An Evaluation Procedure for Crack Driving Forces in Mismatched Welds and its Applicability to ECA Analysis of Undermatched Clad Pipe Girth Welds

Abstract

Structural integrity of submarine risers and flow lines transporting corrosive and aggressive hydrocarbons represents a key factor in operational safety of subsea pipelines. Advances in existing technologies favor the use of C-Mn steel pipelines (for example, API X65 grade steel) either clad or mechanically lined with a corrosion resistant alloy (CRA), such as Alloy 625, for the transport of corrosive hydrocarbons. However, while cost effective, specification of critical flaw sizes for their girth welds become more complex due to the dissimilar nature of these materials. In particular, effective fracture assessments of undermatched girth welds remain essential to determine more accurate acceptable flaw sizes for the piping system based upon engineering critical assessment (ECA) procedures. This work focuses on development of an evaluation procedure for the elastic-plastic crack driving force (as characterized by the J-integral) in pipeline girth welds with circumferential surface cracks subjected to bending load for a wide range of crack geometries and weld mismatch levels based upon the GE-EPRI framework. The study also addresses the effects of an undermatching girth weld on critical flaw sizes for a typical clad pipe employed in subsea flowlines having an Alloy 625 girth weld. The extensive 3-D numerical analyses provide a large set of solutions for J in cracked pipes and cylinders with mismatched girth welds while, at the same time, gaining additional understanding of the applicability of ECA procedures in welded cracked structural components.