A methodology is developed to account for the effect of crack face pressure on the reference stress in high pressure, thick walled, flowlines and risers. Risers and flowlines in production service may be exposed to corrosive attack resulting in pitting. In such cases, cracks can initiate and fitness for service assessment, including fatigue crack growth and fracture analysis, is required. For such thick walled lines, the effect of high internal pressure on the crack face is significant and must be taken into account. Current industry standards, including API-RP-579 [1] and BS7910 [2], are silent on the effects of crack face pressure on the reference stress.

Reference stress, a quantity similar to the net section stress, can be used to predict the local plastic failure of the remaining ligament ahead of the crack tip, or the collapse of the full cross section. The reference stress is also used in the abscissa on the Failure-Assessment-Diagram (FAD) to determine the allowable stress intensity ratio for fracture assessment.

The methodology described in the paper follows from systematic detailed finite element analysis (FEA) conducted using linear material properties. The analysis is conducted over a range of pipe thickness ratios (D/t), crack depths (a/t) and crack aspect ratios (c/a). The results presented permit an accurate calculation of the effect of crack face pressure that can be implemented within the framework of existing methods of fitness for service analysis without the use of FEA. This paper presents a detailed analysis of the effect of crack face pressure on the reference stress for axial and circumferential internal flaws. The results from this study provide expressions to estimate the reference stress with crack face pressure loading, thus, allowing an improved fracture prediction for high pressure risers and flowlines.

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