Advanced Strain-Based Design Pipeline Welding Technologies

Abstract

To meet the increasing worldwide demand for natural gas, there is a need to safely and economically develop remotely located resources. Pipeline construction is a major activity required to connect these remote resources to markets. Such pipeline routes may cross areas containing geohazards such as discontinuous permafrost, active seismicity and offshore ice gouging. These pipelines may be subjected to longitudinal strains above 0.5%. To safely design pipelines for such conditions, a strain-based design (SBD) approach can be used in addition to conventional allowable stress designs (ASD). Significant pipeline construction cost savings can be achieved with the use of higher strength steels (X70+) due to reduced pipe wall thicknesses (less steel) and faster girth welding. However, a robust welding technology for higher strength SBD pipelines is often a technology gap depending on the target level of longitudinal strain that needs to be accommodated, since such applications often demand excellent weld toughness at low temperatures (−15°C) and high tensile strength (>120ksi). This paper discusses the development of an enabling welding technology that offers a superior combination of strength and toughness compared to commercially available technologies. Acicular ferrite interspersed in martensite (AFIM) has been previously identified as a useful high strength weld metal microstructure that can be applied in field pipeline construction. This paper describes how this microstructure has been used to create welds with excellent strength overmatch and good ductile tearing resistance for X80 SBD pipelines. This approach has been implemented for mainline, double-joining and repair welding applications. This paper describes the welding procedures, mechanical properties achieved, estimated strain capacities, and the results of a full-scale pipe strain capacity test.