| As a "lifeline" connecting the surface floating body and the subsea wellhead,the marine flexible pipeline is the key equipment for deepwater oil and gas development.Its structure is composed of several metal layers and polymer layers,which combines the advantages of two types of materials.In practical applications,when high-temperature oil and gas media are transported by flexible pipelines,the long-term use of polymer materials in high-temperature environments is prone to aging,which may cause material failure,pipeline damage and leakage,resulting in environmental pollution.Therefore,this paper studies the design of polymer materials under high temperature characteristics,analyzes the high temperature aging behavior of polymer materials at the material scale to explore the mechanism,and tests the flexible pipes made of polymers at the equipment scale to verify its actual application.Firstly,the functional requirements of high temperature medium for polymer materials are comprehensively analyzed,and the selection system of polymer materials at high temperature is formed.Among them,high temperature resistance,aging resistance,fluid compatibility,mechanical properties and processing properties are key properties.Polyvinylidene fluoride(PVDF)is the optimal sheath polymer material,and its high temperature mechanical properties are tested.The results show that the high temperature will effectively accelerate the movement of PVDF molecular chain.With the increase of temperature,the elastic modulus and tensile strength will gradually decrease,and the elongation at break will gradually increase.Secondly,this paper presents the study of this phenomenon by using the XRD,FT-IR,DSC,SEM and mechanical tensile tests to obtain the behavior of PVDF for 60 days thermal aging at 130°C.The result shows that its degree of thermal degradation becomes greater,and its apparent color is deepened with the extension of thermal aging time.The sample is equivalent to the annealing treatment during the initial stage of thermal aging.The annealing process promotes the amorphous to crystalline phase transformation.The crystal perfection and crystallinity of the sample are obviously improved.The melting temperature of the sample is shifted towards the high temperature.The tensile strength of the sample increases,while the elongation at break decreases.As the thermal aging test progresses,crystalline region is destroyed by the high-temperature inducing,the crystallinity of the sample decreases,the melting temperature decreases slightly,and the tensile strength and elongation at break decrease.The crystal structure of PVDF was preserved during the entire thermal aging process.Based on the characteristics of PVDF structure,the mechanism of thermal aging is studied,and the root cause of the influence of thermal aging on its structure and function is analyzed.The results show that the side group removal occurs when PVDF is aged at high temperature,and with the HF removal,a conjugated double bond chromophore will be formed to cause the color to become darker;And with the volatilization of the plasticizer and the change in crystallinity,the macroscopic properties are correspondingly degraded.Finally,taking a flexible pipe made of PVDF as a sample,a new temperature cycle experimental device was designed to study the practical application effect of PVDF in the pipeline at a specific temperature.The results show that the increase of temperature and pressure will promote the overall elongation of flexible pipe.PVDF has higher linear expansion coefficient than steel,which causes its elongation to be greater than that of adjacent metal layer.And stress has been created at the joint of the flexible pipe.After 25 temperature and pressure cycles PVDF has no obvious surface discoloration,breakage,cracking and other failure phenomena.The PVDF performance and the sheath layer structure seal fluid function are maintained well,and the flexible pipe joints and structural integrity are preserved. |
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