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

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

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 1936⫺1976 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:

⫽ cargo ⫹ consumables

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

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,

some “squaring off” of the hold spaces, with the result that considerable wing space is available for ballast tanks As a

FUEL OIL

OR BALLAST

FUEL OIL

OR BALLAST

FUEL OIL

OR BALLAST

INNER BOTTOM THIRD DECK

SECOND DECK MAIN DECK

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.

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

FUEL OIL OR BALLAST FUEL OIL OR BALLAST

FIGURE 2 Outline midship section through container hold, typical container ship.

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 ⫹ 2 meters, where L ⫽ 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

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

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 −260⬚F 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 ⫺50⬚F, 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 275⬚F 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

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

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

Notes:

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 carcar-riers, 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

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 configuracombina-tion 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 proceproce-dures

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 casindivid-ualties 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

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 ⫻ 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

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 6shows, 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” tankbuild-ers 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

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.

FORK PEAK

FIGURE 5 Outline arrangement of typical product carrier

Source: U.S Department of Commerce, Maritime Administration.