Article 19.03(7) and Article 19.03(9)(d) - Calculation of intermediate stages of flooding (free surfaces)
Article 19.03(7) stipulates that the calculation of damage stability is to be based
on the method of ’lost buoyancy‘.
Article 19.03(10)(a), stipulates that:
"The calculation of the free surface effect in all intermediate stages of flooding shall be based on the gross surface area of the damaged compartments."
If one uses the lost buoyancy method, that means the “loss” of the damaged part of the vessel as regards the displacement ensuring buoyancy and the moment of inertia. The waterline area and the moment of inertia reduce when faced with a constant displacement. Communication between the damaged compartment and the surrounding waters means that the water located within the damaged compartment is attributed to the vessel’s environment (flooded spaces).
That is why no free surface effect attributable to the water occurs in the damaged compartments, and there is no reason for corrections.
One possibility would be to perform the calculation using the “added mass method”. This method takes account of the “intact” hull. The damage compartment filled with water until the water level in the damage compartment corresponds to the vessel’s waterline. With this method, the waterline area and the moment of inertia remain intact, but the displacement of water increases. With this method of added mass, it is important to correct for the free surface effect.
Are the intermediate stages of flooding to be calculated using the “lost buoyancy method”? If so, the text concerning the free surface effect in article 19.03(10)(d) is to be deleted.
Is it possible to use the “added mass method” as an alternative for calculating the intermediate stages of flooding? “See also question 4a below) If so, the free surface has to be corrected for the damaged compartments.
If there are grounds for correcting the free surface for the damaged compartments (method of added mass), why not make this correction based on the “real” free surface? Currently, accurately calculating the correction based on the real free surface is no problem thanks to the use of computers. The maximum values approach is rather obsolete and dates from the time when all calculations were performed manually. Indeed, it is even difficult to force the computer to make “incorrect” calculations.
Overall, the lost buoyancy method yields a result that is a closer to reality for a major breach. In the case of a limited breach, the added mass method is more accurate at the onset of flooding but the lost buoyancy method is more accurate when determining the final equilibrium state after flooding. The added mass method is to be used for the intermediate stage of flooding. This is why this latter method is advocated for stability in the final state. The added mass method is to be used for the intermediate stages of flooding.
Even in the lost buoyancy method, a distinction can be made between net surface and gross surface The aim is to define the flooded compartment with some degree of accuracy. If it is defined only in terms of its walls, this is tantamount to calculating the gross surface area within the vessel environment (flooded spaces) If one wished to perform a more accurate calculation, corresponding to the net surface area, one would have to bear in in mind that permanently installed elements within the compartment, such as machinery or pipes, remain integral to the vessel and are not part of the compartment. If one is working with software, one would have to digitise these elements and include them in the model of the vessel. The notion of “permeability” would then be redundant. But what complexity!
Conclusion: even on this assumption, the accuracy of “calculating based on gross surface area” is useful.
The statement that “accurately calculating the correction based on the real free surface is no problem thanks to the use of computers” is somewhat exaggerated. Indeed, that would require the digitisation of elements complex in shape such as an engine and this would have to be reviewed each time an element was added to or removed from the compartment. Taking as a basis the gross surface area is tantamount to increasing it as compared with reality and thus taking as a basis a stability that is less than that which actually exists. This solution goes with the grain of safety as well as simplifying matters.
RV/G (07) 84 rev. 1
The explanations are not proposed amendments of the requirements but comments. These explanations are correct.
RV/G (07) 90
CCNR Inspection Regulations Working Group RV/G, stability, damage stability, flooding