Feature 3 | RO-RO FERRY REPORT


from 20m to 30m. Using a channel angle of 2degs, the corresponding maximum channel thickness would range from 0.5m to 0.8m respectively. Tis aligns reasonably well with the measured channel thickness, which indicates values of 30cm to 40cm for the latter two measurements, which were measured at the margins of the channel and could be expected to increase to 50cm to 60cm at the channel edge. The mid-channel thickness


corresponded approximately to the level ice thickness, in the range of 25cm to 35cm. Te old brash ice channels were typically 100% ice coverage.


Ice interactions From the observations of ice breaking formations some general correlation between the ice loads in different ice thickness and manoeuvres may be made. When navigating in level ice, the initial


point of contact was the stem. Here there was crushing of ice for a short length. Tis was followed by the bending of ice once the inclined angle of the side shell promoted bending failure of the ice. Te bending of ice started by crushing the ice edge until enough vertical bending force was created. Te inclined sides created this force and the bending failure of the ice created a cusp breaking pattern. Tis bending failure created an ice load patch which is long horizontally and of quite small height. This effect was visually observed: Figure 2 shows the breaking ice in a cusp as a result of the inclined surface of the ship acting vertically down. On failure of the cusp, the ice piece


was then displaced. The displacement depended on the hydrodynamic flow around the ship and also the interaction with the other ice pieces and surrounding ice, as well as the hull form. In general, the ice was either submerged or pushed up on the level ice. During the process of displacement, the


ice cusps were oſten broken into smaller segments, due to the interaction between the ice cusp and edge of the ice sheet, although more typically, the ice cusp and ship. This process predominantly took place from the shoulder region and towards midships. During the process, the pieces were oſten tipped onto their side, due to the hull angle, and this, coupled with the


54


Figure 14: Ice blocks being entrained into the propeller during a stern-first manoeuvre.


forward movement, pushed pieces onto the surrounding level ice. Te ice breaking pattern along the ship


was quite random; however, some general observations may be made. It was noted during the morning a trial at Lumparn that the distance the ice broke from ship side was relatively small, which is indicative of the thickness and strength of ice, but also implies a small load length. As noted previously, the average length of the cusp may indicate the load length, being larger forward and smaller midships due to the subsequent breakage of ice. While the process of


ice breaking


is a complex phenomenon, a further investigation of the ice breaking cusp pattern may provide an indication of the ice load contact dimensions. Another effect noticed during the ice trials


was the break up of ice from the resulting wave form when operating at full power in 0.17m level ice. Although this does not influence the ice loads in the forward region, it may be noted that the wave form would have been established by the midship and stern region and would be assisting in the ice breaking for these regions. During ship turns, it was observed that the


ice breaking principally occurred along the inside shoulder and outside midships to aſt shoulder regions. Te ice interaction along the outside edge


at the aſt shoulder resulted in a large amount of water spray pushed up onto the ice edge and was broken in large ice pieces. The large ice pieces were then pushed under the hull due to the movement of rotation. Aſter


breaking the ice, the wash from the propellers moved the ice pieces under the outside edge of the channel (Figure 3). Te forward shoulder on the inside edge


also broke the ice in large pieces, although these were made much smaller with the interaction with the hull at midships, resulting in a relatively straight curve when compared with the jagged edge on the outside channel edge. During the 90degree azimuth angle turns,


the ice was broken in the same regions; forward to midships on the inside edge and midships to aſt on the outside edge. Te ice interaction mechanism was much the same as the larger radius turns, except for the additional wash from the propeller, which moved the ice away, and the impact with floes on the completion of the first rotation.


Boroscope observations Te following provides a summary of the video images captured using a borescope which were made at two locations; the port azimuth space and starboard void in the azimuth space. Tese locations provided a transverse profile view across the thruster housing and an oblique fore and aſt view from the hull towards the thruster housing. Te borescope was deployed through a


customised hollow bolt (M35 HST) with a sapphire glass observation window in order to be scratch resistant and to prevent damage to the thin-walled borescope. Two bolts were used; one with a 45degree and one with a 90degree orientation. Te bolts provided 360degree viewing around their axis, thus allowing observations from ahead to astern. Video images were captured under natural daylight, although an artificial light source was also tested, utilising the borescope’s fibre-optic light path. This proved to be ineffective in the prevailing conditions due to reflections from the ice blocks and from suspended particles in the water. The main observations of propeller-ice


interaction were captured during backing and ramming runs, where the ship was backed a short distance (approximately a ship length) from the unbroken ice sheet and then driven into the level ice. One ahead observation from the azimuth


space location, which was above the waterline at rest, provided a short view of the breaking ice sheet. However, as the floe tilted under the weight of the ‘bow’ and passed along the


The Naval Architect April 2011


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