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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.

The objective of the employment of a sophisticated finite element method (FEM) program for structure fire resistance evaluation is to compliment, if not to substitute for altogether, the expensive fire tests of the natural size structural elements. Unfortunately the temperature dependent material options, provided by FEM programs, can be directly applied only to the steel and other materials with the similar hardening behaviour under compression and tension, For such materials as concrete with a hardening under compression and softening (cracking) under tension only cold numerical models exist and they cannot be used for the structure fire resistance evaluation. Instead the temperature dependent concrete model can be created in a separate subroutine and then inserted into the main program. Actually the thermoelastic-plastic concrete material was developed in two versions: first, the experimentally defined temperature functions of compressive and tensile strength. Young's modulus and thermoplastic (transient) strains were used as input data and then the standard thermoelastic stress analysis (ABAQUS) was performed. Second, the temperature dependent strength and Young's modulus were input as previously and the thermoplastic flow was computed in a subroutine following the ABAQUS algorithm for the compressive and tensile stress state taking into account the temperature dependence of the parameters involved. The computed results (displacements) were compared to the measured ones with resonable difference between.

In the process of ice-structure interaction, elastic waves propagate outward from the contact points into the depth of a moving infinite ice sheet carrying away with them a certain amount of energy.Although the equations of motion do not contain dissipative terms the damping effect occurs due to the border geometry.If not taken into account this geometrical damping can cause substantial errors in the load evaluation.Actually to demonstrate a phenomenon in the time domain, the method of potentials is used to solve a dynamic problem of the elastic half-plane subjected to the arbitrary force acting on its boundary edge.The dynamic properties of the elastic ice sheet are also studied in the frequency domain.This research, as part of a joint project, was carried out in cooperation with the Norwegian Geotechnical Institute, Oslo, Norway.

Ice field measurements around the concrete cylindrical lighthouse in the Gulf of Bothnia were carried out in the middle of March 1984 employing sensors of different design and purposes. Relatively low ice activity at the period enabled the use of linear ice/structure interaction models for evaluation of the ice loads transferred to the lighthouse as well as linear constitutive material relations when dealing with ice rheology. In the present analysis the information coming from the strain sensors embedded into ice is interpreted into forces on the basis of hereditary viscoelasticity with references to the same data from other sources. The advantages and flaws of the method are discussed.

Results are presented for several events of ice movement acting against a concrete cylindrical lighthouse in the Gulf of Bothnia. A comparison is made of stresses in the ice at several locations around the structure with those experienced by the structure. The transfer of ice stresses into force upon structure is shown.

Prediction of the structure response to the random excitation and proper interpretation of the observed data can be aided by a beforehand knowledge of the structure behaviour subjected to the certain excitation type prevailable under known circumstances.This knowledge can be also useful in determining unknown parameters of the excitation on the basis of the response measurements.The publication studies the response of the pipes and cylindrical shells to the turbulent flow excitation employing the mode superposition method.The research was carried out in the Vibration and Seismic Laboratory of CEN, Saclay, France, within the framework of the Franco Finnish scientific cooperation.