The practical method to minimize the introduction of unwanted organisms from the discharge of ballast water. The exchange procedure shall be carried out in an “open ocean condition” at least 200 nautical miles from the nearest land and in waters at least 200 metres in depth. It can be accomplished by either the sequential empty-refill method, by flow-through method or by or dilution method whereby tanks are overfilled by pumping in additional water. Due to limited biological efficiency the ballast water exchange at sea is to be regarded as an interim measure.
- Flow-through method – This method involves pumping open-ocean water into a full ballast tank. Ballast equal to approximately three times the tank capacity must be pumped through the tank to achieve 95% effectiveness in eliminating aquatic organisms. Applying the flow through method does not alter the stability, stress, and ship attitude.
- Sequential method – This method entails emptying ballast tanks completely and refilling with open-ocean water. During ballast water exchange sequences there may be times when, for a transitory period, the criteria for propeller immersion, minimum draught or bridge visibility cannot be met. Emptying of certain tanks may lead to major reduction of stability , higher stresses, high sloshing pressures and increased probability of bow slamming.
Further reading: MSC.Cir. 1145 Precautionary advice to masters when undertaking ballast water exchange operations, ABS Advisory Notes on “Ballast Water Exchange Procedures” – can be downloaded from www.eagle.org