Research On Fitness-for-service Assessment Of Oilfield Non-metallic Pipeline

Non-metallic pipeline is widely used because of its good corrosion resistance, long life, and reliability advantages. At present, the early pipelines all have the problems of fracture, oil leak, and conveying incapability. Therefore, it is so significant and necessity to start fitness-for-server assessment for the defective nonmetallic pipelines.The fitness-for-server assessment includes two aspects, one is residual strength evaluation, and another is remaining life prediction. In this paper, a systematic method including the process of integrating theoretical analysis, numerical simulation, modeling simulation and example of verification is then developed for studying the residual strength calculations and remaining life prediction of defective non-metallic pipelines.1) According to the theoretical basis of finite element analysis, using the software of ANSYS, this paper conducted the mechanical model to simulate the defective pipeline and calculated the maximum safe operating pressure allowed by the pipeline. By drawing the curves of relationship between defect stress and defect thickness and relationship between defect stress and pipeline pressure, the connection between pipeline pressure, defect thickness and defect stress was abtained.2) An experimental system used for testing the limit load of defective pipeline under combined loads was designed. The experimental results of limit load test are consistent with those from finite element analysis, which show that the modeling method and calculation of finite element analysis are correct and it is effective to calculate the limit load by finite element analysis.3) Two kinds of methods were proposed to predict the remaining life of defective pipeline. One is based on reliability theory and probabilistic statistics; another is based on extreme distribution. The technique procedure and the main processes of pipeline life prediction were described in this paper and the applied condition was also discussed. The methods of reliability predict the pipeline remaining life by determining the average corrosion rate calculation mode and the limit corrosion defect size formula. The method of extreme distribution using the probability theory gives maximum corrosion depth of corroded pipelines and pipeline remaining life also based on the localized corrosion Progress empirical formula.4) On the basis of the residual strength and remaining life research, combined with the standards at home and abroad, the software "Oilfield Non-metallic Pipe Assessment and Selection System" was developed and applied to evaluate the relevant examples successfully.