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Defect assessment is an essential component which is integral to the integrity management program of pipelines. By processing and analyzing the data collected by in-line inspection (ILI) tools and from other sources such as historical records, operating control and monitoring system, and offline/aboveground inspections with various models, formulas and numerical algorisms, the defect assessment provides information about performance condition of the pipelines, including prediction of failure pressure, determination of fitness-for-service (FFS), and further, estimation of the remaining service life. The defect assessment also contributes to failure risk and reliability evaluation, and recommendations of proper measures and actions for pipeline failure mitigation and control. The pipeline defect assessment technique has evolved in the past several decades, experiencing development of three levels of technical progress, i.e., Levels I, II, and III methods. Targeting determination of the stress and strain distributions at the defects and evaluation of pipeline FFS and failure pressure, the three levels of methods distinguish themselves mainly by improved accuracy of the defect sizing, inclusion of the interaction of multiple defects, and solving highly nonlinear problems in defect assessment on pipelines, respectively. Nowadays, Levels I and II methods have been extensively used in industry for improved integrity management, while the Level III method, which relies on finite element (FE) modeling and analysis for solving nonlinearity at pipeline defects, has found its applications mainly in engineering research community due to background knowledge requirement and computational complexity. In the last decade, my research group has been focusing on Level III defect assessment on pipelines, developing various FE-based models and methods to determine the stress and strain distributions at corrosion defects based on accurate definition of the defect dimension under pipeline operating conditions, evaluating their effect on FFS of the pipelines and predicting the failure pressure. Moreover, the assessment targets not only a single corrosion defect on pipelines, but also multiple defects between which a mutual interaction may exist to further degrade the pipeline integrity. For various orientations the corrosion defects are aligned with each other, critical spacings between them are defined to determine if an interaction exists so that they should be assessed either together or separately. The major contribution of my group to development of the Level III defect assessment technique is, based on the mechano-electrochemical interaction theoretical concept I proposed in 2013, to integrate the mechanical force with electrochemical force, developing a multi-physics field coupling model for defect assessment while considering the dynamic nature of corrosion defects in actual service environments. Prior to that, the corrosion defects have been usually treated as metal-loss features, while ignoring the dynamic process of defect growth due to corrosion reactions. This is regarded as “revolutionary” to pipeline defect assessment techniques. The novel Level III defect assessment method, at the first time of its kind, enables prediction of the rate of corrosion defect growth on pipelines under the synergism of mechanical and electrochemical forces, reproducing the reality and thus providing more accurate and reliable results. In addition to corrosion defects, the mechano-electrochemical interaction integrated Level III assessment method has also expanded its use to other types of surface anomalies such as dents, buckles and wrinkles, as well as combinations of different types of defects. Moreover, the defect assessment applies on both straight pipes and pipeline elbows where the defects experience different mechanical and corrosion conditions. Criteria and methods are developed to evaluate the pipeline performance and predict burst failure. The book starts with an overview of pipeline integrity management program in Chapter 1, where the basic principle, main components and methods, and design pathway of integrity management of pipelines are introduced. Various threats to degrade the pipeline integrity in the field are reviewed, and common ILI tools for detecting surface defects are summarized. Chapter 2 introduces the historical development of defect assessment techniques, while focusing on the principles, criteria, and applications of Levels I and II methods. Commentary remarks are given to analyze the limitations of the two levels of assessment method. In Chapter 3, the FE-based Level III defect assessment method is detailed in terms of the principles, criteria, and applications for pipeline FFS determination and failure pressure prediction.