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Ballast Water Flushing of Multiple Tanks on Board Sea-Going Bulk Carriers
Stephen D. Hall and Kathryn F. Wilson

Australian ports have large quantities of ballast water discharged into them every year, and this is a major source of aquatic nuisance species, particularly micro-organisms. To prevent the introduction of these nuisance species in ballast water, ships are required to exchange their ballast water by effectively flushing the ballast water tanks in deep water. Flushing of the ballast tanks is essentially a mixing problem in which the clean water removes and dilutes the contaminated water, so that the concentration of contaminated water is reduced exponentially. This study investigates the proportion of clean flushing received by different tanks when dissimilar tanks are pumped simultaneously. During a quarantine inspection visit to a bulk carrier, details of the ballast pumping system were obtained from ships plans and direct measurement. Flow rates were then calculated using computational fluid dynamics. The results show that, in general, when dissimilar tanks are pumped there will be a difference in flow rate and one tank will be inadequately flushed. While current Australian ballast water pumping guidelines recommend only similar tanks be pumped together, it is none the less common for dissimilar tanks to be pumped simultaneously. It is recommended that only simultaneous pumping of similar tanks, and not dissimilar tanks, be acceptable for ballast water exchange, or that the tanks are pumped for the necessary additional pumping time required to achieve the necessary dilution under ballast water management guidelines.

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