Oil And Gas Pipelines Residual Strength Evaluation Studies

The assessment of acceptability of flaws in gas and oil pipeline system consists of two parts: remaining strength and remaining life. The assessment of remaining strength includes the assessments of planar flaws, non-planar flaws and shape imperfections. In the thesis, a whole set of method has been given on assessment of fitness for service. Three different levels A, B, C are adopted during the process of assessment and principles, specific methods and approaches of three different levels are compared to show which one is appropriate for material preferences given and special conditions. Based on assessment , the conclusion of whether a flaw is safe or not is made; safety abundance of toughness and primary press % limited stress are calculated and possible failure models is predicted.The bending stress due to angular misalignment and axial misalignment in pressure pipes is calculated by software ANSYS to test formulas of bending stress presented in Standard API579 and BS7910. The result shows variations of bending stress along with pressure, spiral angle, angle of angular misalignment and amount of axial misalignment. By analysis, formulas which should be more appropriate in Figure FAD, a method used to evaluate flaws in pipes, are recommended when calculating bending stress due to angular misalignment and axial misalignment in pressure pipes. Such performances of material as tensile strength, yield strength, Charpy energy and COD fracture toughness are tested in experiment.The assessment of risk in gas and oil pipelines is conducted. In this part, the definition of risk, methods of risk analysis and risk evaluation are illustrated. The relationship of risk analysis, risk evaluation and risk management is showed by framework. A risk assessment approach, analytic hierarchy process is presented. It is a flexible and robust multicriteria decision-making methodology, which formulates the problem in a hierarchical structure. The priorities of the elements of each level are calculated by pairwise comparisons of importance with respect to a common attribute. The solid mathematical foundation of AHP ensures that the assessment process is correct and reliable. Based on AHP approach, the final risk value is calculated.