Equivalent ice thickness for evaluating ship resistance in ice

• Main Author: Niiler, Hardi
• Other Authors: Suominen, Mikko, Insinööritieteiden korkeakoulu, Kujala, Pentti, Aalto-yliopisto, Aalto University
• Format: Master Thesis
• Language: English
• Published: 2015
• Subjects: equivalent ice thickness; ship resistance in ice; Antarctic ice thickness measurements; stereo camera system; actual thrust; Antarctic; The Antarctic; Antarc*; Sea ice
• Online Access: https://aaltodoc.aalto.fi/handle/123456789/16335

The current thesis investigates the Antarctic sea ice and found an equivalent value for evaluating ship actual resistance in ice. The full-scale measurements were conducted on board S.A Agulhas II in the Antarctic waters during 2013-2014. For ice resistance calculations the Lindqvist’s method was used. The equivalent ice thickness concept was taken as the geometric method, where all the ice forms in some certain area are taken into account and the result is given by one simple thickness value. The equivalent ice thickness of the ice was found using the real measurements data for level ice thickness, ridges height, snow thickness, ice concentration and the porosity of ridged ice. The work was based on the ten pre-selected cases, where each case was one hour period. Ice thicknesses were measured by stereo images, electromagnetic sounding and visually for all ten pre-selected cases. These three method’s results were compared by probability distributions and they show quite similar results, which confirms that the results are realistic. Ridges heights were measured from electromagnetic sounding figures. Snow thickness was measured in stereo images. The ice concentration was taken into account on the basis of visual observations. The calculated resistance of equivalent ice thickness has been compared with the ship average actual thrust, which was measured from the shaft line. After that, four suitable cases are selected for more detail analysis, where the periods were shortened to get smoother navigational parameters for ice resistance calculations. The results indicate that the ship actual thrust is similar with calculated ice resistance, although the final phase of the work the four different cases results have differed up to 33%. This result can be considered as good as the ice conditions were not as smooth as they might be and ice mechanical properties were not measured as tight as the thicknesses of the ice.