Trans RINA, Vol 152, Part A4, Intl J Maritime Eng, Oct-Dec 2010
this valve is fully submerged a larger coefficient is needed. The different flow conditions are illustrated in the video captures in Figure. 8. Consequently, two separate values are used for this opening, depending whether the discharge is to water (Cd = 0.70) or to air (Cd = 0.41). All the applied discharge coefficients are listed in Table 2.
Table 3: Applied permeabilities Room SOLAS
side tanks
equipment room 0.85 pump room
0.95 0.85
store 0.60
Rough estimation 0.95 0.90 0.85 0.95
3.3 PERFORMED SIMULATIONS
Figure 8: Different flow conditions of the valve between the side tank and the equipment room
Table 2: Applied discharge coefficients for water flow Opening:
damage hole
hole eqp room/store 3.2 (b) Permeability
Rough estimation 0.60
0.60 0.60
Detailed analysis 0.78
valve side tank/eqp room 0.60 0.41 & 0.70 manholes 0.60 open door
0.68 0.70 0.60
The applied permeabilities are usually taken from the SOLAS
regulations without any further analysis.
However, a variable permeability is sometimes used in vertical direction in order to model the fact that most of the equipment is not usually evenly distributed.
The large tanks in the pump room were modelled as separate
rooms and thus
remaining part was increased to compensate this. The direct modelling of large
the permeability of the impermeable equipment
provides a realistic distribution of the permeability, also in transverse and longitudinal directions. All the loose objects and also the insulation were removed before the tests. Especially, the store room was practically empty (Figure. 9), and thus a very large permeability was used. All the applied permeabilities are listed in Table 3.
The hull form in the numerical model was created on the basis of a laser scanning. Before the tests, the floating position of the ship was checked from the draft marks. In addition the weight of the ship was measured after the tests when she was lifted from the dock. Based on these measurements, the displacement, draft and trim could be determined. The ship had a small initial heeling to port side. The centre of gravity was determined by performing an inclining test with all the necessary measurement and pumping equipment
installed onboard. These data determined the initial condition for the simulations.
The quality of the numerical results depends on the accuracy and reliability of the applied input data. In the case of flooding simulation, the precise values of the permeabilities and discharge coefficients are not usually known. Furthermore, these parameters are always based on some simplifications. In order to study the effect of the input data, simulations were also performed with rough estimations for discharge coefficients (Cd = 0.6) and permeabilities.
Constant time step of 0.1 s was used for the flooding case with restricted ventilation (section 4.1) and 0.2 s for the cases with slow progressive flooding. The applied convergence criterion corresponds to a water height difference of 0.01 mm. It was checked that neither a shorter time step nor a stricter criterion had any notable influence on the results.
Trim and vertical motion (sinkage) were considered to be quasi-stationary but the roll motion of the ship was calculated by assuming linear damping (ξ = 0.1) and a rough estimation for the natural roll period (Tφ ≈ 15 s).
The time that it took to open the “damage hole” valve (typically about 5 s) is taken into account in the simulations by linearly increasing the effective area of the opening.
3.4 CO-ORDINATE SYSTEM Figure 9: Empty store in the forward compartment
All calculations were done with the NAPA software, using a co-ordinate system, where heeling towards the damaged side (SB) is negative and bow trim is positive. The water level heights are presented as the vertical distance between the horizontal water level and the measurement point.
Detailed analysis 0.95 0.94 0.90 0.97
©2010: The Royal Institution of Naval Architects
A-201
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