the general manoeuvring runs. Te location of the maximums is frame 10.5c, and the midships (although one plate instance is on Frame 7c and 12.5d), and one instance on 12.c for the frame. Te plate strains are all in the longitudinal direction except one which was in the 45degree direction. Figure 6 shows an overview of the
Figure 25: Colour intensity plot for distribution of maximum measurements in ice (for illustrative purposes and not to scale).
shell plating it impacted on the observation bolt and jammed into a recess created to protect the sapphire glass. Few clear images were thus obtained from this location due also to the amount of ice passing between the observation position and the thruster. Te alternative observation location was placed deeper and at a frame position between the thruster axis and mid-ships, thus allowing an oblique view across the propeller. Te ramming and manoeuvring runs
also created more open water around the propeller and good images of subsequent propeller-ice interaction were captured for short periods of time before the broken ice blocks obscured the propeller and reduced the ambient light. Te ‘normal’ navigation mode during the
trials was ‘stern first’, since the borescope positions were fitted close to the thruster at frame zero. When approaching the ice edge the
first interactions were through broken ice containing small ice pieces (approx ½ blade chord) which passed around the thruster housing and made contact with the propeller blades, as shown in Figure 4. Te clarity of the water reduced continuously due to suspended air bubbles, slush and smaller blocks of broken ice until the ice sheet was reached. Larger ice floes generated by hull-ice
contact travelled along the hull surface and were felt to impact on the protruding M35 observation bolt and, in the latter stages of the run, were seen to make contact with the azimuth strut before being displaced
The Naval Architect April 2011
downwards towards the propeller tips or travelling along the hull plating. These large ice floes had dimensions up to 2/3 times the blade chord by length and the ice thickness by depth. During the backing and ramming runs
the thruster was used to manoeuvre the vessel in partially broken ice, close to the edge of the unbroken sheet. In this case large blocks of ice were forced under the hull in an oblique, transverse direction towards the propeller. Figure 5 shows such a case, where the dark surface of the propeller is shown outlined, for clarity, with smaller ice pieces above its tips and with a large block of ice being entrained towards the blades. Te size of the larger blocks was of the order of the blade radius.
Preliminary data analysis A summary of the top five maximum measurements is shown in Table 2 in rank order: top being the highest. It may be noted from Table 2 that the highest measurements occur during the largest ice thicknesses. Tey are also a mixture of straight line and turning runs, with a number of occasions due to general manoeuvring. Turning with 25degs, 75/75 power setting is also the highest run for plating and framing on more than one occasion. Te highest measurements on the longitudinal members are in at the channel edge at 100/100 power, which is also the highest for the frame gauges. Te longitudinal members also generally experience the high measurements during
measurement distribution through a colour intensity plot for the runs in ice. The maximum measurements generally follow the waterline, as expected. Tere are, however, a few noticeable features, one of which is the longitudinal girder at frame 7, which shows a higher stress on the upper longitudinal member than the lower one; this may be attributed to the scantling size of the member, although the plating also shows a higher value. A later conversion to ice pressure may identify this effect in more detail. Also of interest is the frame P10.5, which shows the highest measurements. Tis may be attributed to the increased loads due to turning, since the equivalent gauges on the starboard side are lower, which would indicate the increase would be due to turning to the port side. The maximum measurements were
plotted against the measured ship speed for each run in ice conditions. Teoretically, the ice loads should increase with increased ship speed and in general this is seen for the thicker level ice conditions, but significantly less so for the thinner level ice conditions. Equally, it would be envisaged that the loads would increase with ice thickness and again this is also supported for level ice runs, but again less so with thinner ice conditions. However, it may be noted that both the channel edge and mid channel runs have the opposite trend.
Conclusions Te project is one of the few measurements undertaken with controllable pitch azimuth thrusters coupled with a comprehensive measurement system and with large combinations of trial runs. Te preliminary results have provided a unique insight into ship-ice interaction and ice loads. Te ice data measurements have also provided greater knowledge of the environmental conditions in the Åland Islands and the shipping activities in the Baltic, as well as high technology systems that may be used in ice navigation research. NA
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