Fracture mechanics methodologies for calculating fatigue lives have been successfully applied by pipeline operators to estimate integrity reassessment intervals. Their application in the definition of pipeline system fatigue lives has been overly conservative in actual practice. The source and magnitude of the conservatism inherent in the calculated fatigue life estimates needs to be identified so operators have a better indicator of when reassessments should take place. The pipe life estimation is especially critical for Electric Resistance Weld (ERW) and Electric Flash Weld (EFW) pipeline systems with longitudinally oriented defects. Prior work on improving fatigue life was initiated through studies completed by Pipeline Research Council International, Inc. (PRCI) to evaluate the sources of differences between fatigue life estimates produced by industry fatigue analysis software and different metallurgists. Two significant sources of conservatism in the fatigue life estimation process were identified: the fatigue crack growth rate (da/dN) and the bulging correction factor applied to axial surface flaws. The experimental and numerical simulation techniques considering the impact of these factors on rate of fatigue crack growth of pipeline axially oriented defects are described in this paper. Finite element modeling was used to simulate pipe bulging in the presence of axial flaws. The effect of the pipe thickness, diameter and flaw geometry was compared with treatments included in existing defect assessment standards. The results illustrate that for longer and deeper flaws existing treatments over represent the local bending due to pipe wall bulging. This results in unnecessarily conservative (shorter) fatigue life estimates. The crack growth rate (da/dN) was measured in a compact tension specimen material fatigue testing program. The test results included a range of ERW and EFW pipe materials with varying vintages and grades. The measured fatigue crack growth rate for the materials tested was found to be lower than that recommended by existing industry standards. This adds to the over conservatism of current approaches. The numerical simulation and materials testing results and related recommendations presented in this paper are compared to existing codified treatments to quantify the level of conservatism inherent in the current state of practice. Recommendations are provided to enhance the precision and better manage conservatism in fatigue crack growth rate calculations. Increased accuracy serves to improve integrity management and would be of interest to pipeline operators, consultants and regulators.
Pipeline dents can be developed from the pipe resting on rock, a third party machinery strike, rock strikes during backfilling, amongst other causes. The long-term integrity of a dented pipeline segment depends upon parameters including pipe geometry, indenter shape, dent depth, indenter support, secondary features, and pipeline operating pressure history at and following indentation. US DoT and other standards include dent repair and remediation criteria broadly based upon dent depth, dent location (top or bottom side), pressure cycling (liquid or gas), and dent interaction with secondary features (weld, corrosion, cracks). These criteria are simple and easily applied, however, they may not direct maintenance appropriately and be overly conservative or, in some cases, unconservative. Previous IPC papers have discussed the full-scale dent fatigue testing and dent modelling efforts to support integrity management criteria development by collecting material and structural response during dent formation and pressure loading. The present paper will present the results of this extensive dent structural and fatigue life numerical simulation program using a validated finite element (FE) analysis process. The paper describes the numerical simulation technique, as well as, the development of the novel engineering tool for integrity management, eliminating the need for numerical simulation of individual dent features to assess the relative integrity threat they pose. The development of the engineering tool presented in this paper considers the dent formation, re-rounding and through life response to pressure fluctuations to evaluate the fatigue life of dent features. The results of these analyses are used to develop fatigue life trends based on dent shape, restraint condition and operating pressure. These trends were used to develop models to predict dent relative severity and life based upon ILI inspection dent shape data for single peak dents. Dent shape has also been used to determine the restraint condition of a dent and its influence on the dent feature fatigue life. The tools were developed to address many of the uncertainties inherent in existing regulatory repair and remediation criteria. Current and future applications of the integrity assessment model are described along with recommendations for further development and testing to support pipeline integrity management, industry guidelines and standards. The results of this research will be of use in improving integrity management decisions and support further development of industry guides and standards. As such the information presented in this paper will be of interest to pipeline operators, integrity management specialists, in-line inspection (ILI) organizations and regulators. The recommendations presented in this paper may be used to influence the direction of pipeline standards in their direction in the disposition of dent features.
Onshore pipeline industry has deployed in the last decade comprehensive integrity management programs in a constrained environment. These programs address all types of threats and resulting defects, yet the most complex defects are those due to mechanical damage, as they can combine local pipe deformations (dents) with metal removal (gouges) or even cracks. These programs are first placed in the broader risk management perspective that justify the whole approach and provide a view of the context. Then, operational threat management programs for mechanical damage as implemented by operators are briefly described here, and serve as a basis to identify the main gaps in terms of technology and knowledge. Finally, both incremental and more game-changing innovations as produced by R&D performed by PRCI and consultants, are described in subsequent sections as possible options to fill the identified gaps. Examples of roadmaps are provided that explain the coverage in terms of existing and evolving knowledge and technology, as provided by these R&D programs, to fill these gaps. These various levels of representations are complementary tools to communicate about links between operations, R&D, and their contributions to public safety.
Most recently, as a complement to the ongoing efforts to monitor and document improvements in EMAT ILI technology, PRCI conducted an extensive study of NDE inspection technologies for characterizing SCC in pipelines using various in-ditch technologies and methods. The test pipes used for the study were cut outs from an operating pipeline where SCC features were identified and sized using EMAT ILI technologies. These are now sized with the NDE study and correlated with EMAT data to support an improvement of EMAT technology in characterizing SCC features. More importantly, the test pipes were burst tested to failure, with post failure analysis completed to fully characterize the crack features, including detailed length and depth measurements. This complete data set provides a comprehensive view of the current capabilities of NDE inspection technologies and EMAT ILI technologies to detect and characterize SCC and crack-like features. In this paper, the approach used for the evaluation of in-ditch NDE and EMAT ILI technologies is presented first. The in-ditch NDE technologies used for evaluation which were commonly used for SCC characterization are then described. SCC characterization results from in-ditch NDE and EMAT ILI are summarized and compared to those directly measured from fracture surfaces exposed by burst tests. The findings and its application to pipeline integrity management programs are discussed.
A significant amount of research and development has been carried out on the mechanism of the stress corrosion cracking of underground pipelines. This paper describes the results of a study, co-funded by PRCI, the US DOT, and pipelines companies, to bring together the results of these various studies in the form of a set of guidelines that will assist companies in identifying the most likely SCC locations on their systems and in predicting how frequently inspection or other mitigation is required. The guidelines have been developed along mechanistic lines, and are divided into four “steps” representing: susceptibility to SCC, crack initiation, early-stage growth and dormancy, and crack growth to failure. For each step, a series of Research Guidelines has been derived from the results of individual research papers or studies. These Research Guidelines may or may not be easily validated against field data. The SCC Guidelines were then developed based on one or more Research Guidelines. Wherever possible, the SCC Guidelines have been validated against field data, but in some cases currently un-testable SCC Guidelines were defined because they offer a potentially unique opportunity to identify where and when SCC might occur.
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