Trans RINA, Vol 152, Part A4, Intl J Maritime Eng, Oct-Dec 2010
A stationary wave probe was positioned near the wave maker to measure the water surface profile and hence determine the incident wave elevations and frequencies. Data logging was conducted at 100 Hz for each run period of approximately 15 seconds.
A visual record of the experiments was achieved using both still and video photography. A photograph of a test run is shown in Figure 2.
4. RESULTS & DISCUSSION
The results from the towing tank experiments were converted into response amplitude operators (RAOs) with the translational motion of heave being non- dimensionalised by wave height, while angular motions of pitch and roll were non-dimensionalised using wave slope, as follows:
non-dimensional heave: non-dimensional roll: non-dimensional pitch:
ζ η η3
3′ =
η η k
4′ = 5′ = 4
η η k
ζ 5
ζ (3) (4) (5) where k is the wave number and ζ is the wave amplitude.
Figure 2: Models during RAS test in the towing tank 3.3
TEST CONFIGURATIONS
The operational profile of the supply tanker will clearly involve significant changes to its displacement due to the transfer of fuel. Whilst the supply tanker was designed to operate for the majority of its time at a specific full- load draft, replenishment ships will often be required to operate at other than optimal displacements.
In light of
this, the supply tanker was tested at two drafts to represent full load, and minimum operating to maximise the difference in displacement and the associated influence on vessel motions. The conditions tested are summarised in Table 2 and Figure 3.
All tests were
conducted at a speed of 0.87 m/s, equivalent to 14 knots full scale. The wave height was set at 30 mm model scale for all conditions tested, equivalent to 2.1 m full scale. A transverse separation between the centrelines of the vessel of 72.52 m was used in the study, this is a typical separation for RAS
operations. For this
experimental program two longitudinal separations were studied. The “short” separation was 11.13 m full scale between the
vessels’ midships, whilst the “long”
separation was 45.78 m. These distances, with the frigate aft of the supply vessel, equate to 0.159 and 0.654 m respectively in model scale.
The range of wave frequencies over which the tests were conducted there were no interference effects from the tow tank wall was calculated [18]. Due to the change in separation between the models varied with the conditions.
this frequency range
For certain frequencies three repeat runs were completed to assess experimental uncertainty. Repeatability was found to be very good;
for example for the frigate
motions the repeat runs were mostly within 0.5% of the average value.
In Figure 4 the experimental model results are compared to the numerical predictions for Condition 1. The experimental RAOs for the supply tanker are of a typical form for head seas RAOs, while when considering the motions of the frigate it is apparent that the vessel is behaving very differently to the way it would in isolation. For comparison,
the motion RAOs for the
frigate operating alone in head seas may be found in Andrewartha et al. [15]. Of most significance is the presence of considerable roll motion for the frigate, with the roll RAO having a resonant peak of approximately 7.5. This suggests that the interactions have a significant impact on the rolling motions of the smaller vessel. An important feature to note is the coincidence of the heave resonant peak of the supply tanker with the roll resonant peak of the frigate. This suggests that the heave motion of the larger vessel is a major influence on the establishment of roll motions in the smaller vessel.
It is interesting to note the increase in magnitude of the roll motion of the frigate when the supply vessel is the tanker, compared to the S-175 [15]; even though the transverse separation is increased for these tests. Although the tanker and S-175 have similar motions, the increased size and displacement of the tanker means that larger radiated waves will be produced through its heave motion, which will promote increased rolling of the smaller vessel.
A-184
©2010: The Royal Institution of Naval Architects
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