Study On Strain Assessment Of Simple Pit Defects On Oil And Gas Pipelines

Oil and gas can be exploited and utilized a large scale, which have become the blood of the nation’s industrial system, and they provide power for the development of all fields. Pipelines have become the mainstream of oil and gas transition mode thanks to the rational use of underground space, closed transportation, economic and green. Pipeline transportation safety is directly related to energy security and life and property safety, which needs to arouse enough attention. As a kind of geometric defects, dents are permanent plastic deformations and cause stress and strain concentration, which pose a threat to pipeline safety. Reasonable evaluation for dents should be made to repair and replace the seriously damaged pipelines timely. In order to evaluate dents by using strain based method, several studies are carried out in this paper:In the first place, types, the principles of classifications, detection methods and the existing evaluation standards are summarized. The strain-based method for the assessment of dents in ASME B31.8is recommended.Then, after the material nonlinear and geometry nonlinear analysis, finite element models of smooth dent and kinked dent are established. With the help of these finite element models, the strain distributions of the dents in deferent depth are studied, as well as the strain change rules of dents’spring back due to the internal pressure.In the next place, a series of researches are studied to improve the existing strain evaluation standard. First, there is a technological gap between inspection data and quantitative calculations of strains. A fourth-order least-square B-spline approximation method is developed to remove noise and errors from inspection data and approximate the dent curve, which makes it possible to calculate strains using inspection data with noise and errors. Second, arc length method is used to calculate membrane strain, and improved the calculation method of equivalent strain in the standard considering the geometrical characteristics of pipeline of cylindrical shell. The comparison made among the improved method, the criterion method and the FE results suggests that the improved calculation method greatly improves the calculation accuracy. Third, a ductile failure damage model is established using the related theories of fracture mechanics and damage mechanics. With the help of the model, damage degrees of two types of dents in the indent region as well as damage degree of dents in different internal pressure are studied. A fitting relationship of critical failure strain under different pressures is obtained, which provides a basis for the selection of allowed strain selectionFinally, on the basis of the dent strain calculation model and ductile failure strain damage model, an evaluation process is put forward, which concludes defect on line detection, description, strain solutions and allowed strain selection. Lan-Cheng-Yu pipeline analysis shows that the process can meet the engineering practical needs and easily operated.