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Determination of the ballast water exchange sequence for an LNG carrier using a liquid cargo handling Simulator

favourable, capacity of ballast water eąuipment has not decreased and the number of personnel in-volved remains the same. If any of these factors is not as reąuired, the ballast water exchange should be stoppcd or completely given up. Ship’s listing, caused by unsymmetrical emptying and refilling of ballast tanks has to be taken into consideration so that each step takes place when the ship is in upright position (no list). The conducted operations have to be monitored in order not to generate lists during pumping. The steps have to take into ac-count the assumed trim and draft requirements, avoid slamming, ensure that the propeller is sub-merged and that loss of vision from the bridge is minimal.

It is very important to avoid vacuum during stripping or overpressure while refilling a tank. The exchange seąuence may be different for various ship types and different loading States - ship’s safety should be the basie criterion. Emptying morę than one ballast tanks on one side is avoidcd as this creates a risk of capsizing. Two adjacent tanks must not be pumped out at the same time due to large shear forces and bending moments. The ballast exchange seąuence is established in compliance with the regulations and restrictions in force. The seąuential method for each ship is prepared in the form of a Ballast Water Management (BWM) Plan. The plan is worked out specifically for a vessel and approved by a classification society.

Risks to ship safety during ballast water exchange

From the vievvpoint of ship stability-related safety the process of water exchange will be dan-gerous; in addition, risk will become greater in adverse weather conditions. The types of risk that occur during ballast water exchange at sea may vary for various ship types, as underlined in, Inter alia, [4] and [5], Analyses found in a number of publications indicate the major causes of risk:

-    too long time of ballast w ater exchange,

-    incorrect seąuence of tank emptying and refil-ling,

-    inadeąuate operational parametric values during the exchange,

-    adverse weather conditions (wind, high seas).

Of various operational threats, the most danger-

ous are considered to be:

-    loss or significant deterioration of stability,

-    increased ship motions, rolling in particular,

-    emergence of the propeller at too Iow aft draft, which leads to worse propulsion and manoeuvr-ing ability,

-    bow emergence, which results in slamming and worsened visibility from the navigational bridge (blind sector ahead of the ship).

If threats arising during ballast water exchange are not to decrease ship operating safety below an acceptable level, each step of the exchange seąuence should comply w ith mandatory regulations and stability criteria [6]. These reąuirements may vary for various ship ty pes and sizes. In practice, particularly in bad w eather, some reąuirements are not met.

The studies [4, 5] analyze the process of emptying subseąuent tanks and its influence on conse-ąuent changes in ship parameters, but they do not evaluate the impact of weather conditions. In their conclusions, however, the authors draw^: attention to the need for morę comprehensive analysis of ship safety during ballast cxchange by taking into consideration the effect of wavc action and ship motions. Unfortunately, such studies have not been available yet. Shipowners developing Ballast Water Management Plans for ships in service do not ac-count for weather conditions either, and the safety limits for the exchange are set after a subjective evaluation of the ships master.

The following conclusions can be drawn from analy ses of worldwide literaturę on procedurcs amd methods of ballast water exchange, research into ship safety related with such cxchangc and actual measurements conducted on ships in operation:

-    there is insufficicnt research on stability-related safety of a ship exchanging ballast water,

-    existing data do not account for the influence of real weather conditions on ship safety' during ballast w ater exchange,

-    no data are available on the evaluation of changes in ship motions during the emptying and refilling of ballast tanks,

-    phenomena of slamming and propeller emergence during ballast exchange in waves have not been analyzed,

-    no analysis has been madę in reference to the extent to w'hich alteration of course and/or speed will improve ship safety during the empty ing and refilling of ballast tanks in bad w eather.

From critical analysis of the procedurcs used

and the existing knowledge on ship safety during ballast water exchange and operational demands reported by shipowners, the following research problems can be formulated:

-    determine the relations between ship’s speed and course and weather parameters versus ship safety level during ballast water exchange,

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