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Validation of Assessment Methods for Production Scale Girth Welding of High Strength Steel Pipelines with Multiple Pipe Sources
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Validation of Assessment Methods for Production Scale Girth Welding of High Strength Steel Pipelines with Multiple Pipe Sources
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    Issue: 1.0
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    There is an increasing need to deliver energy from sources in remote areas to demand centers. For example, in North America, the delivery of gas from Alaska to demand centers in the lower 48 states is of major economic and strategic interest. This will require the design and construction of large diameter, long distance pipelines through adverse environments. The economics of these pipelines are dependent on the use of high strength steels to reduce the tonnage of steel required and on high productivity girth welding processes to shorten the construction period. Robust inspection methods are required to reliably detect and size any defects which may occur during welding, and an equally robust method is required to assess the impact of those defects on the safe operation of the pipeline. There are a number of methods that are commonly used for the assessment of a girth weld containing a ‘fabrication’ defect. These range from the more generic workmanship (or weld quality) defect acceptance limits to the more complex pipeline specific engineering critical assessment (ECA) methodologies where defect limits are derived based on the pipe size, material properties and pipeline loading conditions. The ECA approach is widely used to derive girth weld defect acceptance limits that are specific to a pipeline. They are based on either semi-analytical methods or on the results of large-scale tests on pipeline girth welds. There is no one standardized method. The guidance produced by the European Pipeline Research Group (EPRG) is an example of an established methodology based on the results of large-scale tests, while commonly used pipeline specific analytical assessment methods include API 1104a and CSA Z662a . Other commonly used semi-analytical methods, which are more generic in application, include API 579-1/ASME FFS-1a and BS 7910a . The application of any of these methods has certain limitations. For example, there is no ‘upper limit’ to line pipe strength specified for use of the ECA methodology presented in API 1104, although there are limitations to some of the equations used within the procedure which limit their range of applicability up to grade X80 line pipe. Similarly, the EPRG guidelines are limited to pipelines constructed from grade X70 line pipe; although much work has been undertaken to demonstrate the applicability of the guidelines to pipelines constructed from grade X80 line pipe, an updated guidance document has yet to be published. The objective of this project was to investigate the applicability of these ‘established’ methods for defining girth weld defect acceptance criteria for pipelines constructed from grade X100 line pipe. BP provided the project with ten girth welds following completion of their full-scale X100 operational trial at GL Noble Denton’s Spadeadam test facility located in Cumbria, England. This BP project involved the construction of two sections of 48in diameter pipeline. The construction process replicated full-scale practice, where the pipeline was welded above ground and then lowered into the ditch and backfilled. The pipeline test sections were then pressure cycled at a frequency to simulate 40 years of operation over a two year period. The project team selected the most appropriate girth welds that they considered would enable the effects of material variability between abutting pipes, different heats, and different pipe manufacturers to be investigated. A materials test program was developed to fully characterize the performance of each girth weld. In total, 217 tensile tests, 108 Charpy impact tests and 54 fracture mechanics tests were undertaken, in addition to weld macro sections and hardness surveys. The test program concluded with 30 curved wide plate (CWP) ‘mid-scale’ tests, of which 19 specimens contained machined surface breaking defects of specified depth and length dimensions. The remaining CWP specimens contained either natural welding defects (e.g., lack of penetration, lack of side wall fusion or porosity), deliberate defects that were introduced during welding, or combinations of natural and deliberate defects. Each CWP test was assessed using the procedures given in API 1104 (Option 2), EPRG, CSA Z662, BS 7910 and API 579-1/ASME FFS-1. The results of the small-scale test program for each weld were used as input into each assessment. The results of the assessments were compared with the results from the CWP tests to assess the limitations of each assessment method. In general, each assessment method performed well, giving a conservative prediction of failure stress. However, the accuracy of the prediction was found to vary significantly.
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