ABSTRACT

Strain-based design (SBD) of oil and gas pipelines has supported the development of frontier energy sources by permitting cost-effective designs that can withstand large longitudinal strains imposed by discontinuous permafrost, active seismicity, and offshore ice gouging. A key element of SBD is the proper characterization and prediction of pipeline strain capacity in the presence of large plastic deformation and ductile tearing. Recently, a strain capacity prediction tool has been developed that accounts for a wide range of input parameters including pipe geometry, flaw geometry, base pipe and weld material properties, and high-low misalignment. The tool has been validated with more than 50 full-scale pipe tests. However, in all of these tests, the girth weld fill and cap passes were conducted using a single filler metal chemistry. Therefore, the girth weld was considered to have similar material properties throughout its thickness.

This paper will review recent work on strain capacity predictions of a pipe whose girth weld fill passes were deposited using both flux-core arc weld (FCAW) and pulsed gas-metal arc weld (P-GMAW) processes. Biaxial full scale testing (FST) was performed on the welded pipe with surface breaking notches located in the OD weld center line. Pipe failure occurred due to significant ductile tearing (~8mm) that predominantly occurred in the FCAW weld. Different combinations of tensile properties and R-curves of the FCAW and P-GMAW welds were considered in the prediction of the pipe strain capacity. It was found that the strain capacity achieved during full-scale biaxial testing could be successfully predicted by considering the lower mechanical properties of the FCAW material. Proper assessment of the various weld material properties can therefore permit the successful SC prediction of a pipe welded with multiple fill pass materials.

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