Fracture Toughness Evaluation of Pre-1980's Electric Resistance Welded Pipeline Seam Welds

In the 1950's and 1960's, economic expansion in Europe and North America led to significant growth in transmission pipeline capacity. Thus, many pipelines in operation today were constructed prior to 1980 and result in an aging pipeline infrastructure. An important mechanical integrity threat affecting oil and gas pipelines is the potential for fast fracture of longitudinal seam welds. A major factor affecting fast fracture resistance involves the welding practices used prior to 1980. The most common method was low-frequency electric resistance welding (LF- ERW). Imperfections were more prevalent using LF-ERW compared with modern high-frequency electric resistance welding (HF-ERW), employed for the last 40 years. Another important factor is the steel quality produced during this era. High tramp element levels, particularly sulfur, led to lower and more variable fracture toughness than steels produced after about 1980. To effectively manage the risk of fast fracture a strong statistical understanding of quasi-static fracture toughness in pipeline steels is important but has been lacking. To advance integrity management of existing pipeline systems, the fracture toughness of vintage pipeline materials has been characterized by conducting 550 fracture toughness tests, collecting data from literature sources (25) [1] [2], and test results supplied from a secondary pipeline operator (45), for a total dataset of 620. The testing program included a wide range of manufacturers and fabrication years based on sampling over 30 individual pipelines. The results suggest that it is possible to treat pre-1980 pipeline steels/welds as a single cohort. This is valuable since toughness data for individual pipelines are often insufficient or not available. The test data was analyzed to provide input for either a deterministic or probabilistic analysis. Consideration for the constraint effects of specimen geometry was also investigated to address the transferability of test results to an axially oriented flaw in a thin walled 0.2-0.4in (5-10 mm) pipeline. Constraint was characterized by using J-Q and J-A(2) theory in a 3D FE simulation. Finally, analysis of the weld microstructure provided fundamental insight into the operative fracture mechanisms responsible for their toughness properties.