The flow-through method, whereby tanks are overfilled by pumping in additional water, has the advantage that it can be used in weather conditions which would be marginal for use of the sequential method, since there is little change to the condition of the ship. However, the flow-through method introduces certain other risks and problems which must be considered before using this procedure.
Safety issues related to the Flow-through Method
The parameters used when the ship is designed always take account of storm conditions and the water on deck which results. Therefore, even at maximum pumping rates, any accumulation of water on deck will be insufficient to affect stability.
Research has established that it is necessary to pump in three times the volume of the tank to achieve a 95% change of water. For the record, pumping in only once the volume of the tank produces a 63% exchange, twice the volume produces 86% exchange, while four times the volume produces a 98% water exchange.
The table below shows the time needed for the required amount of water to be pumped into each ballast tank to achieve the desired percentage change of water, and the pumps to be used.
A step by step procedure follows, listing the order in which tanks are to be processed.
After each step, a positive decision should be made, taking account of the ship’s position, weather forecast, machinery performance and degree of crew fatigue, before proceeding to the next step. If any factors are considered unfavourable the ballast exchange should be suspended or halted.