Trans RINA, Vol 157, Part A3, Intl J Maritime Eng, Jul-Sep 2015
behaviour of the implemented numerical model. The step response result is shown in Figure 7, obtained applying to the ship a step heeling model of about 1500kNm. The transient stage presents the overshoot that
is almost two times equilibrium value of 5.76°: bigger than the final
Figure 4. Roll decay test.
The roll decay test in the numerical simulation model was performed assuming an initial heeling perturbation of 10° (see Figure 4); while, for the pitch decay test, a smaller value for the perturbation was set at 2° (see Figure 5). For the heave decay test, instead, the ship was assumed to be initially over-immersed of 0.20 m (see Figure 6). For clarity, the z value of Figure 6 is defined, in the inertial frame, as the vertical distance of the centre of gravity from the water-plane: a negative value of the z, means the CG above water.
Figure 6. Heave decay test.
Figure 7. Roll step response. Figure 5. Pitch decay test.
From the obtained results it is possible to notice that the qualitative behaviour, of the several motion decay tests, seems to agree with the expected theoretical overall behaviour (Crossland et alt. 1993). The heave and pitch responses present faster decay time than the roll response with a sudden damping of the oscillations: this is in accordance with the lower damping that characterizes the ship roll motion.
In addition to the decay test, also step response test was performed in the simulation, for checking the
A-156 roll
Also this final equilibrium value has been checked, by means of the hydrostatic code “Autohydro”: for the same condition the equilibrium was reached for a heel angle of 5.73°.
4.3 DAMAGE STABILITY AND LOLL ANGLE
The last applications, regarding the reliability of the results, obtained from the developed and implemented numerical method, were carried out in evaluating the final equilibrium after damage. Moreover, the angle of loll was evaluated, introducing an amount of water in the garage compartment: this condition leads to an initial negative GM, due to the high freesurface moments.
©2015: The Royal Institution of Naval Architects
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