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Paweł Chorab

forth by thc IMO. The plan should include. but not be limited to: detailed safety procedures for the ship and personnel connected with ballast water man-agement as reąuired by the Convention, detailed description of actions to be taken to implement thc ballast water management reąuirements and sup-plemental ballast water management practices pro-vided by the Convention.

Each ship trading intemationally should carry a Ballast Water Record Book. This record book is a document that should contain information on each discharge, exchange, or pumping in of ballast water, position of the operation, water salinity, initial and finał volumes in the tanks. pumps used, area depth. This information constitutes evidence that the BWM is obscrved and can be controlled by competent authority. The Convention also provides how and where ballast waters should be exchanged. Besides, the Convention stipulates that relevant national regulations, even if in morę detail address ballast water issues, they should not be in contra-diction to the BWM provisions.

Sequential method of ballast water exchange

The ship that exchanges ballast waters in order to observe technical standards contained in regulations of the Convention should, whenever possible, do so in an area at least 200 nautical miles away ffom the nearest land, in waters of at least 200 me-tres in depth, taking into consideration the guide-lines set forth by the IMO [3],

There are three basie methods of ballast water exchange:

-    Seąuential method: ballast tanks are emptied and then filled with replacement ballast water, one or morę at a time,

-    Flow-through method: ballast tanks are refilled with replacement ballast water that pushes out in-port or near-shore water,

-    Dilution method: replacement ballast water is filled through the top of the ballast tank w ith si-multaneous discharge ffom the bottom at the same flow ratę and maintaining a constant level in the tank.

The first of the methods described is the most commonly used in ships. The fastest and least ener-gy-consuming, the sequcntial method does not re-quirc additional technical Solutions in the existing ballast installations. Discharging and refilling of tanks, however, temporarily decreases ship's stabil-ity and other safety-related properties. In the se-quential method, particular operations make up aspecific sequence, an order in which discharge and refilling of each tank take place. The sequential method is used when the exchange of ballast is connected with the removal of a very large quantitv of water while the ship is en route and refilling the tanks with replacement ballast water in the open ocean. This is a new procedurę, different ffom the method of ballasting in the port, because at sea the ship is exposed to morę risks, particularly the influence of wind and waves.

Methods of establishing the sequence of ballast tank emptying and refilling

The method of sequential discharges and refills is quite commonly used by ships, contrary to the flow-through method. The reason is that existing ballast installations are not adjusted to, inter alia, excessive pressures when replacement water is pumped in. In the sequential method each operation is part of the sequcnce of actions planncd for an individual tank. While establishing the sequence of ballast water exchange, the following procedurę is used. Ship's operational data are first determined: trim, drafts forward and aft, shear forces and bend-ing moments of the hull. The calculations are con-ducted in the process of discharge and refilling of subsequent tanks. Thus calculated values are com-pared with criterial values, and procedurę is re-peated for each tank in tum. This manner of safety assessment refers only to ship^!s parameters in calm water. Besides, thc application of the same procedurę for each ship, regardless of its type and vary-ing ballast installation characteristics, is a simplifi-cation and not fiilly satisfactory.

According to the Convention [3], the sequence of ballast water exchange should be demonstrated at least for typical loading conditions taken from the approved Stability Information. The ballast water exchangc sequence should be divided into steps, with the following data specified in each step:

-    water volume in each tank,

-    pumps used,

-    approximatc time of operation.

-    longitudinal strength as a function of allowable

values,

-    stability information taking into account ffee

liquid surfaces during discharge or refilling,

-    draft values at fore and aft perpendiculars,

-    other information.

It is recommended that return to the initial con-dition should be possible after each step. The deci-sion to continue an operation should be taken after making surę that the predicted ship's position does not differ from the actual one, weather forecast is

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Scientific Journals 25(97)