 130,000 cubic meters.
LPG transport includes the carriage of such gasses as
propane, butane and ethylene, with propane, carried
at about Ϫ50ЊF, the most common.
In a typical LNG or LPG carrier, cargo is car-
ried in an independent, insulated tank or membrane
liner. Double bottom and wing tank spaces are nor-
mally assigned to salt water ballast and fuel oil is
carried in a relatively small portion of the double
bottom and in deep tanks within the machinery
spaces. Geometry is similar to that of a container
ship and clean ballast operation is accomplished
with no difficulty.
(b) High pressure—ambient temperature—LPG may
be carried in pressure vessels, designed to the
A.S.M.E. Code for Unfired Pressure Vessels.
While this mode of containment has been gen-
erally superceded by low temperature transport
for international trade, a considerable amount of
LPG and similar cargoes is carried in this man-

ner on the inland waterways of the United States
and Europe and in smaller coastwise vessels and
barges. The limiting design condition is usually
for propane, in cylindrical tanks designed for 250
psig. In general, vessels carrying cargoes in pres-
sure vessels have sufficient cubic capacity to per-
mit clean ballast operation.
2) Miscellaneous liquefied gasses—Anhydrous ammo-
nia is carried in significant quantities in U.S. inland
and coastal waters. This commodity may be carried
at low temperature or under pressure, in containment
designed for the transport of propane.
Chlorine gas is commonly transported by barges
in U.S. waters, primarily in pressure vessel contain-
ment. Other commodities of importance are primarily
petro-chemicals, including butadiene, ethane, ethyl
chloride, prophylene and vinyl chloride.
3) High temperature commodities—The transport of
molten sulfur at about 275ЊF in heated independent
insulated tanks has become the most common high
TABLE 1
Representative modern U.S. flat tank vessels
NAME EXXON CHARLESTON EXXON BAYTOWN ATIGUN PASS B.T. SAN DIEGO EXXON VALDEZ
Length, B.P., m 185.93 229.82 263.35 278.90 288.04
Breadth, mld, m 32.26 32.26 52.74 50.60 50.60
Depth, mld, m 18.29 18.29 22.86 23.78 26.83
Draft, keel, m 12.80 11.73 17.47 18.08 19.66
Displacement, tonnes 56,970 73,700 200,400 220,800 244,145
Deadweight, tonnes 42,800 58,645 176,160 191,100 214,860
Cargo capacity, m

3
59,200 62,660 184,300 209,980 240,890
Ballast capacity, m
3
18,500 32,000 57,400 59,600 69,600
Cargo Chem. and prods. Crude Crude Crude Crude
Number of cargo tanks 42 14 13 15 13
Propulsion machinery Dir. diesel Dir. diesel St. turbine St. turbine Dir. diesel
Horsepower, max
continuous
17,000 bhp 17,000 bhp 26,700 bhp 28,000 shp 32,240 bhp
Service speed, knots 16 15.8 16.5 14.25 16.25
Year delivered 1983 1984 1977 1978 1986
Notes:
1) Segregated ballast yes yes yes yes yes
2) Double bottom yes yes no yes no
3) Double hull no no no no no
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MARINE SPILLAGE—SOURCES AND HAZARDS 673
temperature commodity carried on international and
inland waters. Internal hull geometry resembles that
of the low temperature LPG or LNG vessel, with
double bottom and wing tank spaces available for
clean ballast.
The transport of asphalt and bitumen in the
molten state is less exacting than the case of molten
sulfur transport and cargo is normally carried in con-
ventional integrated tanks.
Sulfur and bitumen cargoes are relatively dense

and clean ballast operation should be expected. The
use of cargo tanks for ballast services is not feasible,
except for emergency situations.
4) Toxic and corrosive chemicals—A great variety of
hazardous cargoes are carried in relatively small quan-
tities in a variety of containment systems. References
(3) and (4) contain a hazardous cargo classification
and data for specific commodities, with particular
respect to marine transportation.
Virtually all hazardous cargo carriers will be
built with sufficient ballast tank capacity, in the form
of integral double bottom or wing tanks. It is unlikely
that the use of cargo tanks for salt water ballast would
be permitted, except for emergency conditions.
Combination Bulk Carriers
In order to improve the overall utilization of conventional
dry or liquid bulk carriers, combination bulk carriers have
been developed to permit transporting dry and liquid cargoes
within the same cargo hold spaces. A typical voyage, for
example, would involve carrying crude oil from the Persian
Gulf to Maine, ballast from Maine to Hampton Roads, coal
from Hampton Roads to Japan, Japan to Persian Gulf in bal-
last, etc. Cargo operations of this type involve unique cargo
handling and hold cleaning problems, with associated poten-
tial pollution problems.
Two general configurations exist, the ore-oil carrier and
the more common ore-bulk-oil (OBO) carrier. These are
analogous in function and similar in geometry to ore car-
riers and general bulk carriers, respectively, illustrated in
this section sketches in Figure 3. A common modification in

the latter case is the provision of a double skin side shell to
facilitate hold cleaning.
Referring to Figure 3, the ore-oil carrier is equipped to
carry cargo oil in the wing tanks as well as the center cargo
hold. The double bottom space is normally reserved for clean
ballast. The degree to which an ore-oil carrier can maintain
a clean ballast operation, when operating as a tanker, will
depend on the relation of cargo density to cargo volume
available. The OBO will be operated with dry and liquid car-
goes restricted to the main hold spaces, hence such vessels
will normally operate with clean ballast, as a conventional
bulk carrier. In both cases, however, hold cleaning between
cargoes is a major operational problem that will be consid-
ered in later discussions.
The largest dry bulk carrier in existence is believed to be
the 365,000 DWT ore carrier BERGE STAHL, delivered in
1986. The largest combination carrier is the 280,000 DWT
ore/oil carrier SVEALAND, delivered in 1972.
Miscellaneous Commercial Vessels
The great variety of miscellaneous and floating craft that
could be sources of pollution are too numerous to consider
here. In general, all can be considered, with respect to pol-
lution, in one of the categories considered earlier. One par-
ticular case, of current interest, however, is the development
of large, unmanned seagoing barges for the ocean transport
of dry and liquid bulk commodities. Tank barges of 50,000
DWT are in service. The geometry of a tank barge resembles
that of an austere crude oil tanker of comparable deadweight,
with five center tanks and 10 wing tanks. Operational as well
as casualty pollution hazards are comparable to those of a

self-propelled tanker, with the added complication that no
personnel are aboard when the vessel is underway.
POLLUTION FROM NORMAL OPERATIONS
Ballasting and Tank Cleaning
Break Bulk Vessels The major source of pollution from
break bulk general cargo vessels is in the intentional dis-
charge of dirty ballast. As consumables, primarily fuel, are
expended, displacement, draft and stability changes and may
reach the condition that the addition of water ballast may be
required. Some tankage may be available for clean ballast,
but, in general, ballasting of fuel tanks will probably become
necessary at some point beyond the expenditure of one half
the consumables on board.
Since the imposition of the MARPOL regulations, the
use of clean segregated ballast tanks has been mandatory and
the disposition of oily ballast at sea should no longer be a
major problem.
Tank Vessels, Crude and Refined Petroleum Products Until
the MARPOL agreements came into effect, the greatest
source of intentional discharge of contaminated ballast into
the sea was from the operation of tank vessels transport-
ing petroleum crudes and products. Tankers are normally
one way product carriers and return voyages to the cargo
source are in ballasted condition. MARPOL segregated bal-
last requirements for tank vessels were summarized earlier
in Section 2.4. The arrangement of the ballast tanks to meet
operating requirements and to provide some collision and
grounding protection is discussed later.
Tank Vessels, Special Products Carriers The special prod-
ucts carriers described in earlier discussions are predomi-

nantly clean ballast vessels. Sea water will rarely be pumped
into cargo tanks and sufficient tankage is normally provided,
in the form of double bottoms and wing tanks, to serve as
cargo tank protection as well as clean ballast tankage.
Dry Bulk Carriers The typical dry bulk carrier operates in
ballast over a significant portion of the operating life. Many
trade conditions exist in which return cargoes are not available
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674 MARINE SPILLAGE—SOURCES AND HAZARDS
and ballasting is required for adequate propeller immersion
and seaworthiness. Bulk carriers are inherently stable and bal-
lasting is not required for this purpose. As discussed earlier,
sufficient cubic capacity is normally available in the wing and
double bottom spaces to permit clean ballast operation.
Combination Carriers The most common of the combina-
tion carriers, the OBO, has the same general configuration as
the dry bulk carrier and, accordingly, is generally capable of
clean ballast operation. When in petroleum crude or product
service, the OBO operates as a tank vessel and must comply
with all relevant regulations. Hold cleaning between voyages
with incompatible cargoes, however, is an additional source
of pollution. In the example given in earlier discussions, the
OBO discharges crude oil at Portland, Maine, proceeds in
ballast from Maine to Hampton Roads and takes on coal at
Hampton Roads for delivery to Japan. During the ballast
voyage from Maine to Hampton Roads, the holds are cleaned
by conventional means and the dirty oil washings discharged
into slop tanks located in a pair of wings immediately aft of
the cargo holds. A more complex situation arises for the bal-

last voyage from Japan to the Persian Gulf, after discharging
coal. Several days of manual labor are required to remove
coal residue, followed by Butterworth cleaning to remove
the fine coal powder remaining. The solids and wash water
are discharged at sea in unrestricted zones, unless prohibited
by environmental regulations.
In general, combination carriers can be changed over
from liquid to solid cargoes in a comparatively short time,
say a period of 18 hours to two days. The reverse proce-
dure may require more time consuming cleanup procedures.
Operators, accordingly, will tend to prefer maintaining a
given ship in a single cargo trade for seasonal periods, if per-
mitted by the economics of the trade. Operational pollution
problems are reduced in complexity when the occasions for
cleaning between incompatible cargoes are minimized.
Cargo Transfer, Loading and Unloading
Liquid Cargoes Some pollution inevitably occurs as a result
of fuel and cargo oil transfer between ship and terminal or
between ship and lighter alongside. The majority of such
incidents results, directly or indirectly, from human failure.
Typical incidents include overflow through tank vents and
hose failures. In most cases, proper monitoring or automatic
control of cargo transfer and fueling operations will mini-
mize the probability of oil spillage. Normal inspection routines
should permit anticipating most equipment failures.
The rapid development of the large crude oil tankers has
been accompanied by the parallel development of offshore
terminals to accommodate the deep draft vessels. The tankers
moor to large “monobuoy” single point mooring buoys which
are anchored permanently to the sea floor. Oil pipelines are

led to the underside of the monobuoy along the sea floor, from
the shore tanks. Flexible hoses are led from the buoy to the
midship pumping station on the tanker, usually by a tending
launch. Means for mooring the tanker and connecting up to the
oil hoses are under constant development and are reaching a
high level of reliability. It is not expected that operations of
the offshore terminals, under proper control, will represent a
major source of pollution.
The lightering of petroleum crude from deep draft tank-
ers offshore to draft-limited ports is a major activity at U.S.
coastal ports. The majority of the existing shuttle tankers are
relatively small 40,000 DWT to 50,000 DWT vessels. Shuttle
tankers operate between transit vessels and terminals over
one-way distances generally less than 100 miles. In some
cases the service is limited to a lightening operation to reduce
the transit tanker draft to the allowable terminal draft.
It is anticipated that pending U.S. legislation will address
the lightering issue by allowing existing single skin transit
tankers to be served by double hull shuttle tankers discharg-
ing to U.S. coastal terminals. It is understood that the mini-
mum allowable standoff distance between transit tanker and
coastal terminal will be 60 miles.
Shuttle tankers operate on short voyages, with frequent
encounters with large transit tankers and terminals, through
heavily travelled shipping channels. It is anticipated that
requirements for environmental protection for this class of
tankers will be demanding, considering the nature of the ser-
vice and proximity to environmentally sensitive coastal areas.
Dry Bulk Cargoes Earlier discussions of combination oper-
ations included mention of solids pollution from hold wash-

ings when converting from dry bulk to liquid bulk operations.
Of far greater importance is the harbor pollution occurring
at dockside from the simple transfer, by grabs or similar
mechanical devices, of dry bulk products between ship and
shore storage facility. Over a long period of time, dry bulks
spilled between ship and pier accumulate and become a local,
but significant cause of harbor pollution. While many of the
commodities are inert, others, including coal and some ores
have an adverse effect on the ecology. The cargoes involved
are of low value and command low freight rates, hence there
is little incentive to control spillage of small, but accumula-
tive, quantities into harbor waters.
POLLUTION FROM CASUALTIES
The magnitude of a particular oil spill, or other pollution
casualty, is a function of ship type, ship size and nature of
the incident. Tank vessel collisions or groundings involve
the greatest magnitude of pollution resulting from individ-
ual casualties and, accordingly, will be considered in some
depth in these discussions. Other vessel types are considered
briefly.
Break Bulk Vessels
Pollution resulting from rupture of fuel tanks, as a result of
collision or grounding, is the only significant casualty of this
class of shipping likely to result in pollution. Less important
is the potential rupture of deep tanks carrying various special
cargo oils, primarily edible oils. Figure 1, showing a typical
section through a cargo hold, indicates that double bottom
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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 Cargo
Container, 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 Vessels
Fuel 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 ϫ 5 matrix of cargo tanks. A sixth pair of wing tanks, des-
ignated “slop tanks”, may be located immediately forward
of 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 1989Ϫ1990
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 new
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676 MARINE SPILLAGE—SOURCES AND HAZARDS
FIGURE 4 General arrangement, 215000 DWT tanker EXXON VALDEZ.
Source: National Steel and Shipbuilding Company, San Diego, California, Reprinted with
permission of Exxon Corporation, Houston, Texas.
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MARINE SPILLAGE—SOURCES AND HAZARDS 677

MACHINERY
SPACE
AFT PEAK
(SWB)
87
6
5
4
3
2
1
DT
FORK
PEAK
A
P
CARGO
TA NK
CARGO
TA NK
CARGO
TA NK
BALLAST
BALLAST
B
4
MIDSHIP SECTION
F. O .
CARGO TANK
NO. 8 P

CARGO TANK
NO.7 P
CARGO TANK
NO.6 P
CARGO TANK
CARGO TANK
CARGO TANK CARGO T
ANK
CARGO TANK CARGO T
ANK
CARGO TANK
CARGO TANK
CARGO TANK
NO.5 P
S.W. BALLAST
NO.4 P
S.W. BALLAST
NO.4 S
CARGO TANK
NO.3 P
CARGO TANK
NO.2 P
CARGO TANK
NO.1 P
MACHINERY
SPACE
F.O.
NO. 8 C
NO. 7 C
NO. 6 C

NO. 5 C
NO. 4 C
NO. 3 C
NO. 2 C
NO. 1 C
CARGO TANK
NO.1 S
CARGO TANK
NO.2 S
CARGO TANK
NO.3 S
CARGO TANK
NO.5 S
CARGO TANK
NO.6 S
CARGO TANK
NO.7 S
CARGO TANK
NO.8 S
F. O
DT
(SWB)
WL
F
P
LBP
B
15
B
D

BALLAST
FIGURE 5 Outline arrangement of typical product carrier.
Source: U.S. Department of Commerce, Maritime Administration.
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678 MARINE SPILLAGE—SOURCES AND HAZARDS
ENGINE ROOM
ENGINE ROOM
SLOP TANK
STBO
SLOP TANK
PORT & STBO
NO 5 CARGO TANK
PORT CENTER & STBO
NO 3 CARGO TANK
PORT CENTER & STBO
NO 1 CARGO TANK
PORT CENTER & STBO
PUMP ROOM
NO 4 CARGO TANK
CENTER
NO 4 BALLAST TANK
PORT & STBO
NO 2 CARGO TANK
CENTER
NO 2 BALLAST TANK
PORT & STBO
DRAFT
64.5'
987'

166'
NO 5 CARGO TANK
CENTER
NO 4 CARGO TANK
CENTER
NO 3 CARGO TANK
CENTER
NO 3 CARGO WING TANK
STBO
NO 1 CARGO WING TANK
STBO
NO 2 CARGO TANK
CENTER
NO 1 CARGO TANK
CENTER
NO 4 BALLAST WING
TANK STBO
NO 2 BALLAST WING TANK
STBO
NO 5 BALLAST WING TA
NK
STBO
GROUNDED AR
EA
Moderate to Heavy Damage
Light to Moderate Damage
Light Damage (made watertight)
FIGURE 6 Estimated grounding damage to the EXXON VALDEZ. Permission for use
granted by Exxon Corporation, Houston, Texas.
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MARINE SPILLAGE—SOURCES AND HAZARDS 679
FIGURE 7 Hitachi Zosen EPOCH double hulled tanker design.
Source: Hitachi Zosen Corporation.
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680 MARINE SPILLAGE—SOURCES AND HAZARDS
technology. As noted earlier, LNG, LPG and a variety of
special products carriers have all been built with double
hulls, as mandated by regulatory requirements.
Special Hazards in Tanker Operations In addition to colli-
sion and grounding, the following hazards to tank vessel oper-
ation should be considered as potential sources of pollution:
a) Fire—Of the many possible disasters which may
occur to a tank vessel in petroleum trades, fire is
among the most common. Should rupture of a
tank occur incidental to the fire, the cargo may
be burned, thus preventing extensive pollution.
However, gross structural failures may occur as
a consequence of fire damage, resulting in uncon-
trolled release of cargo that may not be consumed
by the fire.
b) Explosion—Despite the many regulations and
operational precautions, explosions occur when
an obscure source of ignition takes place in the
gaseous atmosphere above the liquid cargo level.
Some of the most disastrous incidents have taken
place when tankers in ballast with empty tanks not
gas-free have been involved in minor collisions.
It should be noted that the MARPOL requirements include

the installation of inert gas systems (IGS) on all tank vessels
of 20,000 DWT or greater. IGS systems consist of means for
pumping flue gas or treated gas from an inert gas genera-
tor into the cargo tanks to displace air with a combustible
oxygen content.
Special Products Carriers Casualties to the special prod-
ucts carriers described in earlier discussions involve unusual
FIGURE 8 Burmeister & Wain double hull product tanker design. Source: Burmeister & Wain Skibsvaerft A/S.
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MARINE SPILLAGE—SOURCES AND HAZARDS 681
hazards and pollution problems. The responsible regula-
tory agencies were active in studying and regulating vari-
ous special hazardous cargoes long before oil pollution was
recognized as a major hazard. Accordingly, safety codes and
regulations have been formulated and effectively enforced
for a long period of time. Regulations of the U.S. Coast
Guard, for example, which apply to various hazardous car-
goes, as well as liquefied petroleum and natural gasses, are
covered in References (9), (10) and (11).
Hazardous cargo transportation is given special treat-
ment by the designer and the cognizant regulatory agencies
by consideration of the following:
a) Materials—Where indicated by the corrosive or
otherwise restrictive nature of the cargo, special
materials, subject to rigorous testing require-
ments, are specified. Liquefied gas containment,
for example, requires special steels or other mate-
rials to avoid brittle fracture at low temperature.
Similar restrictions hold for piping and cargo

pumps.
b) Containment—As noted earlier, in all cases where
special hazards exist, it has generally been the
practice to require transport of cargoes inboard
of shell plating, that is, cargo is contained within
an inner hull formed by an inner bottom and
longitudinal bulkheads. Further restrictions in
some cases require that the cargo containment
be structurally independent of the ship’s main
hull structure. This is the case with pressure ves-
sels and rectangular tanks for carrying liquefied
gasses at low temperature and molten sulfur at
high temperature. The requirement for second-
ary containment, that is, the ability to contain the
cargo for indefinite periods of time in the event
of rupture of the primary tank, has long been
established as a basic requirement for the trans-
port of liquefied gases at low temperature.
c) Cargo handling—Elaborate safety precautions to
avoid spillage and sources of ignition are inher-
ent in established design practices and regulatory
requirements.
The above brief discussion indicates a comparatively high
level of design for safety in the case of hazardous cargoes.
In the event of a low energy collision or stranding there is a
reasonable probability that the primary containment system
will remain intact.
For the liquefied gasses, the greatest tonnage category of
special hazardous cargoes, pollution hazards are limited, or
at least of short duration, since the cargoes are volatile and

would be dissipated in a short period of time in the event of
primary tank rupture. For the other cargoes, toxic materi-
als in particular, the pollution hazard remains. Cargoes are
shipped in relatively small quantities, however, and effective
containment is readily achieved with modern construction
methods.
Dry Bulk Carriers
Fuel oil is likely to be carried primarily in deep tanks, and,
to a lesser extent, in selected double bottom and wing tanks.
Accordingly, fuel bunkers comprise a relatively small portion
of total tank space and do not represent an unusual in the
event of collision or grounding.
Combination Bulk Carriers
When transporting oil cargoes, combination carriers func-
tion as tankers. The internal arrangement of ore/oil tankers,
Figure 3, resembles the arrangement of a tanker, with
respect to location of longitudinal bulkheads at least 20%
of the breadth from the side shell. Accordingly, the extent
of cargo oil pollution from side collision is comparable to
the case of the tanker, assuming comparable compartment
lengths.
The ore/bulk/oil, or OBO, carrier, however, is designed
with high capacity cargo holds in order to transport low
density dry bulk cargoes as well as oil. As a result, the
boundary of the center holds is well outboard the 20%
breadth penetration line, for either the single skin or
smooth skin side shell arrangement. In the event of a side
collision in way of a single compartment of a nine-hold
vessel about 1/9 the oil cargo will be exposed to the sea,
compared to about 1/20 the total cargo in the case of a

modern crude tanker. In the case of a low energy collision,
the OBO with double skin side shell will offer a somewhat
greater resistance to penetration than the more common
single skin arrangement.
The double bottom arrangement of combination carries
does provide for some grounding protection, particularly in
way of the center holds. The lower outboard sloping wing
tanks of some vessels, however, are regularly used as cargo
oil tanks.
PREVENTION
Improvements in maritime safety standards have historically
followed some major disaster at sea. This was the case with
respect to loss of the TITANIC, the 1960 SOLAS conven-
tion with respect to the sinking of the ANDREA DORIA,
and now the accelerated IMO activity in the wake of the
1988Ϫ1989 casualties.
It must be emphasized that economic incentives are
the fundamental basis for the increased potential for pol-
lution due to collision and stranding. The trend to larger
tank vessels and toward simplification of the ship design,
all directed to substantial economies in construction and
operation, are not easily reversed. Yet this trend neces-
sarily increases the pollution hazard occurring from an
individual incident. In the light of the overwhelming short
term economic incentives, it is clear that all improvements
must necessarily come from the imposition of require-
ments from outside the industry; that is, from the delib-
erations of regulatory bodies at an international level and
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682 MARINE SPILLAGE—SOURCES AND HAZARDS
perhaps just as effectively, from the pressures of the insur-
ance industry.
Some suggested directions for study in improvement of
design and operation are included in the following discussion.
Operational Measures
Dirty Ballast This source of pollution should be largely
eliminated with the imposition of MARPOL regulations
requiring segregated ballast and the general prohibition
against discharging oily waste at sea.
Fuel and Cargo Oil Transfer Fuel and cargo transfer oper-
ations are, to a large extent, under control of shore personnel.
The ship’s crew is responsible for tank loading sequences
and monitoring the loading operation to avoid overflow
through tank vents. The general adoption of cargo automa-
tion systems involving existing and developmental hardware
should minimize this source of pollution.
Ship Design, Arrangement and Structure
The recently enacted Oil Pollution Act of 1990, Reference (12),
mandates that all tank vessels of 5000 gross tons or larger,
operating in waters subject to jurisdiction of the United States,
must be of double hull construction by the year 2015. The law
includes explicit phase-out time tables for retiring existing tank
vessels, depending upon year of delivery and type of cargo tank
arrangement, that is, single skin, double bottom, or double side
configuration. The law also mandates the establishment of a
research and development program, directed to improvements
in ship design and operations. For example, the following are
among the structural design measures which warrant study:
a) Structural design to increase collision protection.

Wing tanks could be designed to absorb colli-
sion energy and limit penetration. Techniques are
available to study various arrangements empiri-
cally. See References (5) and (6).
b) Structural design to improve grounding resistance.
Double bottom structural systems could be opti-
mized to enhance grounding protection. Empirical
and analytical approaches to this design objective
are reported in References (7), (8), and (9).
c) Mandatory installation of stress measuring instru-
mentation at strategic locations in the hull structure,
with indicators located on the bridge. The objective
here is to monitor cargo, fuel and seaway loading
effects that may cause unacceptable stresses.
Ship Design, Performance and Maneuverability
Maneuverability is a primary means for avoiding collision
and grounding in emergency conditions. Accordingly, the fol-
lowing observations and studies are pertinent to the overall
problem.
Backing Power The conventional crash stop maneu-
ver for the 550,000 DWT twin screw, twin rudder tanker
BATILLUS, Reference (10), required 34 minutes to stop
in 22 ship lengths. With both rudders turned outward, these
values were reduced to 28 minutes and 16 ship lengths,
respectively. Data given in Reference (11) demonstrates that
steam turbine propelled single screw U.S. flag tankers, with
displacement ranging from 25,000 to 77,000 tons, can be
expected to stop in 8½ to 9 ship lengths from the full ahead
condition and periods of 5½ to 9 minutes will be required to
execute this maneuver from the time the crash stop order is

given.
The geared steam turbines installed in the above ships
are fitted with astern elements with a capability of about 80
per cent full ahead torque and 50 per cent full ahead RPM,
i.e., about 40 per cent full ahead power. The time required to
obtain full astern revolutions, from the full ahead condition,
is on the order of one minute. Crash stop characteristics of
the diesel ship may be somewhat better, because full astern
power is available.
It has been shown that tanker sizes have grown at a far
greater rate than the installed power, hence, the stopping
ability has been reduced significantly, with the obvious
reduction in ship maneuverability.
Steering and Directional Control Turning circles of con-
ventional single screw tankers, expressed as turning circle
diameter/LBP, vary from 3 to 4. Current U.S. Coast Guard
regulations require the posting of turning circle and crash
stop trial data on the bridges of ships over 1600 gross tons.
While no minimum standards are included in international
or federal regulations at this time, it is understood that estab-
lishment of such standards is under active consideration.
Miscellaneous Safety Considerations
Regulatory agencies such as the U.S. Coast Guard regu-
larly upgrade national shipping regulations in compliance
with changes in international regulations, and in response
to changes in the nature of particular transportation sys-
tems. Current examples (mid 1990) include upgrading
of safety standards for coastal passenger vessels, and the
comprehensive Oil Pollution Act of 1990. Other current
design and operational trends that are being addressed by

regulatory agencies and classification societies include the
following:
a) Manpower reduction and automation—Crew sizes
on large oceangoing tank vessels average about 19
to 21 persons. Trends to further reductions to 12 to
15 persons within the next few years are conser-
vatively anticipated. These crew reductions must
be supported by integrated command and control
systems and condition monitoring of main and
auxillary machinery systems and components.
b) Maintenance reduction—To maintain a safe oper-
ating condition, inspection and maintenance pro-
cedures must be maintained at acceptable levels,
while recognizing the constraints of reduced
crew size.
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MARINE SPILLAGE—SOURCES AND HAZARDS 683
c) Enhanced navigation systems—Long term objec-
tives include the integration of satellite and terres-
trial navigational aids, electronic updating of charts,
and automatic position plotting. Safety objectives
include installation of grounding and collision
avoidance systems and “dead man” alarms. Problem
areas to be addressed include international standard-
ization of data format and transfer, and barriers to
electronic data transfer inherent in chart copyright
status. Research and development is required in all
areas noted above, to attain a “user friendly” state of
development and to reach the degree of reliability

necessary to satisfy regulatory requirements.
Detailed discussions of potential improvements to tank
vessel design, construction and operation are included in
Reference (13).
REFERENCES
1. Taggart, “ Ship Design and Construction ”, The Society of Naval Archi-
tects and Marine Engineers, 1980.
2. Dillon, Hoffman and Roseman, “Forty Years of Ship Designs Under
the Merchant Marine Act, 1936Ϫ1976”, Transactions, The Society of
Naval Architects and Marine Engineers, 1976.
3. National Academy of Sciences, National Research Council, “Evaluation
of the Hazard of Bulk Water Transportation of Chemicals—A Tentative
Guide,” report to the U.S. Coast Guard, 1970.
4. “Chemical Data Guide for Bulk Shipment by Water”, U.S. Coast
Guard, CG-388.
5. Chang, et al., “A Rational Method for the Prediction of Structural
Response Due to the Collision of Ships”, Transactions, The Society of
Naval Architects and Marine Engineers, 1980.
6. Reckling, “Overall Structural Response of a Ship Struck in a Collision”,
Transactions, Society of Naval Architects and Marine Engineers. Spring
Meeting/STAR Symposium, 1980.
7. Poudret, et al., “Grounding of a Membrane Tanker—Correlation Between
Damage Predictions and Observations”, Transactions, Society of Naval
Architects and Marine Engineers. Symposium on Extreme Loads
Response, October 1981.
8. Poudret, “Collisions and Groundings—Practical Analysis Methods”,
Arctic Section, Society of Naval Architects and Marine Engineers.,
March 1982.
9. Wierzbicki, Shin and Rady, “Damage Estimates in High Energy
Grounding of Ships”, presentation to Committee on Alternatives for

Tank Vessel Design, National Research Council, 25 March, 1990.
10. Laredo, Beghin and Garguet, “Design of the First Generation of
550,000 DWT Tankers”, Transactions, The Society of Naval Architects
and Marine Engineers, 1977.
11. Long, Stevens and Tompkins, “Modern High Speed Tankers”, Trans-
actions, The Society of Naval Architects and Marine Engineers,
1960.
12. Oil Pollution Act of 1990, Conference Report to accompany
H.R. 1465.
13. Tanker Spills: Prevention by Design, Committee on Tank Vessel
Design, Marine Board, National Research Council; National Academy
Press, Washington, D.C., 1991.
DONALD P. ROSEMAN
David Taylor Research Center
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