Encyclopedia of Environmental Science and Engineering, Volume I and II

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.

C013_003_r03.indd 682C013_003_r03.indd 682 11/18/2005 10:41:04 AM11/18/2005 10:41:04 AM