Stemming the Tide Controlling Introductions of Nonindigenous Species by Ships' Ballast Water (1996) / Chapter Skim
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2 BALLAST WATER AND SHIPS
2 BALLAST WATER AND SHIPS
Pages 22-31
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From page 22... ...
This chapter, which provides information on the role of ballast in ship operations, is a necessary prerequisite to the committee' s assessment of proposed strategies for managing ballast water and controlling the introduction of nonindigenous aquatic species without compromising ship safety. A brief summary of ship types and ballast systems is followed by an overview of safety issues relating to ballast operations and typical ballast conditions at sea and in port.
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From page 23... ...
The capacity, location, and flexibility of use of ballast tanks is a focal point in ship design. Consideration of required drafts and trim, hull loading limitations, and required vertical center of gravity establishes the necessary ballast volume and location.
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From page 24... ...
displacement, ship owners tend to use as little ballast as is necessary for the ship's safe, efficient passage when operating either with ballast or in ballast. SAFETY Ballast water is taken on board vessels to achieve the required safe operating conditions during a specific voyage or portion of a voyage.
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From page 25... ...
, particularly under heavy weather conditions; control its draft and trim for maximum efficiency; and control its stability to ensure safe passage. Related factors that determine ballast conditions at sea are summarized below.
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From page 26... ...
. Sailing with Full Tanks Ballast tanks used for controlling trim or heel, some fuel oil tanks, and tanks containing fresh water for domestic use, may be partially full at sea, depending on the stability and strength requirements of the ship.
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From page 27... ...
BALLAST CONDITIONS IN PORT Ballast operations are carried out in port to maintain ship stability, as discussed above. Ballast operations in port also maintain both the clearance under cargo loading or cargo discharge facilities and the under-keel clearance so the vessel remains safely afloat; maintain the hull bending moments and shear forces within safe limits to avoid the catastrophic damage that can result from incorrect loading; and maintain the ship upright by trimming or heeling the ship.
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From page 28... ...
Safe Longitudinal Loading Considerations The shear forces and bending moment on the hull of a ship are established by the distribution along the ship's hull of the difference between the light ship weight, together with the cargo, fuel, ballast and other deadweight items; and the supporting buoyancy force. In a seaway, buoyancy support forces are subject to change as waves move along the hull and as the hull moves relative to the sea surface.
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From page 29... ...
Ballast Condition for Voyages When ballasting a ship for a voyage, the crew, under the direction of the master, defines the amount of water required taking into account the loaded condition, route, predicted weather conditions, and the need to complete the voyage in a safe and efficient manner. In the future there may be some possibility of modifying ballast systems in new ship designs to facilitate cleaning and improve the safety of changing ballast at sea, but the complete elimination of ballast is not currently practicable (see Appendix D)
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From page 30... ...
As was noted earlier, some unpumpable ballast water always remains on board.2 This unpumpable water may form a virtually permanent layer on the bottom of a dedicated ballast tank, with the concomitant capability of supporting 2The amount of unpumpable ballast water remaining on board varies greatly because of vessel diversity. One report notes that vessels claiming to have only unpumpable ballast on board were carrying, on average, 157.7 metric tons of water and sediment (Weathers and Reeves, 1996)
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From page 31... ...
Inbound vessels that have released their ballast water prior to or during cargo loading, and outbound vessels with full cargo loads, may have relatively little ballast water remaining such that the mariner would report a ballasting condition of "no ballast on board." Sediment frequently accumulates on the bottom and on many horizontal surfaces in ballast tanks. Sediment may include the settled mud (silt and clay)
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