Encyclopedia of Environmental Science and Engineering, Volume I and II

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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