Numerical finite element analyses (FE) have been undertaken of circumferential flaws in the girth weld of a pipeline subjected to plastic straining simulating installation by reeling, and J driving force curves derived. These were then compared with J driving force curves derived using the guidelines described in DNV-RP-F108 intended for installation methods involving repeated plastic straining. The assessment procedure is based on BS 7910 FAD methods. Strains up to 2% were considered. Analyses were undertaken for surface breaking weld root flaws located at the weld fusion boundary. The pipe was nominally to API 5L Grade X65 strength, with a diameter of 323.9mm (12¾in) and wall thickness of 20.6mm. Conclusions are drawn concerning the conservatism in flaw assessment procedures conducted to DNV-RP-F108 and the treatment of welding residual stress.
The definition of rational flaw acceptance criteria for girth welds in pipelines subjected to axial straining within the context of existing codified fracture mechanics based assessment procedures is problematic since these are essentially stress based. Although there is no fundamental problem in using such procedures, the solutions provided are not always the most suitable for strain based assessments. Nevertheless, with appropriate modifications, assessments based on BS 7910 (BS, 2005) procedures have been used successfully for a number of years to set acceptance criteria for pipeline installation methods involving plastic straining such as by pipe reeling. The flaw acceptance criteria provided by these methods have, in many cases, enabled larger flaws to be accepted than those based on workmanship standards, such as BS 4515 and API 1104. The benefits to the industry have included reduced repair rates without loss of integrity and increase in lay rates. The demonstration of integrity by means of an existing and well established assessment procedure is considered important since it enables independent third party verification to be undertaken.