Current hydrogen pipeline code ASME B31.12 requires that pipe materials shall be qualified for adequate resistance to fracture in hydrogen gas based on Article KD-10 of ASME BPVC, Sec. VIII, Division 3. In order to assess the integrity of a hypothetical hydrogen pipeline, fracture toughness and fatigue crack growth tests under gaseous hydrogen at up to 21MPa were first conducted using a recent Grade X65 linepipe with fine grained bainitic microstructure. Fatigue crack growth in the pressurized linepipe with semi-elliptical surface flaw was calculated by the procedures described in the Article KD-10 using the da/dN data obtained from the X65 linepipe and the fatigue crack growth equation specified in ASME B31.12. Pressure cycles were applied to the pipe with a surface flaw to investigate the effects of pressure range and design factor. The critical crack size was analyzed using the failure assessment diagram (FAD) concept which is also specified in Article KD-10. Significant fatigue crack growth was not observed under the lower design factor such as fD = 0.5 with small pressure range, while fatigue crack growth was drastically accelerated under the higher design factor and large pressure fluctuation. Integrity assessment by FAD analysis for longitudinal semi-elliptical crack and girth weld flaw clarified how the toughness value affects the critical condition.

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