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


The expected extreme roll angles for the frigate, with respect to the vessel separation, are shown in Figure 11. The reduction in roll angle with increasing longitudinal separation can be clearly seen. For example in sea state 6, at a separation of 11.13m the extreme roll of the frigate was 30.27 degrees, reducing to 23.1 degrees at a separation of 45.78m, equating to a reduction of 23.7%. The numerical predictions, whilst significantly smaller in magnitude, produced a reduction of 25.7%, a very similar value to the experiments.


Increasing the longitudinal separation tended to increase the extreme relative separation between the vessels, as shown in Figure 12. This is probably due to the RAS location point being used on the frigate; for the larger separation the forward RAS point was used, while the smaller separation used the midship RAS point. The pitch motions will have a much greater effect on the change in relative motions when the forward point is used, due to its distance from the LCG. Thus, while the extreme roll angle is shown to decrease as the frigate is moves aft, the extreme change in relative motion does not necessarily follow the same trend. The theoretical predictions only predict a small increase in relative motion with an increase in longitudinal separation and the overall


relative motions are significantly under


predicted. Overall,


the experimental and numerical results show


that significant reductions in the extreme roll are likely to occur with an increase in longitudinal separation. Increasing transverse separation has previously been found to reduce the change in relative motion [15]. However increasing longitudinal


• Since the heave resonant peak of the supply vessel coincides with the roll resonant peak of the frigate, the heave motion of the larger vessel will significantly influence the roll motions of the smaller vessel.


separation tends to


increase the relative motion, since the forward RAS point on the frigate must be used. This means that the resulting reduction in roll by moving the frigate aft is offset by the increased effect of pitch, therefore producing a resultant increase in relative motion.


It is therefore apparent that to determine the optimal vessel separation it is vital that the motions of the vessels are not considered in isolation. To study the appropriateness of a RAS configuration all three main motions (heave, pitch and roll) need to be considered for both vessels simultaneously.


to conduct


seas. This may highlight any issues with the interaction of vessel wakes with irregular incident waves. pseudo-random encounter


• With an increase in displacement of the supply vessel (minimum operating to full load) the roll motion of the frigate increased significantly. The


heavier


displacement supply vessel experienced larger heave motions around the resonant frequency, although the other motions were not affected.


• When the longitudinal separation between the vessels was increased the motions of the supply vessel changed little from


frigate These comparisons have


been conducted using a statistical approach in which a sea spectrum is applied to the vessel RAOs. It may be beneficial


further experiments in irregular The


frequencies experienced by


vessels in irregular seas may result in large magnitude waves forming due to superposition with the interacted wave forms. This may impact on the vessel motions, and is not accounted for by simply applying spectra to the RAOs. In addition the linearity of the vessel motions during RAS operations should be studied by performing model experiments in a range of wave heights.


separation condition. In contrast the heave and roll motions of the


the smaller longitudinal reduced appreciably. The


reduction in motions is probably due to the offset position of the frigate, so that the influence of the radiated waves from the larger vessel is reduced.


• When the motions of the vessels in irregular seas were examined in terms of the extreme value that would not be expected to be exceeded in a 3 hour period with a confidence of 99 percent; in sea state 6, at a longitudinal separation of 11.13m the extreme roll of the frigate was 30.27 degrees, reducing to 23.1 degrees at a separation of 45.78m, equating to a reduction of 23.7%.


The following conclusions may be drawn through comparing the theoretical predictions from a 3-D zero-


In the current investigation only the magnitude of the change in relative motion was investigated. While this characteristic will impact on the physical RAS set up in terms of the length of the span wire, the velocity and acceleration of this parameter will possibly have greater implications as these characteristics will impact on the performance of both the RAS rig and the safety of the operators.


6. CONCLUSIONS


The motions of two vessels travelling side by side into head seas, a common operating condition for naval vessels when undertaking RAS activities, have been studied. Model


tests were conducted using a supply


tanker and a generic frigate model travelling in close proximity. The following conclusions, for the vessels and conditions tested, may be drawn from the experimental study:


• The smaller vessel was found to experience


significant roll motion when travelling side by side with the larger supply vessel, with the roll RAO having a resonant peak of approximately 7.5.


A-188


©2010: The Royal Institution of Naval Architects


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