discharged into the same environment from whence it came. Although this is an extremely attractive scenario from a commercial perspective (full ships at all times), this approach to controlling introductions of nonindigenous species through ballast operations would be difficult to implement in practice because of different cargo requirements, the geographic separation of loading and distribution points, and the imbalance of trade.

Under certain circumstances many types of ships—some container vessels, for example—can retain ballast water on board by moving it within the ship's ballast system to compensate for cargo deployments that affect vessel trim and stability.

IMPROVING BALLAST CHANGE

Currently, one of the best available options for controlling ballast water is changing it in open ocean by deballasting and reballasting. A new ship design with increased structural strength and stability would make at-sea ballast change safer. This option would require that the proposed plans for changing ballast water be approved by the vessel's classification society and the regulatory authority of the flag state.

If the strength or stability of a vessel cannot sustain complete discharge and refilling of ballast tanks at sea, the continuous flushing method for changing ballast water change could be used. Some typical design modifications that could be used are as follows:

  • Tanker-type hatches could be fitted to the weather deck in way of the topside tanks, and the topside could be interconnected with the lower wing tanks. This modification would be suitable for bulk carriers.

  • This design would be the same as above, but instead of fitting tanker hatches, larger ventilation pipes would be fitted with cross-sectional areas capable of handling flows equal to the largest ballast water pumping capacity.

CLEANER BALLAST TANKS

To assist in the complete removal of ballast water and sediment, the internal features of the ballast system structure could be designed to facilitate draining of ballast water to the ballast suction and stripping suctions. Horizontal, flat longitudinal surfaces could be fitted with drain holes located along their length and adjacent to vertical surfaces. Alternatively, horizontal surfaces could be designed to shed water or sediment by slight angling downwards, with the return angles or face angles fitted to the underside of the longitudinals. Smooth tank coatings would also reduce clingage and allow for better discharge of ballast sediment.

For vessels that carry small quantities of ballast water in relation to their size, the double-bottom or side tanks could be fitted with additional divisional



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