Occurrence of Continuous and Intermittent Crushing During Ice-Structure Interaction

This paper presents analyses of full-scale data from the Norstromsgrund lighthouse in the Gulf of Bothnia. From load and response patterns, intervals with ice crushing were decided to be either intermittent or continuously crushing. Details of the initialization of intermittent crushing were studied and two different types of triggering were discovered, namely triggering with circumferential cracks and triggering with internal cracking. Another issue of the analyses was to figure out a range of drift speeds where intermittent crushing was more likely to occur. From all the selected intervals, intermittent crushing occur between 0.02 m/s and 0.08 m/s. Further studies revealed that the transition speed between continuous and intermittent crushing was slightly increasing both with increasing ice thickness and a 24 hour averaged air temperature. The reason for the increasing transition speed with increasing temperature is that both speed and temperature can cause either brittle or ductile failure of the ice. The increasing transition speed with increasing ice thickness was addressed to the failure of independent zones during continuous crushing. A reason for that effect might be that the number of zones is decreasing for increasing ice thickness and that the zones synchronize easier when the ice gets thicker, simply because there are fewer zones.