Study Of Crushing Ice Forces On Vertical Structures Based On Analysis Of The Data From Tests

The essential physical process of crushing ice force on vertical structure is analyzed, and it is concluded that crushing ice force is a subject coupled with damage mechanics of solid material and structure mechanics/dynamics. Since modern damage mechanics is not able to describe the crushing process of ice sheet accurately, it is fairly difficult to study crushing ice force using theoretical or numerical methods. Therefore, the crushing ice force problem should be investigated according to the following approach:firstly prototype and model tests are conducted to obtain the real data of ice force, structure's response and other related information; then the qualitative model of ice crushing and ice force calculation methods are developed based on data analysis; finally the model and calculation of crushing ice force are verified using data from tests.Ice mechanics and development of ice-resistant structures are briefly summarized, and then the research on static and dynamic crushing ice forces is reviewed. The topics on crushing ice force on vertical structures which need further study include:mechanical behavior of ice under compressive state, calculation of peak static crushing ice force, mechanism and model of dynamic ice force, model test on dynamic ice-structure interaction and so on, which are the topics discussed in present thesis.Because of the remarkable scale effect of ice's mechanical properties, data from prototype tests is the most reliable information for ice force research. Typical prototype tests on ice force study are reviewed firstly, and then the field test system on jacket platforms in Bohai Sea is described in detail. The research of ice force is supported by the field tests information to a great extent.Based on analysis of the field tests data, crushing ice forces on vertical structures are classified into three modes:quasi static ice force, steady state dynamic ice force and stochastic ice force. According to the phenomenon that three ice force modes take place in different ranges of ice velocity, a set of physical mechanism is developed to explain the three ice force modes:the three modes accord with mechanical behavior of ice under different loading rates respectively, which include ductile behavior under low loading rates, ductile-brittle transition behavior under intermediate loading rates and pure brittle failure under high loading rates. Besides, the essential mechanical problems involved in the three modes are discussed thoroughly. Model test is another important way to study ice force. Previous model tests conducted in many countries are reviewed briefly, and problems of similarity and scaling are analyzed. According to the prototype tests results, it is concluded that relative velocity between structure and ice sheet is the dominant parameter for similarity of dynamic crushing ice force. Design principles of model tests which aim to dynamic crushing ice force are given, and a model test system is designed and constructed in the laboratory of DUT. A series of model tests are performed and different crushing ice force modes are obtained successfully, in addition, ice forces on jack-up leg which has gear teeth are also simulated using model tests.Based on ice force data from field and model tests, static crushing ice force calculation is discussed detailedly. The scattered results from existing ice force formulas are due to simultaneous/non simultaneous failure hypothesis. The field and model tests data indicate that ductile failure or so called simultaneous failure hypothesis is valid for static crushing ice force on narrow and compliant vertical structures, accordingly, the static crushing ice force on narrow and compliant structures should be remarkably higher than conventional low level ice force formulas.There are two modes of dynamic crushing ice forces on vertical structure:steady state ice force and random ice force. The field test data indicates that ice induced steady state vibration is a type of self excited vibration, and the physical mechanism is developed to explain ice induced self excitation. It is proved that during self excitation process, compressive strain rate in ice sheet enters ductile-brittle transition range, and the formation and failure of micro cracks in ice controls ice force's fluctuation and results in lock in phenomenon. In order to develop criterion and mathematical model for ice induced self excitation, further method is presented based on the physical mechanism. Based on previous research work on random dynamic ice force, the dominant factors on frequencies of random ice force are analyzed, and it is suggested that fracture mechanics is applied to describe the brittle failure size of ice sheet, and then the frequency distribution of random ice force could be predicted.