Structural Design And Validation Of Umbilical Of Subsea Production System

The subsea production system is an effective and economic development mode for the deepwater offshore oil exploitation. With the rapid progress of the oil development in the South China Sea, it has a good application prospect. Dynamic umbilical that connect the top floating platform and the subsea device is one of the key equipments of the subsea production system. It provides power, data transmission, hydraulic and chemical injection for the subsea device. Only a small number of foreign companies have the ability to design and manufacture umbilicals. All the umbilicals that used in China were bought from foreign companies. The application and development of the subsea production system are seriously restricted by the expensive price and long cycle of umbilical.The design of deepwater dynamic umbilicals has a higher requirement of the structure performance. It needs to meet big tension due to self-weight, bending and high fatigue life due to rugged ocean environment and floating movement. Therefore, in order to prevent the tension, bending and fatigue failure, the umbilical structure should be designed with better tension performance, bending performance, fatigue performance and optimal layout. Due to the umbilical characteristics of multi-unit, multi-material and helical wound structure, the predicting model of the structural performance is complex. The theoretical and numerical models are imprecise. So the design needs to be verified by experiment.The support for the research is provided by the subject "The Key Technology of Umbilical of Subsea Production System"(No.2009AA09Z301) of the marine technology field of the "863" High Technology Research and Development Program of China. The analysis method and experimental research, which aimed to improve the structure performance of deepwater umbilicals, are developed. The predicting models and experimental verification platform of tension performance, bending performance, and fatigue performance are studied in the dissertation. Quantitative method of the optimal layout design is proposed. The main works and research results are summarized as follows:(1) Tension stiffness design and analysis of umbilicalThe top of deepwater dynamical umbilicals will be subjected to large tension loads during installation and operation. If the tension stiffness of the structure is too low, it will lead to a large deformation, which may lead to structural deformation failure. Umbilicals tension stiffness is an important design indicator to measure the performance of the structure tension. Due to the radial contraction characteristics of the umbilical core, a semi-analytic model of the tension stiffness that considered the radial stiffness of the core is proposed. The model combined with theory analysis model and numerical analysis model. The radial stiffness of the umbilical core which can be got from the two-dimension finite element analysis model of the cross-section is the input parameter of the theory analysis model. The tension stiffness of the umbilical can be rapidly and accurately calculated by the semi-analytic model. To verify the applicability of the proposed model, the analysis results were compared with results of the tension test in the lab. Parametric studies are conducted to investigate the effect of the umbilical design parameters that consist the radial stiffness, diameter and laying angle of armored wires on the results of tension stiffness.(2) Bending stiffness design and analysis of umbilicalThe top and bottom of dynamic umbilicals will be subjected to large bending loads. Thus reducing the curvature load can avoid bending failure. The bending stiffness of the umbilical is an important input parameter of dynamic load analysis. It has a great effect on the curvature load response. So the bending stiffness is an important design indicator to measure the performance of structural bending. The full three-dimension finite element analysis model of umbilical that considers the friction effect between components is established. The nonlinear bending behavior from no-slip stage, part-slip stage to full-slip stage can be accurately simulated by the model. The analysis results were compared with results of the bending test in the lab to verify the correctness of the finite element model. Parametric studies are conducted to investigate the effect of the umbilical design parameters that consist of the frictional coefficient, laying angle of components and the material of sheaths on the results of bending stiffness.(3) Fatigue life design and analysis of umbilicalAt the top of dynamic deepwater umbilicals, it is subjected to big tension load and cyclic bending load due to the periodic wave loads and platform motion. Alternating stress of components are produced, which can lead to cumulative damage and fatigue failure of the umbilical. The fatigue life analysis method of dynamic umbilical is presented. One of the important structural characteristic of the umbilical is the contact and friction between components. During the process of the fatigue life analysis, nonlinear bending stiffness is considered in the fatigue load analysis and friction stress are considered in the fatigue stress analysis. Based on the fatigue load analysis model and the fatigue stress analysis model, more accurate fatigue life can be predicted. Four-point bending fatigue test is proposed to verify the fatigue performance of the umbilical.(4) Cross-sectional layout design of umbilical Only general principles of such as symmetry of the layout requirements are given in the umbilical specification ISO13628-5. But methods and tools which can be specifically operated did not give. The optimal layout design method of umbilical cross-section is proposed based on component symmetry and structural mechanical properties. Firstly, in order to make sure the symmetrical arrangement of components in each layer, mathematical description model and the appropriate design software of symmetry is developed based on the rotational symmetry and the maximum number of the axis. Then after the symmetry arrangement of each layer of different layout design concepts, the quantitative analysis results based on finite element analysis of mechanical properties to determine the final optimal layout design is presented.