Trans RINA, Vol 154, Part A2, Intl J Maritime Eng, Apr-Jun 2012


alongside the FHT compared to when it is located inside the FHT well dock.


The pitch motions of the FHT did not vary appreciably between the cases when the feeder vessel was located alongside the FHT or inside the well dock (refer Figure 7).


located inside the FHT well dock at the nominal wave heights of 2.0 and 4.3 metres, as can be seen in Figure 9. It appears that the heave motion for both the FHT and feeder are, in general, increasing linearly with increasing incident wave height, which agrees with linear ship motion theory, Lloyd (1998).


Interestingly, the pitch motions of the feeder vessel


are quite similar to that of the FHT while it was located inside the well dock, with typical maximum values around 0.4 to 0.5 degrees. However, when the feeder was located alongside the FHT its pitch motions became significantly larger, by almost an order of magnitude, with the peak value approaching 4 degrees. In each case, the peak pitch angles occur at incident wave periods in the region of about 10s to 11s.


As might be expected, the roll motions of the FHT were generally relatively small for head sea conditions, as shown in Figure 8. However, in wave periods greater than 8 seconds, the FHT rolls more when the feeder is moored alongside than when it is inside the well dock, suggesting that the presence of the feeder alongside adversely affects the motions of the FHT. The roll motions of the feeder vessel whilst inside the well dock is similar to the roll motions of the FHT, but considered to be relatively small with values generally less than 0.2 degrees at all wave periods investigated. Of significant concern are the notable roll motions of the feeder vessel when moored alongside, which are found to exceed 3 degrees (around a wave period of 9s) which, similar to the pitch motions, is around an order of magnitude greater than found when the feeder is located inside the well dock.


In summary, the results presented in Figures 6, 7 and 8 highlight the potential reduction in heave, pitch and roll motions that can be achieved by ‘sheltering’ the feeder vessel within the aft well dock of an FHT. It is acknowledged that the motions of the feeder vessel when alongside the FHT could be controlled (reduced or potentially increased?) to some extent by the manner in which it is moored to the FHT, however, in certain circumstances this may be impractical.


Further experiments were conducted in head seas at greater nominal incident wave heights (up to 7.7 m) in order to determine the effect this has on the motions of both the FHT and feeder vessel. Cross-plots of the motions as a function of increasing wave height are provided in Figures 9, 10 and 11 for the nominal full scale wave period of 10s. This wave period was selected as the maximum motions were found to occur at or close to this wave period in the results presented in Figures 6, 7 and 8. It should be noted that tests on the case with the feeder vessel alongside the FHT were limited to incident waves of approximately 4.3m in height unacceptably high motions at higher wave heights.


due to


The heave motion of the feeder when located alongside the FHT is approximately twice that of the feeder when


The pitch motions of the FHT and the feeder vessel, when located inside the FHT well dock, are very similar over the entire range


of wave heights investigated


(Figure 10). However, when the feeder was located alongside the FHT the pitch motions are approximately eight times higher than the case with the feeder located inside the FHT well dock. The pitch motions of the FHT and feeder (when inside the well dock) can be seen to follow a general trend of increasing magnitude relative to the increasing incident


wave height, i.e. a linear


relationship exists, as was found for the heave motions. Relatively small roll angles


(<1 degree) were


experienced by the feeder vessel when located inside the FHT well dock at all three wave heights (Figure 11). In contrast, when the feeder was located alongside the FHT the roll angles were found to exceed six times this level. The roll motions of the FHT were marginally greater when the feeder was side-by-side the FHT than inside the FHT well dock.


Both the rotational motions (pitch and roll) of the feeder vessel when moored inside the well dock are significantly


less at all incident waves heights


investigated (2.0m, 4.3m and 7.7m) than those measured at just 2.0m high incident waves for the alongside case. As previously mentioned, it may be possible to utilise alternative mooring arrangements to reduce the alongside motions, however, there will be a practical limit as to how effective and safe this will be.


Further analysis of the experimental data is presently underway to investigate the relative heave motions at other critical locations of both vessels and different wave headings, as are investigations of the practicality of implementing a more substantial system to moor the feeder vessel within the well dock to further reduce the motions. It is also planned to conduct additional experiments in various irregular seaways.


6. CONCLUDING REMARKS


The concept of a Floating Harbour Transhipper (FHT) for transferring bulk cargo offshore in open seas


is


outlined and discussed, including the use of smaller shallow-draught feeder vessels to transport bulk goods and


equipment from small harbours benefits of being sheltered from incident waves. or unprepared


beaches to the FHT. A novel aspect of the FHT is the aft well dock in which the feeder vessels are moored during the transfer operation,


thus taking advantage of the


A-100


©2012: The Royal Institution of Naval Architects


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