In-depth | ICE-GOING SHIPS
of vessels sailing in a limited number of navigable channels accessible for larger ships. In accordance with traditional ice
navigation tactics, vessels move in a convoy formation led by an icebreaker making its way along navigable channel through ice cover. In this case the order of ships in the convoy is fixed and established in accordance with the ice-going capability and engine power of specific vessels. Overtaking is forbidden. Icebreakers are used to restore and
maintain existing navigable channels in the Gulf of Finland when these are frozen in winter season. Tese channels in ice are relatively wide and practically have the same width as a normal ship waterway. The vessels navigating in these ice channels have different sizes and speeds; therefore vessels often overtake each other or sail in opposite directions. Te number of large oil and product tankers is significant. Accidents with laden oil tankers manoeuvring in ice channels may result in great oil spill and environmental disaster. Te pattern of navigable ice channels
made by icebreakers is changing over time. Initially, the “fresh” ice channel is filled with broken ice pieces measuring across from 2m up to 20m (Fig.1a). Later, under the action of two processed - ice freezing in the channel and continuous ice breaking by ships - the ice channel is gradually filled with small ice cake and brash ice. Te ice pieces in the “old” ice channel are not larger than 2m across. Te brash ice layer in the channel could be much thicker than the intact level ice around the channel (Fig.1b). Navigable ice channels of different widths experience a similar evolution in their patterns. In analysing the safety of navigation
in an ice channel with manoeuvring constraints, it is necessary to consider ship interactions in the cases of opposite and overtaking movement. Te hydrodynamic interactions between vessels in open water have been studied in sufficient detail, but the effects related to distance from an ice channel’s boundaries and ice pieces in the channel, which fill space between ship hulls, have not been investigated so far. Under these conditions, the decisive factor is the effect of ice pieces on the ship’s
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Figure 3. Schema of experiment in the ice-towing tank: narrow (a) and wide (b) variants of the navigable ice channel.
Under the MS GOF Project, the special-
purpose model investigations regarding ice effects on an overtaking ship in an ice channel were carried out in the Ice Basin Laboratory of the Krylov Shipbuilding Research Institute for the first time in the world. Te experiments were performed on two special ice-going ship models of L1 = 3.8m and L2 = 3.0m. Te longer model was fixed, while the other model was towed by tank carriage. Te loads (forces and yaw moment) were measured only on the towed model by a dynamometer mounted on the carriage. The studies focused on the navigation scenario in the fresh ice channel. Two ice fields of 30mm and 50mm thickness were modelled. Before each experiment, a channel was
Figure 4. General view of test in the ice-towing tank: the narrow ice channel variant.
hull combined with the hydrodynamic forces and moment. The ice forces and moment are expected to play the dominant role in the interaction of vessels in ice channels because ship speeds in these channels are low in relation to open water conditions. Te experimental and theoretical studies initiated in the MS GOF Project focus on the ice forces and moments versus the ice cover properties, ice channel dimensions and kinematic parameters of ship motion. A prior analysis of emergency situations,
which may arise in ice channel, indicates that apart from mutual attraction of ships known from open water experience some other effects may take place. First, one or both vessels could be pushed against the ice channel edge when overtaking (Fig. 2a) or passing by in opposite directions (Fig.2b). Tis incident can be caused by a significant yawing moment, which is not compensated by rudder deflection in a timely fashion.
made throughout the full length of the tank. Te ice in this channel was broken into pieces of approximately equal size. The fixed ship model was positioned either by the channel edge, one meter away from the tank side (Fig. 3а), or by the tank side (Fig. 3b). In the first case a narrow channel was modelled, while in the second case a wide channel was modelled. Te tests were carried out at towing
speeds of 0.1m/sec, 0.2m/sec and 0.3m/sec. In the narrow channel case, the distance between the model’s sides was 0.51m and the distance of the towed model from the ice edge on the opposite side of the channel was 0.2m. In the wide ice channel case, the distance between model’s hulls was enlarged to 1.80m and distance of the towed model from the channel border was 0.4m. Aſter each test run the ice pieces forced apart by model hull were brought back to ensure their uniform distribution on the channel surface. The towed model was connected to
the carriage via a three-component dynamometer to measure two forces (resistance and side force) as well as yawing moment in the model water-line plane. Upon completion of tests in the ice channel, the models were tested in open water at the same speeds to measure models’ interaction in open water. Te most interesting part in the analysis of ships’ interaction in the ice channel is the variation of the side force PY and yawing moment MZ at the time when model hulls are passing each other.
The Naval Architect February 2009
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