Burst Strength of Corroded Pipe: "Flow Stress" Revisited

Abstract

SUMMARY If corrosion damage occurs in a pipe, assessing the remaining burst strength is essential in operational safety management. In this paper a model is presented to predict the burst strength for defects that take the form of a large area of metal loss, where a continuum mechanics approach is more appropriate than a fracture mechanics approach. The model results in an explicit criterion for failure which can easily be applied in an operational setting, and provides fundamental insight in the effect of the depth and the circumferential length of the defect, the material grade and the axial stress. A recipe is available which shows how a burst strength analysis can be done with a pocket calculator. Apart from data on the depth and circumferential length of the defect, it assumes that both the yield strength and the ultimate tensile strength of the pipe material are known (or estimated). The first comparisons between the model predictions and test results are very encouraging. It thus could form the basis for the development of a reliable defect assessment procedure for pipes with patches of metal loss. There is remaining work that needs to be undertaken. It would be useful to analyse the effect of different stress" strain curves, and to incorporate additional curve fitting formulas in the model. The next step is to further verify this burst prediction tool by means of an extensive test programme, preferably in a joint industry project. INTRODUCTION Corrosion damage in the form of larger areas of metal loss is not uncommon in pipes, transporting oil and gas. With pipelines and risers approaching their design life in increasing numbers, adequate fitness-for-purpose assessment procedures for various circumstances should be available. Even for new pipes these procedures should be in place, in view of the trend to transport untreated hydrocarbons where corrosion monitoring is performed during operation [1]. The evaluation of the remaining strength of these pipes, with respect to both collapse and burst, is essential in operational safety management. Using the ANSI/ASME B31G guideline [2] to assess the burst strength for this type of corrosion is questionable, since it would then be used beyond the test data on which it was based. It may lead to substantial underestimation of the strength [3], but may be unreliable for high grades of steels or under unfavourable levels of axial stress. In this paper a model is presented to predict the burst strength of a pipe with an idealised corrosion patch that has an infinite axial length. This continuum mechanics model incorporates the necessary effects of finite strains and displacements. The analysis results in an explicit formula for the burst pressure that provides fundamental insight in the interrelationship between depth and circumferential length of the defect, the material grade, and the axial stress.