The following table describes a safe sequence for the exchange of ballast water using the empty-then-refill procedure, known as the sequential method. The process requires the removal of very large weights from the ship in a dynamic situation, and then their replacement.
This is a new procedure, and a sense of familiarity with the mechanics of ballasting in port should not be allowed to induce complacency.
The table indicates the status of the ballast water in every tank at the start of each step, and indicates an assumed weight of fuel and domestic drinking water [aft of the engine room bulkhead], estimated draughts, bending moments and shear forces. The action to be taken and tanks involved in each step are then specified.
It will be noted that the original condition is restored after each pair of steps. A positive decision should be made at that time, taking account of the ship’s position, weather forecast, machinery performance and degree of crew fatigue, before proceeding to the next pair of steps. If any factors are considered unfavourable the ballast exchange should be suspended or halted.
Heeling effects due to asymmetrical emptying or filling have been taken into account so that all steps represent upright conditions. Actual operations must be managed so that lists do not develop during pumping.
The steps in the table meet trim and draught requirements of propeller and rudder immersion, to avoid any possibility of slamming while changing ballast, and to maintain the bridge visibility within tolerable limits.
It is as important to avoid underpressure in a tank due to emptying, as it is to avoid overpressure when filling.
The consequences of bulkhead damage, or even tank collapse, at sea will be even more significant than in port.
Each step has been checked for conformity with strength and stress limitations. Checks have been made that the minimum intact stability requirements of the ship are met at every stage, and that the allowable limits for bending and twisting moments are not exceeded.
Each step is therefore safe for the ship at sea in fair weather. The figure given under bending moments is the percentage of the maximum allowable at the end of each step, before commencing the next step.