Ice Load Identification And Ice-induced Vibration Simulated By Physical And Numerical Tests

Dynamic interactions of sea ice with offshore structures, ice-induced vibrations and ice loads identification, are still the hot and key problems in ice engineering. However, due to the complexity of sea ice mechanics, a widely accepted physical model describing the mechanism of dynamic ice-structure interactions and ice-induced vibrations has not yet been well developed, especially the methods for identifying dynamic ice loads are not so satisfactory. Therefore the main work of this thesis is to deal with the following topics: (1) methods for identifying dynamic ice loads during the process of ice-flexible structures interactions; (2) Simulation of ice-induced vibrations of flexible conical structures by laboratory tests; (3) Numerical simulation of ice-structure interactions by DDA method. Accordingly the contents of this thesis include three parts as follows.There are two ways to get the dynamic ice loads in ice engineering, i.e., direct measurement and indirect identification. Both of them have advantages and disadvantages. Chapter 2 mainly treats with different methods for dynamic ice load identification. First, several kinds of methods in frequency and time domain for identifying dynamic ice loads are introduced, summarized and compared. Then the emphasis is concentrated on improving of the present methods in time domain. These methods have the advantage that they can provide time history of the unknown dynamic force directly, with no need performing Fast Fourier Transformation (FFT), and therefore are welcomer by engineers compared with those methods in frequency domain. However, the methods in the time domain also have disadvantages that the processes of calculations are much too heavy and complicated, as well as with no strong anti-noise ability. So based on more reasonable assumption that the unknown external forces vary linearly within the discrete time interval, an improved method for dynamic load identification for proportional damping system is presented by authors. A set of concise recursive formulae for determining the values of these forces is derived, only any kind of structural response: displacement, velocity, acceleration or strain needed for the input. Practical examples show in detail that compared with other formulae given in recent literature, the approach given by the authors is simple, effective, with strong anti-noise ability and can be applied to identify all kinds of dynamic loads in the engineering.Performing laboratory tests is an effective way to explore the mechanism of ice-induced vibrations of conical structures. So Chapter 3 introduced a laboratory test program designed by the authors for the simulation of ice-induced vibrations of a flexible conical structure, in which DUT-1 model ice sheet are driven by the multi-function trailer at different velocity. The elastic modulus and flexural strength of both ice sheet and ice samples are measured. The responses of the model structure, including accelerations, displacements and strains are measured and some typical photos recording the ice failure process are presented. According to the method of dynamic load identification developed by the authors in Chapter 2, time histories and Fourier spectrums of the global ice loads exerted on the conical structure model are given. From the measured and calculated results, some conclusions are drawn, i.e., The ice-induced vibration of aflexible conical structure is a kind of intermittent free-damped vibration after a forced vibration, dominated by modes of low frequencies; The predominant failure mode of DUT-1 ice sheet before the conical structure is bending failure, mixed with other modes such as crushing and shearing failure, which are illustrated by typical photos taken on the spot; the magnitude of the identified ice loads coincides with that of the measured ice force in Bohai Sea, However, variation of ice loads with time and velocity, while other parameters are set, cannot be expressed by a simple function; The trend of maximum ice load increasing with the ice velocity is obta...