Trans RINA, Vol 154, Part A2, Intl J Maritime Eng, Apr-Jun 2012
published studies is that the smaller (feeder) vessels experience notably greater motions than the larger vessel (mothership).
Both Hong et al. (2002) and van der Valk and Watson (2005) investigated the forces and motions of both side- by-side and tandem vessel arrangements (where one vessel is aft the other) through the conduct of physical scale model experiments. A similar study using a higher- order boundary element method was conducted by Choi and Hong (2002). Each of these studies indicate that the aft positioned vessel has lesser motions due to the shadow effect of the windward vessel. The tandem position is not under consideration in the present study as it is generally used for the transfer of liquid products, not solid materials.
Recent work by some of the present authors has involved the measurement of the motions of a landing craft within a flooded well dock while in a seaway, however in this work the size of the well dock is considerably larger than the vessel inside it, Cartwright et al. (2007). This work is primarily related to military applications and unfortunately very little is presently available in the public domain. But the process of undertaking such experiments has been of considerable benefit
FHT and; (b) with the feeder vessel located inside the well dock of the FHT.
For the study with the feeder alongside the FHT, the feeder was located on the portside of the FHT with the longitudinal location defined by lining up the midships of both vessels and the bows
orientated in the same
direction, as can be seen in the photograph shown in Figure 3. The starboard side of the feeder model was attached to the port side of the FHT model using a pair of vertical slides and universal joints, refer Figure 4. The aft vertical slide/universal joint also incorporated a short horizontal slide. This arrangement allowed freedom in heave, pitch and roll, whilst constraining the feeder model in surge, sway and yaw (relative to the FHT model).
in the
development of suitable techniques and procedures for undertaking such work, which has been utilised in the present study.
4. EXPERIMENTATION
The model experiments were carried out in the shallow water wave basin at the Australian Maritime College in Launceston, Tasmania. The water depth in the basin was simulated to represent a typical coastal region having a constant depth of 15 metres. A variety of incident wave conditions and vessel headings were investigated as part of a comprehensive test program. Experimental results presented within this paper concentrate on a series of head sea tests in
nominal wave heights corresponding to 2m, 4.5m and 7.7m. A wide investigated.
In the case where the feeder was located within the well dock, the stern of the feeder model was attached to the internal end wall of the FHT well dock using a vertical slide and universal joint and the bows facing opposite directions. This allowed freedom in heave, pitch, roll and yaw, whilst constraining the feeder model in surge and sway (relative to the FHT model). The photograph in Figure 5 provides a general view of this set up. Fenders were attached to the inside walls of the FHT well dock near the entrance to limit the yaw movement of the feeder.
A simplified mooring system was adopted to ensure that the FHT model maintained the required nominal heading to the incident waves. This mooring system included a pair of mooring lines, one each from the bow and stern of the FHT model (connected at the still waterline). The stern mooring line incorporated a bridle so as to avoid contact with the feeder model.
regular sinusoidal waves having range of typical wave periods were
The primary particulars of the FHT and feeder vessel, in both model (scale 1:44) and full scale, are provided in Table 1. Results presented here are for the case with the FHT in ballast condition and the feeder vessel at half load condition. These
lighter conditions were
investigated to simulate a common worst case scenario. Full displacement conditions in general produce smaller motions; hence it is common practice for ships to ballast down during storm conditions.
The motions of both vessels were measured using a non- contact
optical tracking system based on infrared
cameras (supplied by Qualisys). Two specific cases were investigated; (a) with the feeder vessel alongside the
It is acknowledged that there would be value to also assess the mooring and restraining loads, however the primary focus of these initial physical experiments was on proof of concept through an evaluation of the relative motions of the two craft. An assessment of these loads is planned as part of further research into this concept.
5. RESULTS AND DISCUSSION A comparison of the resultant heave, pitch and roll
motions for an incident wave height of 2 metres at a heading of 180 degrees (head seas) is shown in Figures 6, 7 and 8 respectively. A range of wave periods from 4 to 12 seconds were investigated with both the feeder inside the FHT well dock and the feeder alongside the FHT.
As can be seen in Figure 6, the heave motion (at the LCG) of the FHT did not vary appreciably between the cases when the feeder vessel was located alongside or inside the well dock. In contrast to this, the heave motions of the feeder are significantly greater when it is
©2012: The Royal Institution of Naval Architects
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