Research On Residual Ultimate Strength Of Damaged Submarine Pipeline Under Combined Loads

As important strategic resources,oil and gas always play a dominant role in the energy system.Land and ocean are the main sources of oil and gas resources.Submarine pipeline,which has the characteristics of safety,efficiency and economy,is an important component of the development of oil and gas.Because of this,it is usually called lifeline of the sea.Defects,which could decrease the ultimate load-carrying capacity of pipe structure,often occur on the surface of submarine pipeline due to the complex environment.These defects could cause safety accident,resulting in huge economic losses and serious environmental pollution.Therefore,it is necessary to evaluate the residual ultimate strength of defective pipeline and formulate the corresponding measures to guarantee the safety and avoid unnecessary repair or replacement at the same time,which will improve the economic benefits of pipeline.In the present research,residual ultimate strength of submarine pipeline with notch,dent and combined defects under combined loads was studied by means of theoretical analysis,pipeline test and finite element calculation.Ultimate load-carrying capacity of damaged pipeline bearing different load cases was analyzed.Specific research work can be summarized as follows.(1)A full-scale of four point bending test of seamless steel pipes applied in actual engineering was designed and carried out.The effect of internal pressure on the ultimate moment capacity of pipes was analyzed by considering two types of load condition which are the single action of bending moment and the combination of internal pressure and bending moment.Three kinds of defect including notch,dent and the combination of thereof were induced on the pipe specimen respectively,in order to explore the effect of different defect type and defect location on the residual ultimate strength of pipes.(2)Finite element calculation of the residual ultimate strength of pipes under combined loads was carried out through the software ABAQUS.The obtained ultimate moment capacity,failure mode and the relationship of bending moment and curvature during pipe bending were compared with the test,verifying the validity of finite element model and method.In addition,the procedure of manufacturing dent and combined defect on pipe specimen was simulated numerically,analyzing the rerounding behavior.The validity of finite element model for re-rounding was verified through the comparison of numerical and experimental results.(3)Parametrical study on pipes with different type of defect was carried out through nonlinear finite element method.A serious of finite element model of pipes were established by changing the length,width and depth of defect.A serious of finite element calculation of damaged pipes were carried out by controlling the internal pressure.Based on the work,the ultimate moment capacity of different model under different load cases was obtained.After this,the effects of various factors on the residual ultimate strength of pipes were analyzed and summarized.The present research has positive significance for evaluating residual ultimate strength of submarine pipelines accurately,assessing the applicability of damaged pipelines effitively and improving pipelines' safety as well as economic benefits.