Assessment of flaws in pipeline girth welds - a critical review

摘要

Currently, there are a number of fracture mechanics-based procedures for assessing the acceptability of flaws in pipeline girth welds. These are designed to relax workmanship flaw acceptance criteria, which are often based on flaw length alone, and to provide a means of deciding acceptability of flaws found by ultrasonic testing where information on height, position through the pipe wall thickness, as well as length are provided. The potential benefits of these procedures are a reduction in costs by minimising the need for repair and delay imposed. This is achieved without compromising the integrity of the weld. The established procedures are designed for applied stresses below the yield strength of the material. Methods based on BS7910 and API 1104 are introduced. Strain-based procedures, where the applied strain exceeds the yield strain, are described and compared. These are the DNV RP F108 procedure, which is primarily concerned with pipeline installation, and the more recently developed strain capacity methods developed by ExxonMobil and PRCI CRES for pipelines subjected to internal pressure combined with axial plastic loading. Finally, some comments on how the results of inspection can be used to help develop assessment methods and inform Engineering Critical Assessment strategies.