On dynamics of ice-structure interaction: Dissertation

Ice-structure interaction is a complicated dynamic process of the failure of moving ice against an offshore structure often resulting in violent vibrations, thus endangering the normal exploitation operation. Beside the nonlinear ice crushing in the contact zone, the entire ice field behaviour as a part of the ice-structure system should be studied. In the process of ice-structure interaction, elastic waves propagate outwards from the contact area into the depth of a large moving ice sheet. These waves carry away a certain amount of energy. If the ice sheet is not infinite, the waves, reflected from the boundary, return and interfere on their way back towards the source with still outgoing waves. The ice sheet boundary conditions (finite or not) should be taken into account in any ice-structure interaction model. Actually the ice sheet presented as an elastic half-plane is subjected to the unit impulse force acting on its boundary edge. The method of potentials is used to find a Green function for a half-plane in the time domain. The convolution theorem enables one to extend this solution to the arbitrary force. The dynamic properties of the ice sheet are also studied in the frequency domain. The effect of the large (infinite) ice sheet may be described by its complex dynamic stiffness matrix and the notion of radiation damping is introduced. The stochastic approach to the problem is proposed next as an alternative to the direct material and interaction modelling. The structural response to spatially random excitation of ice crushing is studied. The notion of characteristic spatial dimensions of excitation is formulated and its effect on the structural response is examined as well. The quantitative relation between dominating ice crushing frequency, the ice sheet velocity and thickness, and the structural stiffness is found. This relation enables spectra of ice load to be established for various structures and ice environment on the basis of the available experimental or computed data. Having such a spectrum as an input excitation, the structural response can be easily found using the linear spectral analysis and the mode superposition methods, thus avoiding ice nonlinearities. Assuming an exponential correlation in space of the ice crushing forces, the spectral analysis can be applied to large structures.