1992

Eranti, Esa

A theory of dynamic ice-structure interaction is described with transverse and torsional vibrations included. The theory is applicable to cases of pure crushing of ice against vertical structures as well as combined crushing and shearing of ice against inclined structures. The zonal ice failure approach is adapted, the structure being locally in contact with solid and broken ice alternately. Simple rules are given for the development of the zonal ice force in the interaction process. The structural response, the zonal ice force components and the global ice force are found in an iterative step-by-step procedure based on the development the theoretical distance between the zonal contact point and the ice edge. A number of medium scale ice-structure interaction tests were performed in the VTT ice tank using freshwater columnar grained ice. The mass and the stiffness of the vibrating system, pushing velocity, and the aspect ratio, shape and inclination of the contact face were varied. Highest effective ice pressures were measured for flexible systems with vertical contact face. Interaction coefficients that quantity the zonal interaction process are obtained based on the test results. Simulations of the interaction tests using these coefficients give response, acceleration and ice force records that are in good agreement with measured ones over the whole range of cases. Moreover, simulations of ice interaction with full scale structures using these same coefficients provide results that are in reasonable agreement with available field experience. The study concludes that the common practice of using aspect ratio curves or pressure-area curves to predict effective ice pressures caused by columnar grained level ice is generally invalid and may in some cases be even dangerous. Each situation is a special case of dynamic ice-structure interaction and the effective ice pressure and the structural response records depend strongly on the geometric and dynamic properties of the structure. The proposed approach provides an upper bound solution for global ice force and structural response, in the sense that it assumes pure crushing or combined crushing and shearing failure of ice, and ignores reductions that may result from other failure modes.