Encyclopedia of Environmental Science and Engineering, Volume I and II

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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