| Hydrocarbons are mainly transported by underground or undersea pipelines. Metal loss due to corrosion is one of the most common situations leading to the loss of pipeline integrity. There are various mechanisms for both external and internal corrosion which may produce local reductions in wall thickness. These reductions can be in the form of single pits, multiple pits or general corrosion over a large area, with or without pitting.; In the early 1970s, a criterion for the prediction of the burst pressure of corroded pipe was developed through research sponsored by the American Gas Association Pipeline Research Committee and the pipeline industry. This criterion, commonly referred to as the B31G criterion, has been embodied in pipeline design Codes that are part of the ANSI/ASME B31 Code for pressure piping. While B31G has been very helpful in evaluating the integrity of corroded pipe, it has been found to be overly conservative and ambiguous to apply in many situations encountered during pipeline inspections.; In the past, conservatism of the B31G criterion was appropriate due to the low resolution of inspection tools used in pipeline examination. Currently, high resolution in-line inspection tools are being introduced which permit the accurate measurement of corrosion damage in pipelines. Such tools provide data that are sufficiently accurate to allow estimation of corrosion growth rates from subsequent inspection and development of long-term maintenance plans. However, present corrosion assessment procedures are too simplified and conservative to allow such a procedure to be economically viable. Therefore, a multi-level assessment procedure is proposed, which would reduce the degree of conservatism in the assessment for increasing accuracy in the evaluation procedure and knowledge of the situation, the lowest level incorporating B31G to provide continuity with previous assessments.; Burst testing demonstrated that failure of older pipe from natural corrosion occurs by plastic collapse. A two-criterion approach, used with nonlinear finite element techniques, was successful in predicting plastic collapse of pipe with single and multiple corrosion pits, and complex-shaped corrosion, and is proposed for the highest level of assessment. Unfortunately, such an assessment, while accurate, will be time consuming, require large computing resources, accurate definition of the material behaviour and experienced control of the iterative solution process and would only be justifiable where the costs of repair or replacement or the consequences of leakage are high. Therefore, a simplified technique which aims at producing a suitable stress field for the lower bound theorem of limit analysis at the current yield locus is proposed as a transition from an empirical (Level 1) to a nonlinear (Level 3) assessment of corroded pipe. The concept of a modified elastic modulus has been incorporated in an elastic finite element analysis as a simplified nonlinear analysis. |
