There is increasing demand for subsea transport of well-produced fluids that requires the use of corrosion resistant pipelines such as stainless steel or more recently bi-metallic pipes. The latter are made of carbon steel (CS) pipe and a thin (typically 3.0 mm thick) internal layer of corrosion resistant alloy (CRA) such as 316L, 625, 825 or 904L. Bi-metallic pipe joints are girth-welded together to make a subsea pipeline with nickel-based consumables which often undermatch the parent metal at yield. Such pipelines are often installed with the reel-lay method which is very efficient but subjects the pipe to cyclic plastic straining. In this context, weld metal undermatch may be problematic as engineering critical assessment (ECA), undertaken to derive weld flaw acceptance criteria, typically requires that the weld metal overmatches the parent material when applied loads are in excess of yield.
In the subsea industry, it is common practice to determine a fracture toughness resistance curve from specimens in the as-received condition and use it for the ECA of reeled pipes subjected to cyclic plastic straining. This is because it has been shown that cyclic plastic pre-strain has a negligible effect on the fracture toughness of overmatch welds. There was concern, however, that this may not hold for undermatch welds. Therefore, the authors carried out a fracture testing program to quantify the effect of cyclic plastic pre-strain on the fracture toughness of undermatch welds in bimetallic pipes. It is shown in this paper that such pre-strain has a negligible effect on toughness for undermatch welds.
Although most pipeline girth weld flaws are embedded, they are typically approximated by surface breaking flaws in the integrity assessment because the available reference stress solutions for embedded flaws are too conservative when used to estimate the crack driving force. This approximation, however, may not always be appropriate. To address this deficiency, the authors have recently proposed a set of new reference stress solutions for the assessment of embedded flaws in even/overmatch welds. In continuation of this work, a procedure for the ECA of embedded flaws in undermatch bi-metallic pipeline girth welds is proposed and numerically validated in this paper.
