In-depth | ICE-GOING SHIPS Te model was towed in two ways: bow
first to model the case when it is overtaking the fixed model (Fig. 4) with measurement of loads on the overtaking vessel; and stern first to model the situation when the towed model is overtaken by the fixed model with measurement of loads on the overtaken vessel. Te results obtained should be treated as
preliminary because they are applicable to a limited range of parameters characterising ship manoeuvres in ice channels. In addition, the stochastic nature of ice floes/ model (ship) hull interactions requires statistically representative samples of measured loads to allow evaluation of their average values and regression analysis of variable ice channel and ship motion parameters. Fig. 5 presents the graphs with some
experimental data: side force PY and yawing moment MZ data for the bow-first towing mode in the 50mm ice field. Tus, these data show the side force and yawing moment induced by the fixed vessel on the overtaking vessel. Te side force is positive in the direction from the fixed vessel to the moving vessel. Te yawing moment is assumed positive when it turns the moving vessel bow sideways from the fixed vessel. Te x-axis indicates relative time – t/Т describing the time taken by the ship models
to pass by; t - is the real time of force and moment recording. Т is estimated on the basis of model lengths (L1 – length of shorter model, and L2 – length of longer model) and towing speed V, i.e. Т = (L1 + L2) • V-1. In this case the passing of the towed model past the fixed model is characterised by the following range: 0 ≤ t/T ≤ 1. The results presented in the graphs
indicate that the side force PY and yawing moment MZ in the process of overtaking in ice channel are subject to significant fluctuations in value and changes in sign, and this pattern of parameters’ behaviour is very different from the pattern observed during manoeuvres in ice-free open water. The absolute values of lateral force and yaw moment are significantly higher when compared to the values measured on the same models at the same towing speeds but in open water conditions. If the models used in the experiments had
scale 1:30 in relation to real ice-going vessels, then experimental results should be related to the ice thickness of about 1.5m. Te scale up of test data indicates that the maximum side force in the process of overtaking can be from 1.5MN up to 1.7MN, while the maximum yawing moment may reach 85MNm. Te following conclusions can be drawn from the results obtained:
1.Te test results have provided conclusive evidence that the interacting forces and moments generated when ships are overtaking or passing each other at opposite headings in ice channels have a very different type of variation when compared to the case of well-studied hydrodynamic interactions in open water.
2. The side force and yawing moment when ships are passing each other in ice channel attain significant values, have considerable value fluctuations, and change their direction.
3. Based on the results obtained it can be concluded that the side force and yawing moment values and type of variations during manoeuvres in the ice channel are mainly dependent on speeds of passing ships, thickness and size of ice pieces, overall width of ice channel as well as the distance between ships’ sides and ice channel edge.
Furthermore, ice tank experiments
are needed to study ship interaction processes during manoeuvres in ice channels to cover variations of all relevant parameters. It is also necessary to improve test procedures further to involve factors that may have been omitted at this stage of the project. NA
Figure 5. Example of the experimental results: the side force measured on towing model during overtaking for various velocities of tow. (Dashed line refers to overtaking on open water).
20
The Naval Architect February 2009
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