Probability and fracture mechanics applied to ice load estimation and associated mechanics.

摘要

Ice considerations dominate the design of offshore production systems and are of great importance for sub-sea installations in arctic water areas and subareas. The design for ice loads generally requires consideration of local loads and global loads. This thesis aims to study the methods of ice load estimation and the associated mechanics due to crushing failure of ice. Multiyear ice and icebergs are the main subjects of this thesis.; The study of the associated mechanics provides the theoretical background for the methods of ice load estimation. In this thesis, the fracture of ice is studied - in particular, the types of fractures and conditions of fracture occurrence. Both experimental and numerical work are performed to assist the investigation. The experimental work is designed to study the conditions for the occurrence of different fractures. The numerical analysis is conducted to study the mechanisms of different fracture occurrence. Finite element analysis is used to aid the investigation. The investigations also include the trajectory of fractures, and consequences caused by fracture such as load reduction and scale effect.; Methods for both local load estimation and global estimation are studied in this thesis. In the local load estimation, the present design criteria based on a discrete method is first applied to analyze the field data from the 1982 Polar Sea Beaufort Sea trials. Then, a new more mathematically rigorous method is developed based on the theory of up crossing rate. The present design criteria can be evaluated based on the comparisons between the results using the two methods. Recommendations are given in this thesis based on the comparisons. The studies of global load estimation in this thesis include the method for estimation of global load governed by kinetic energy and by limited ice failure stress. In the first case, a global pressure-area relationship is applied and the parameters involved in the model are random and calibrated with existing ship ramming data. The application of this global pressure-area relationship is based on the study of mechanics as described in the previous paragraph. For the limited stress conditions, the effect of probabilistic averaging is simulated with a space-time autoregressive model. Both spatial and temporal correlations among the local pressures are considered in the model. The duration effect, pressure-area relationship and pressure-aspect ratio relationship are explored using the model. Finally, a practical model to estimate the global load for the limited stress conditions is developed to include the effect of probabilistic averaging and duration effect. The model is also based on the theory of up crossing rate and can be applied to estimate the global pressure when given information of local loads.