Encyclopedia of Environmental Science and Engineering, Volume I and II

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  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.

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

C013_003_r03.indd 671C013_003_r03.indd 671 11/18/2005 10:40:59 AM11/18/2005 10:40:59 AM