Study On The Buckling Collapse Mechanism Of "FRP-Concrete-Steel" Sandwich Pipe Under Complex Stress Conditions

Deep-water subsea pipelines are the key equipment for the exploitations of deep-water oil and gas fields,which plays a significant role on enhancing energy self-sufficiency rate and guaranteeing energy security of China.As the key equipment required for offshore oil and gas development,deep-water subsea pipelines are responsible for the important mission of oil and gas gathering and transportation,and are called the "lifeline" of the offshore oil and gas production system.Although traditional "Steel-Filler-Steel" sandwich pipe has been used in s deep-water oil and gas pipelines with good buckling resistance and thermal insulation performance,but it is easy to produce ellipticity under the coupled effects of bending load and has poor corrosion resistance.Therefore,this thesis proposes a novel deep-water sandwich pipe composed of concentrically placed outer GFRP pipe,inner steel pipe and ECC sandwich layer,which can effectively adapt to the external pressure of the deep-water environment.Compared with traditional deep-sea oil and gas pipelines,this novel sandwich pipe has the advantages of small bulk density,good durability,and good buckling propagation resistance which means it is a novel offshore oil and gas pipeline with great application prospects.Since deep-water pipelines are often subjected to the coupled effects of bending and external pressure at the stage of pipeline construction and operation,they will be in a complex stress state.Under such loads,subsea pipelines are damaged due to buckling and collapse,thereby causing disastrous consequences.Therefore,this thesis focuses on the mechanical properties and buckling mechanism of the novel sandwich pipe under complex stress states by numerical simulation and pipe pressure experiment.The main work and results are as follows:(1)Analysis of the buckling mechanism of the new sandwich pipe.By summarizing the relevant design methods of traditional deep-water subsea pipelines,the pure external pressure action and bending external pressure coupling action,which need to be focused on the design of deep-water subsea pipelines,and the key structural parameters including diameter-thickness ratio,initial defects,material properties and interface bonding performance,which have important influence on the buckling performance of pipeline under these two conditions.The work in thischapter provides the research basis and research direction for the mechanical properties of the new sandwich pipe.(2)The establishment of the finite element model of a new type of sandwich pipe.In this thesis,the finite element software ABAQUS is used to finely model each part of the structural parameters of the new sandwich pipe,which lays the foundation for the subsequent finite element parameter analysis.(3)Numerical simulation study of novel sandwich pipeline under pure external pressure.In this thesis,the mechanical response of the novel sandwich pipe under external pressure is analyzed by the finite element numerical simulation method,and the influence of initial geometric defects,interface bonding performance,cross-section geometric dimensions and material properties on the mechanical properties of the new type of sandwich pipe under external pressure is studied.The results show that different structural parameters have great influence on the buckling properties of novel sandwich pipe in the elastic stage,and have great influence on the buckling properties in the plastic stage.The diameter-thickness ratio of the inner steel tube has the most obvious effect on the buckling behavior of novel sandwich pipe under external pressure.(4)Numerical simulation study of novel sandwich pipeline under the coupled effects of bending and external pressure.In this thesis,the finite element numerical simulation method is used to conduct a comprehensive study on the mechanical response of a new type of sandwich pipe under the coupling action of bending and external pressure,including the loading path,initial geometric defects,interface bonding properties,cross-section geometric dimensions and material properties.The impact on its mechanical properties under this working condition.The results show that the interfacial bonding performance and the thickness of core layer have the most significant influence on the buckling resistance of the new sandwich pipe under this working condition.(5)Design of novel multifunctional pipeline pressure chamber.This thesis has developed a new type of multifunctional pipeline pressure chamber.This novel pipeline pressure chamber adopts a semi-closed structure,which realizes the real-time collection of internal test data of the pressure chamber during the pressure test of pipeline.Furthermore,it also realizes the coupling load mode under bending and axial pressure by setting axial and horizontal loading devices.The research and development of new multifunctional pipeline pressure chamber has established a reliable and powerful test platform for equal scale pressure test of sandwich pipeline.(6)Hydrostatic pressure test of novel sandwich pipeline.By using the novel multi-functional pipe pressure chamber,the hydrostatic pressure test of the new sandwich pipe specimen is carried out.This test not only verifies the effectiveness of the test device,but also preliminarily obtains the mechanical response and failure mode of the new sandwich pipe under hydrostatic pressure.The buckling load of the pipeline in the pressure test is 31.45 MPa,and the buckling load in the numerical simulation is 32.1MPa,the error is only2.07%.The successful reproduction of the test results preliminarily proves the accuracy of the new numerical simulation method and the numerical simulation results.