Evaluation of strain based ECA method in DNVGL-RP-F108

Abstract

Pipelines may be subject to strain-controlled external loads exceeding yield strength of materials due to many causes, e.g., soil settlements and ground movement. There are several strain based fracture mechanics assessment methods to determine the maximum tolerable defect sizes in girth welds in pipelines. One of these methods is the DNVGL-RP-F108 procedure which was developed from a joint industry project for assessing reeled pipes and has been used for both offshore and on-shore pipelines. However, as with other methods, it requires the weld strength either even- or over-matching the parent steel which may be difficult to achieve for high strength steels. There are also uncertainties about the adequacy of the DNVGL-RP-F108 guidance addressing the effects of internal pressure and welding-induced residual stresses. In this study, the maximum tolerable defect sizes in the girth welds of an X70 spiral welded pipeline were determined using the DNVGL-RP-F108 approach. The effects of several variables on the assessment results, including weld strength mismatch, stress-strain curve magnitude, internal pressure level, fracture toughness, residual stresses and misalignment, were also investigated. A comprehensive finite element analyses were performed to verify the crack driving force determined by the DNVGL-RP-F108 approach. It was found that the DNVGL-RP-F108 approach can be conservatively applied to girth welds with a weld strength under-match by 5% in all cases, and even by 10% for most cases, for the conditions investigated. It was also revealed that use of this guidance could be unduly conservative for strength over-matching welds.