Experimental Testing and Evaluation of Crack Defects in Line Pipe

Pipelines are in wide use throughout the world, and aging pipelines may experience defects such as environmental or stress-induced cracking. The evaluation of crack defects is important for continued safe operation of pipelines. At present, there are several assessment methods for crack-like defects in pipelines including API579, BS 7910, NG18, software applications, as well as numerical modeling approaches. All have been used successfully to evaluate crack defects, but the degree of conservatism and sensitivity to the various input parameters is not known. To address this need, a series of full-scale burst tests was undertaken on end-capped, seam-welded pipe specimens. The tests were carried out on 508 mm (20 inch) diameter Grade API 5L X60 line pipe with a 5.7 mm wall thickness. Elliptical cracks were created by first cutting a longitudinally oriented narrow slit in each pipe and then pre-fatiguing the pipes to create sharp cracks of different depths. Rupture tests were conducted by pressurizing the pipes to failure and the failure pressure was evaluated using current assessment methods. Examination of the fracture surface showed that the pipe sections failed by ductile tearing, as expected for the material and crack sizes. It was found that the Level 3 FAD for API 579 (J approach, using the cylinder equations) and CorLAS provided the most accurate prediction in comparison with the other methods i.e. BS7910 and NG-18.