Study On Corrosion Direct Assessment Of Oil And Gas Pipelines

Direct Assessment is a new method to assess integrity of pipelines. It includes External Corrosion Direct Assessment and Internal Corrosion Direct Assessment. It is a four-step process consisting of pre-assessment, indirect inspection, direct examination and post-assessment. External Corrosion Direct Assessment adopts mature external inspection techniques, and determines priority of pipeline maintenance through analysis of inspection results, which avoids blind maintenance. At the same time, it integrates remaining strength assessment and remaining life prediction, which guarantees integrity of pipelines. We can accurately locate internal corrosion areas by Internal Corrosion Direct Assessment. It can optimize current inspection techniques and provide the best monitor locations. Direct Assessment can not only determine locations that have occurred corrosion, but also locations that will occur corrosion. Integrity Management Plan can be drawn up by it. Through practice validation, it can determine integrity levels of the pipelines instead of other assessment techniques (In-Line-Inspection techniques or Hydrostatic testing), or as a complimentary tool for them.Considering status in quo of inspection techniques and assessment methods at home, this paper studied Direct Assessment. During the study of Internal Corrosion Direct Assessment, the flow regime in long-distance gas pipelines was analyzed. This paper studied inspection techniques of internal corrosion, and perfected assessment steps. For External Corrosion Direct Assessment, it made detailed illumination of each step, and studied inspection techniques of external corrosion. Otherwise, it exemplified establishment processes of criteria: division of region, definition of indications, severity classifications of indications, priority of excavations needed by repairs, long term performance metrics.On the basis of in depth analysis to External Corrosion Direct Assessment, the Direct Assessment was combined with Structural Reliability Analysis in view of uncertain elements in material behaviors, loads, computing methods and inspection data, which can improve the reliability of the assessment method, and reduce assessment cost.In addition, remaining strength assessment and remaining life prediction are involved in direct examination and post-assessment of the Direct Assessment process. Considering the importance of two aspects, this paper respectively studied remaining strength and remaining life. In the study of remaining strength assessment, it made study on the method of improved API 579 by theory of double-shear yield strength, and provided graded assessment procedure of different corrosion defects. In study of remaining life prediction, it established electrochemistry model of corrosion rates under the effect of polarization based on electrochemistry corrosion theory and electrochemistry dynamics. Then, combined with remaining strength assessment, it determined the model of remaining life prediction.On the basis of the study on remaining strength and remaining life, it programmed software 'remaining strength assessment and remaining life prediction system' using VB6.0.