Recent Concepts for the Welding of High Strain Pipelines

Standard allowable stress-based pipeline designs (strain demand ≤ 0.5%) are now giving way to more complex strain-based designs (strain demand higher than 0.5%) as the locations of future pipelines move into regions of increased strain demand. The increase in required levels of strain demand are attributed to seismic activity, soil movement, soil liquefaction, frost heave, thaw settlement, ice scour or a combination thereof. Pipelines in high strain demand regions are typically limited by the strain capacity of the girth weld. As strain-based pipeline design has matured, it has become evident that specific material properties (both weld metal and line pipe), defect size, defect location, misalignment, and operating pressure each affect the strain capacity of the pipeline. This paper reviews proposed design and testing methodologies for the qualification of strain-based design welding procedures. These methods have been applied in the development and qualification of welding procedures for the construction of pipelines subject to longitudinal tensile strains in excess of 2%. Strain-based design requires considerably more effort than traditional design in terms of girth weld qualification and testing. To ensure adequate girth weld strain capacity for strain-based design the testing includes large scale and full-scale pressurized testing for design validation.