Localized pressures during ice–structure interaction: relevance to design criteria

Abstract

Crushing is a prevalent mode of ice failure during ice–structure interaction. This failure mode frequently occurs with vertical structures and over local areas on sloping structures. During ice crushing, the interaction zone is characterized by three distinct regions of pressure; critical zones, regions of background pressure, and areas of recently spalled ice. Critical zones may be defined as local regions of ice where intense pressures occur over short time periods. Critical zones influence significantly the crushing process. The parameters associated with critical zones are quantified by examining three types of field-scale interactions; medium-scale indentation tests, ship ramming trials of the Louis S. St. Laurent and CanMar Kigoriak, and an ice–structure interaction with the offshore structure Molikpaq. Critical zones occur regardless of the scale or type of interaction. Despite the highly random nature of critical zones, basic parameters such as zonal size, force, pressure, and spatial density are quantified. Critical zones are found to be approximately 0.10 m 2 in area and may exert forces ranging from 0.1–4 MN. Spatial densities of the critical zones, defined as the number of zones per unit meter, range from about 0.6 to 0.8 zones/m 2 and appear to be influenced by confining pressure and scale effects. Critical zones provide an explanation for the exhibited reduction in average pressure with increasing contact area. Two kinds of pressure–area relationships are presented; one in which the contact area increases over time, the other in which large, unconfined contact areas contain smaller, highly confined regions. The importance of aspect ratio in relation to the pressure–area trend is discussed. Reduction in contact area due to spalling and the orientation of the critical zones within an impacted area are examined with respect to the aspect ratio.