Fatigue And Dynamic Stochastic Optimization Analysis Research Of Deepwater Steel Catenary Riser

In recent years, need for exploitation of deep water gas and oil reserves becomes increasingly strong. Steel Catenary Risers (SCRs), as one of the key facilities for deep water exploitation, has an obvious nonlinear characteristic in its global configuration and local components, because of which, we face various challenges in its design, analysis, installation and safe operations.Stress joint of marine riser makes the connection between risers and platforms, and it is one of the key components in riser system. The joint can provide the bending stiffness and hanging angle in the top region of the riser, at the mean time, it is also subjected to extreme alternating tensions and bending moments. So, fatigue damage is easy to occur, as well as oil/gas leaks, and the safety of the entire system is endangered. Because there exists very complex contact effect and apparent state nonlinearity between riser and joint, this region is one of the most important and difficult points in the numerical analysis for deepwater steel catenary riser.Besides, the cost of the riser accounts for larger proportion of the whole system as the water depth increases. Thus, optimization for the steel catenary riser to reduce the cost under the premise of structural safety reliability is of practical significance. Marine risers in sea environment have stochastic characteristic in several aspects, e.g. material properties, dimensions, and loadings, which bring the uncertainties of the analysis results, and also means that the traditional deterministic optimization design for the riser structure may face the problem of inadequate safety reliability. Therefore, an optimization considering the stochastic effects is highly important for the safety of marine risers.This research performs a detailed discussion on stress joints based on the global dynamic analysis for a deepwater steel catenary riser, and the optimization for global and local model is also studied. The main work of the research is as follows:1. Global dynamic and fatigue analysis for the Steel Catenary Riser under environmental loadingsA combined use of analytical and numerical method helps to establish the initial static equilibrium configuration, based on which, transient dynamic analysis is performed, and then fatigue analysis. Results show that weak points of the SCR are generally located at the top hanging and bottom touchdown zone.2. Fatigue life prediction for two types of stress joints of SCRThere mainly talks about two types of stress joints, namely tapered stress joint (TSJ) and sleeved stress joint (SSJ). Responses from global analysis provide the input of local loading applied on stress joints, and dynamic analysis as well as fatigue analysis according to multiaxial fatigue theory is performed. Moreover, parametric design study is made to show the effects and trend brought by changing geometric dimensions, and this can provide reference for the optimization of stress joints structure.3. The study on stochastic design optimization of SCRThe dynamic stochastic design optimization for SCR is studied. The use of single-loop method of reliability based design optimization reduces the iteration steps during the optimization process and the use of meta-model approach can largely improve the optimization efficiency by reduce the steps calling for FE code. The used methodology improves the optimization efficiency apparently, and the optimal design declines the structural cost with satisfying the required reliability. Considering this, the proposed method takes both the risers'economic and safety requirements into account scientifically, and makes up the drawbacks that the deterministic optimal design may face a problem of low safety reliability.Through the global analysis for the SCR, this research definitely shows that the stress joint near the top hanging zone of riser is one of the weak points of the riser structure; a parametric analysis for the two types of stress joints illustrates their own mechanical characteristics; research on method for dynamic stochastic analysis and dynamic stochastic optimization design devotes on efficient and scientific design method for SCR and satisfying requirements from the view of economy and safety, and the method can provide reference for practical engineering design.