System Reliability Analysis And Optimization Of Subsea Pipeline With Distributed Buoys

The uncontrolled abnormal lateral buckling is a major factor leading to the failure of subsea pipeline system.The distributed buoyancy method can stimulate the controlled buckling deliberately to release the excessive axial force and reduce the risk of abnormal buckling using the buoys.The reliability analysis is necessary because there are so many uncertainties in the operation of the subsea pipeline with distributed buoys,such as parameters of pipeline,pipe-soil contact and initial imperfections.The reliability analysis model of serial buoys system is proposed based on the analytical solution for buckling of the subsea pipeline with distributed buoys,and the reliability analysis is carried out.Meanwhile,The optimization design of layout schemes of subsea pipeline with distributed buoys is performed.Firstly,based on the analytical solution for lateral buckling of the subsea pipeline with distributed buoys,the failure modes of single buoy are defined in combination with several major uncertainties.Considering the strength failure and buckling failure of the pipeline structure,three failure modes are defined by introducing the out-of-straightness(OOS)and load controlled criteria(LCC): abnormal buckling occurs at the position of OOS,no planned buckling occurs at the position of buoy,and the pipeline does not meet the LCC.No failure occurs at the single buoy only when none of the three failure modes occurs.The failure modes in the paper are more reasonable than the previous research.Furthermore,the axial force indexes are defined in the process of constructing performance functions,which are simplified using the surrogate model method.Secondly,based on the performance function and layout scheme of multi-buoy,the reliability analysis model of serial buoys system is proposed.The system reliability is calculated by judging three failure modes at the positions of each buoy one by one in single Monte Carlo calculation with the above surrogate model of the pipeline,fully considering the post-buckling state of pipeline and the interaction between adjacent buckling.The variation of axial force before pipeline buckling is analyzed.The results show that the axial force before buckling is always fluctuating up and down,and the amplitude of fluctuation gradually decreased.The influence of OOS on the system reliability is discussed.The research found that the bigger the distance between OOS and rear buoy,the higher the system reliability.The system reliability is independent of the number of buoy and pipe length when OOS is not considered.Finally,based on the reliability analysis model of the serial buoy system,the optimization design for layout schemes of the subsea pipeline with distributed buoys is carried out.According to the relative design criteria and the post-buckling state of subsea pipeline with distributed buoys,the safety performance index and the control performance index are defined.Taking the parameters of buoys as design variables,the single objective optimization design to maximize the system reliability is carried out.Then,the reliability based multi-objective performance optimization design is conducted,considering both the safety performance and the control performance.In the end,the reliability based multi-objective cost optimization design is performed,considering both the control performance and the cost of layout schemes.The optimization results provide an important reference for the practical design of the subsea pipeline.