Dynamic Analysis Of Deepwater Umbilical And Reliability Analysis Of Bending Stiffener

Umbilical is the key apparatus of subsea production system and it has obvious mechanical nonlinear characteristic and random characteristic. As the offshore oil and gas exploration are moving into deeper water, the umbilical will face more challenges during the design, analysis, installation and operation.First of all, the top tension of dynamic umbilical would increase as the water depth grows. Buoyancy modules are required to reduce the high tension loads. But the deployment method of buoyancy modules would affect the global configuration of umbilical. To ensure the safety and economy of structure, the optimization for arrangement of buoyancy modules is required. Meanwhile, the static umbilical on seabed subjected to marine environmental loads easily result in large displacement, which would cause the instability failure of umbilical. It will bring about the failure of subsea oil and gas production system and leak of oil and gas. In addition, bending stiffener is employed to protect the upper connection of umbilical against damage from overbending. However, it is susceptible to failure from accumulation of fatigue damage which sustains cyclic loads and environmental corrosion.Some uncertainties existing in the umbilical structural analysis, such as the variations in geometric size, material properties, environmental loads etc, have the effects on the result of analysis. This will make original design has the possibility of failure. It is very important to improve the structural safety when considering the effects of uncertainties during the analysis of umbilical and bending stiffener. To ensure the safety and reliability of stuctural system, it needs to perform on-bottom stability analysis and fatigue reliability based design optimization (RBDO).This work analyzed the three critical problems of umbilical in detail: the middle part of dynamic umbilical where deployed the buoyancy modules, the bottom part of static umbilical on seabed and the top part of umbilical connecting with platform. The dynamic analysis and fatigue reliability analysis of bending stiffener were studied. The uncertainties during the analysis of structure are taken into account in this work, the main tasks are given as follows:1. Multiobjective optimization for deployment of buoyancy modules of dynamic umbilicalThe global analysis of umbilical is performed considering the environmental loads. The effects of different arrangements of buoyancy modules and wave and current parameters on umbilical global configuration are discussed. Then, the deployment of buoyancy modules is optimized. Approximation model is used to reduce the computational expense of optimization. And the best compromise solution is selected from Pareto solution set with multiple attribute decision making method. The optimal results show that approximation model can improve the efficiency of multiobjective optimization and reduce the computational cost without sacrificing the accuracy. The results also show that the best compromise solution can be selected effectively and quickly using the multiple attribute decision making method.2. Reliability analysis for umbilical dynamic stabilityConsidering the effects of uncertainties on subsea umbilical stability analysis, a design analysis method of subsea umbilical stability based on reliability is developed. The stability is analyzed using the dynamic stability method. The hydrodynamic loads are calculated with the Fourier coefficient model method. Monte Carlo simulation is used to estimate the reliability of stability with approximation model. The parameters have influence to the stability are studied by sensitivity analysis and the effects on the stability are discussed. The results can offer the reference for the design of umbilical stability.3. Fatigue reliability design optimization for bending stiffenerTaking the effects of uncertainties on structural fatigue life into account, a fatigue reliability design optimization method of bending stiffener is developed. The optimization for bending stiffener is performed with fatigue probabilistic restraint. The fatigue analysis of bending stiffener is based on the methodology from the global analysis to local analysis. The sensitivity analysis and interaction effect analysis are performed to study the fatigue life performance of bending stiffener. And the effects of parameters on the fatigue life are discussed. The RBDO employs a single loop method to reduce the number of iterations. Approximation model is used to improve the computational efficiency of fatigue RBDO. The optimization results demonstrate that comparing with the deterministic optimization, fatigue RBDO consider the effects of random variables and balance the requirements of economy and safety more perfectly and reasonably.