Toughness Evaluation and Integrity Assessment of Modern X80 Linepipe Welds for the Application of High Pressure Hydrogen Pipeline

Abstract The fracture toughness test under 100% hydrogen with testing pressure of 21 MPa was conducted on a recently produced Grade X80 linepipe and its seam and girth welds. For comparison, two X70 linepipes and one X65 linepipe and their welds were tested as well. For the X80 linepipe, the seam weld heat affected zone (HAZ) exhibited the same level of fracture toughness as the base metal. The toughness is higher than the minimum requirement in ASME B31.12 when it is measured by the crack initiation toughness not by the conventional 0.2mm offset value in a J-R curve. The girth weld HAZ of the X80 linepipe showed higher fracture toughness than the seam weld HAZ. Using the fracture toughness values obtained from the weld fracture toughness tests in 21 MPa hydrogen, the critical depth of planar axial flaw was determined for different measured values of toughness and design factor using assessment procedures in API 579. The critical flaw depth strongly depends on the residual stress assumption used in the assessment. The allowable planar flaw dimensions for girth welds were determined using the assessment procedure in Annex A of API 1104. It was shown that the girth weld of the X80 linepipe would have workable allowable flaw dimensions due to the high toughness measured in the tests. However, girth welds with the required minimum toughness of ASME B31.12 would have allowable flaw dimensions much smaller than the workmanship criteria in API 1104 or equivalent standards. There can be multiple causes for this outcome which should be investigated further.