What is Sequential Method of ballast water exchange and factors taken into account?

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

  1. Stability
  • Intact Stability
  • Damage Stability
  1. Longitudinal Strength

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
  1. Sloshing

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.

  1. Free Surface Effects

When using the sequential method, the free surface moments of all tanks are to be set to the maximum for stability and strength calculations.

  1. Forward and Aft Drafts

Forward Draft

In all sequences of ballast water exchange, the forward draft of the vessel is to be greater than the prescribed minimum forward draft as shown in the Trim and Stability Booklet. If, for a transitory period or in a specific sequence, the minimum forward draft cannot be satisfied, the Ballast Water Management Plan is to identify the stages of the exchange process that do not meet the criteria for minimum forward draft, and a footnote documenting this is to be assigned to the relevant stages. The following is an example of such a footnote:
“Exchange of ballast in tanks …. is to be carried out in favorable sea conditions such that the risk of forward slamming is minimized (Beaufort Force less than or equal to 7).”

Aft Draft

Full propeller immersion is assumed as the minimum requirement for the aft draft.

  1. Propeller Immersion

The aft draft of the vessel in each sequence of ballast exchange shall be adequate to fully immerse the propeller. If, for a transitory period or in a specific sequence, 100% propeller immersion can not be satisfied, the sequence is to be identified in the Ballast Water Management Plan with a footnote documenting this limitation. The following is an example of such a footnote:

“At this stage of the ballast water exchange operations, the full immersion of the propeller is not possible. This may be considered satisfactory provided the ballast water exchange operations are undertaken in fair weather and sea conditions – such that the Master will be satisfied with the control of the vessel’s maneuverability, and that there is sufficient ballast capacity available at all stages of the voyage to enable propeller immersion in a reasonably short period of time should weather and sea conditions begin to worsen.”

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About the author

Amit Sharma

Graduated from M.E.R.I. Mumbai (Mumbai University), After a brief sailing founded this website with the idea to bring the maritime education online which must be free and available for all at all times and to find basic solutions that are of extreme importance to a seafarer by our innovative ideas.

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