Trans RINA, Vol 153, Part A1, Intl J Maritime Eng, 2011 Jan-Mar


X Kong, GVA Consulting AB, Sweden The authors’ work focuses on the


AUTHORS’ RESPONSE comparison of


experimental and numerical investigation of a naval ship subject to progressive flooding. The significance of this kind of work is notable. Firstly, the complex geometry and/or interactions between to exterior and interior flow domains make CFD simulation impossible for practical time consumption. Besides, the flooding flow involves salient viscous effects, for instance flow separation. This can introduce additional error if one uses the Froude scaling to perform model test.


This study takes advantage of exterior domain in calm water condition and non-violent flow (therefore assuming quasi-steady) in the damaged compartments due to flooding-induced ship motions. However, it is still expected the ship can have two-time scale motions, with one caused by floodwater loads on the ship and with the other by the ship’s own motions in ‘calm’ water (actually the ship motions and flooding will definitely excite waves). This may explain some inconsistence between the presented results. Fortunately, the tank’s sloshing period (assuming length and depth are 8 and 4meters) is much lower than the ship’s natural


roll period.


Otherwise, the nonlinear or interaction effect has to be considered.


A Scott, Maritime and Coastguard Agency, UK


This is an extremely interesting and valuable piece of research which will increase overall confidence in the use of numerical flooding simulation as a method of assessing


real-time survivability. It is quite a rare


opportunity to be able compare simulation results with a full-size ship rather than a model and the high degree of correlation is impressive. The variation of discharge coefficient between air and water is interesting and will be of help with future computer modelling and simulation work, as will be the effects of air pressure, permeability and local


structure on the degree of


correlation. It is encouraging to see that even the rough method gives quite good correlation indicating that the theoretical basis of this method is sound. I really have no negative comments and congratulate the authors on an excellent piece of work. The only question


I have is


whether any further confirmatory work on more complex flooding scenarios is planned?"


REFERENCES


5. Papanikolaou, D. Spanos, E. Boulougouris, E. Eliopoulou, A. Alissafaki, Investigation into the Sinking of the Ro-Ro Passenger Ferry Express Samina, Proc. of the 8th Inter. Conf. on Stability of Ships and Ocean Vehicles, Madrid, Sep., 2003, Journal of International Shipbuilding Progress, Vol.51, No.2-3, 2004


Mr Schreuder asked about the relative impact of the parameters. The applied permeabilities had much smaller effect that is visible especially in the final condition. The discharge coefficient of the valve between the side tank and the equipment room had the most significant effect on the results. This opening was clearly a bottle neck that affected the flooding rate to many other compartments.


Dr Valanto, Dr Spanos and Mr Ypma brought up the discharge


coefficient for the second valve, located


between the side tank and the equipment room. Direct analysis of the effective discharge coefficient, based on the measured water heights, is not possible since the flow rate in this valve was not measured. That is the reason why experiments with the valve were carried out in the laboratory of the Water Engineering Group at Aalto University. The results of these tests are presented in Figure. 20. As the discussers noted, in reality the discharge coefficient is a function of the pressure height,


Firstly, we would like to thank all the discussers for their comments and interest in the paper. As noted by Dr Corrignan and Dr Valanto, the presented study was limited to still water condition due to practical reasons. Furthermore, due to the small size of the damage hole, it was rightfully assumed that there would not be large transient motions. Consequently, roll decay tests were not carried out either for the intact or damaged ship. The authors agree with Dr McTaggart that this remains a significant


challenge for seakeeping analyses. Also,


comparative CFD calculations for the flooding process were not performed. This would have been very interesting, but also very time-consuming and expensive.


Dr McTaggart asked also about the computing times. On a modern laptop the simulation of the most extensive flooding case takes about 12 min (i.e. 4 times faster than real time). For simpler flooding cases the relative computing


speed is much better. Naturally, faster


calculations can be achieved with a longer time step. Mr Ypma asked about the effects of applied time step and convergence criterion. These parameters were selected so that it was certain that they had no effect on the results. In practice, somewhat larger time step might be applied. Nevertheless, it is a good practice to try a shorter time step and a stricter convergence criterion in order to ensure that the iterations are fully converged and the numerical error does not accumulate.


The principal idea behind using the rough estimations was to test how accurately a typical flooding simulation, based on values in literature, corresponds to the measurements. After all, the ship designer or accident investigator can only do simulations that are based on simplifications and assumptions. The detailed analysis for discharge coefficients and


permeabilities was


performed in order to validate the applied simulation algorithm, using as accurate input data as possible.


A-66


©2011: The Royal Institution of Naval Architects


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