ERDC/CHL CHETN- IX-7
December 2001
waves were all unidirectional. Unidirectional waves were used because wave transformation
processes would be more important in the entrance channel than wave directionality. Since it is
a well-known fact that ships respond differently in a directionally-spread wave environment, it
would be a reasonable next step to examine the effect of directional spreading with a
multidirectional wavemaker.
Laboratory Scale Effects: One of the goals of these laboratory experiments was to
demonstrate that laboratory scale effects do not significantly impact the measured results. The
wave forcing for the Barbers Point data set was small, with prototype wave heights in the
channel in the range of 0.30 to 0.58 m. One would not expect large ships like the World Utility
to respond much to such a small wave height. Indeed, this was the case as the field
measurements indicated vertical motions at the CG in the range of hWA,CG = 0.05 to 0.17 m.
These measurements correspond to a maximum vertical response of 30 percent (i.e., 0.17 m/
0.58 m) of the wave height.
The worst case for the laboratory data was an underprediction of 25 percent for the outbound
fully-loaded World Utility transit. When compared to the larger laboratory dataset for all the
outbound fully-loaded transits, the underprediction was reduced to only 12 percent. Many of the
laboratory values of hWA,CG were actually larger than the field values. In general, the laboratory
is a conservative predictor of the maximum vertical motion as they are all larger than the
corresponding field values.
The fact that the laboratory values were within an order of magnitude of the field values for such
small wave heights is fairly remarkable. Most of the time ships experience larger wave heights,
up to their design limits, during transits in entrance channels. The prototype wave heights
measured at Barbers Point were smaller than the maximum possible values. Larger wave heights
in the laboratory will only reduce any possible laboratory scale effects due to signal to noise or
measurement tolerances. Thus, considerable confidence can be placed in this system as a tool
for accurately measuring and predicting ship vertical motions.
SIGNIFICANCE TO UNDERKEEL CLEARANCE: The field data are based on an average
from the three GPS sensors at approximately the vessel CG. All of the comparisons so far have
been for wave-induced vertical motions at the CG. The CG does not experience as much vertical
motion as other locations on the ship because it does not incorporate the wave-induced, vertical
motions due to pitch and roll. Ship locations such as the bow and stern, and the forward and aft
port and starboard sides experience the most hWA. When calculating ships underkeel clearance
hUKC, these locations where the larger values of vertical motion occur should be used. Presently,
we have software to calculate hWA at these other locations only for the model ship (plans are to
develop this capability for the field data). As an example, the hWA,Bow at the ship's bow for the
laboratory data from Figure 12 was calculated and is shown in Figure 14. In general, the hWA,Bow
are approximately three times larger than the corresponding hWA,CG values and exhibit the same
trend. Thus, there is a significant difference between the wave-induced motion at the CG and the
bow.
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