The sequential method entails completely emptying ballast tanks of the coastal waters and refilling with open-ocean water. Emptying of certain tanks may lead to significantly reduced stability, higher vessel structural stresses, high sloshing pressures and/or reduced forward drafts which may then increase the probability of bow slamming. Margins are to be provided for stability and strength for all seagoing conditions, as specified in the vessel’s approved trim and stability booklet and the loading manual. The loading conditions for the selected ballast water exchange method or methods are to be taken from the approved loading manual or trim and stability booklet.
The following items are to be evaluated when the sequential method is selected for ballast water exchange
- Intact Stability
- Damage Stability
At any longitudinal location, a typical output of longitudinal strength includes the following:
- Calculated still-water bending moment (SWBM) and still-water shear force (SWSF)
- Maximum and minimum values of SWBM and SWSF
- Calculated still-water torsional moment (SWTM), where applicable
- Maximum permissible value of SWTM, where applicable
- Calculated SWBM and SWSF in flooding conditions, where applicable
- Maximum permissible values of SWBM and SWSF in flooded conditions, where applicable
In general, sloshing is unavoidable for a vessel using the sequential method. However, it can be controlled to within acceptable limits by restricting the sea state in which the ballast water exchange operations are conducted. Where necessary, an approach is to be proposed, subject to the approval of the Flag Administration, to establish a maximum sea state or significant wave height for partial filling of ballast tanks where sloshing occurs.
An assessment of the potential for liquid resonance in the tank as a function of the ballast water filling height. For low filling levels (less than 25%), the resonance in general is not a concern. However, confirmation is required. For filling levels greater than 50%, resonance can be an issue that may require a sea state limitation during ballast water exchange sequence.