Ice Crushing Global Ice Pressures and Contact Area Geometry

Abstract

The ice load predictors in present codes tend to be empirical. The empirical relationships are heavily influenced by data from relatively small areas and thickness ranges. One uncertainty in applying the predictors is related to the effect of different contact zone geometries, as these variations tend to get “smeared” over the whole contact zone in the present empirical predictors. An approach (termed the Two-Zone Model) was developed to investigate the effect of various contact zone geometries on global loads and pressures. The contact zone was divided into two independent zones based on their proximity to a free edge. Lower pressures were prescribed for the Outer Zone on the presumption that these would be governed by spalling and flaking of the ice. Various approaches were explored to develop the Two-Zone Model to provide insights and identify sensitivities; and two Cases were used for sample analyses. The Two-Zone model was run for three scenarios producing significantly different contact zone geometries. Of course, the results are sensitive to the assumptions made regarding the pressures within, and the extents of, the Inner and Outer Zones. The work serves to highlight some of the uncertainties involved in estimating ice loads associated with severe ice-structure events. It is hoped that it will help to point a way forward for taking more direct account of the variations in contact zone geometries created by major ice-structure interaction scenarios. Further work in the form of both modelling improvements and large-scale measurements, would be beneficial to quantify the key inputs and relationships for the Two-Zone model. In fact, exercising this type of model helps to highlight the uncertainties and emphasizes the need for full-scale data at larger areas than measured to date and over a range of aspect ratios design ice loads will be determined by the ice crushing pressure applied over the contact area. It is also usual to assume that the maximum average ice crushing pressure will decrease with increasing contact area. Such a trend is well supported by the empirical treatment of measured values from many sources including indentation tests, ship rams, instrumented structures and large scale experiments such as Hans Island. Many theoretical approaches and concepts can also be invoked to support such a trend. These include, fracture theory for brittle solids, statistical treatment of high pressures zones and multi-zone failures. Even plasticity theory can demonstrate a downward trend with indenter width. Data and observations show that contact pressures are not uniform within the contact zone, and uncertainty remains regarding how pressures are distributed within the contact zone. This is of concern because different ice-structure interaction scenarios can produce contact zones that vary significantly with respect to their width, depth and geometry. For example, the interaction of a first-year or multi-year ice sheet with an offshore structure will tend to create a contact zone that is considerably wider than its depth. In contrast, an iceberg impact is likely to generate a contact with much larger depth. A multi-year ridge interacting with a wide offshore structure would generate nonuniform contact zone conditions, as the contact zone depth would vary across the width of the structure. Although these cases could generate contact zones with the same overall area, they would differ substantially. The iceberg impact case is more confined as more of the contact zone is farther away from a free edge. Intuitively, one would expect that this would have an effect on the contact pressures, and hence, the total loads produced by the crushing interaction. There is uncertainty in applying data from one interaction shape to another.