Fracture Assessment of Flaws in Undermatching Welds

Abstract

The criticality of girth weld flaws in subsea pipelines, installed by methods introducing plastic strains such as reel-lay, is usually evaluated through an engineering critical assessment (ECA). Most ECA methodologies require weld overmatch for pipes subjected to plastic deformation. This, however, is not always achievable for corrosion resistant or even carbon steel pipelines. In this case, a material and geometry-specific ECA is often applied. However, this ECA approach necessitates many 3D finite-element (FE) fracture analyses to be performed. Therefore, the authors propose a simpler screening assessment, which requires undertaking only a limited number of 3D FE fracture analyses. If the significance of a weld undermatch is shown to be negligible then a material and geometry-specific FE-based ECA is deemed unnecessary. Instead, flaw acceptance criteria can be determined under the assumption of weld evenmatch using a material-specific analytical ECA. The work was undertaken to define and validate the screening assessment process. Subsequently, DNV-GL endorsed the proposed approach which has since been successfully applied on several projects allowing optimization of the project cost and schedule. This paper describes the screening assessment methodology and discusses its application range and limitations. The conclusions and recommendations from a validation program are also provided.