Trans RINA, Vol 152, Part A4, Intl J Maritime Eng, Oct-Dec 2010


similar to those for the lighter displacement, although with notable exceptions.


The experimental heave and


pitch motions of the frigate are similar to those with the lighter tanker displacement, but the roll motion of the frigate has increased significantly from a resonant peak of 7.5 to one of over 9.0. This clearly demonstrates that the operating displacement of the supply vessel can have a significant influence on the motions of the smaller receiving vessel.


The numerical predictions again correlate well with the experimental results for the supply tanker heave and pitch motions. Interestingly the theory has predicted a significant reduction in roll motion of the tanker with the increase in displacement; again the actual roll motions of the tanker are insignificant.


The numerical heave and pitch frigate RAOs match relatively well with the experiments, but once again there are significant oscillations in the


predictions at


Figure 3: Test configurations including tanker loading (full scale)


a = 72.52 m, b = 11.13 m, c = 45.78 m (model scale)


a = 1.036 m, b = 0.159 m, c = 0.654 m


Comparing the experimental results with the predictions it appears that the theory may be over predicting the viscous roll damping of the frigate resulting in a reduced roll magnitude, although


the resonant


frequencies greater than the resonant peak. The magnitude and frequency of the resonant peaks are predicted quite well. The frigate’s roll motions are again under predicted; this may stem from over estimation of the viscous roll damping, or inaccuracies in the predicted radiated and diffracted waves emanating from the supply vessel.


A summary of the theory’s performance in predicting the motion RAOs as determined through the experiments is shown in Table 2.


frequencies


correlate reasonably well. The under-prediction may alternatively stem from inaccuracies in the prediction of the diffracted and radiated wave patterns and hence the interaction effects. The theoretical heave and pitch RAOs for the frigate correlate relatively well with their respective experimental results. However there are some fluctuations predicted which were not apparent in the experiments; there also appears to be a small shift in both the heave and pitch resonant frequencies.


The correlation for the supply tanker heave and pitch motions is excellent apart from at the lowest frequencies. The peak magnitudes and frequencies line up well with very little deviation experimental results


between the theoretical and across the


frequency range


examined. The roll motions of the tanker are not well predicted.


Numerically a relatively large peak is


apparent at a wave frequency of approximately 0.45 rad/s which is not visible in the experimental data. Clearly the theory is predicting that the interaction between the vessels will cause the tanker to roll, but this does not occur in reality with the interaction effects on the tanker being minimal.


With an increase in the displacement of the supply tanker, the results for Condition 2 (see Figure 5) are


©2010: The Royal Institution of Naval Architects


The numerical method can remove the effects of irregular frequencies by adding an ‘inner free surface’ panel layer on the waterplane of the vessels similar to the treatment proposed by Lee and Sclavounos [19].


The


calculations presented in the paper used this technique; however the


effect of not removing frequencies was found to be insignificant.


Another possible explanation for these oscillations could be the simulation of resonant waves trapped between the two vessels whilst they are in close proximity. Since the velocity potential theory cannot directly simulate any effects of viscosity and wave breaking, it predicts an exaggerated trapped wave and obtains an over estimated RAO at these frequencies. One of the possible options to decrease these trapped wave effects would be to add an artificial damping patch on the free surface between the two ships.


The effect of supply ship displacement is more clearly shown in Figure 6 which contains experimental heave, pitch and roll RAOs for both the supply tanker and the frigate for Condition 1 (tanker operating at minimum operating (MO) condition) and Condition 2 (tanker operating at full load (FL) condition).


Heave is the only motion mode of the supply tanker which is significantly affected


by the increase in the irregular


A-185


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