High Strength Line Pipe Research Committee organized by The Iron and Steel Institute of Japan carried out two full-scale burst tests on X 70 line pipes, 48 in. o.d. × 0.720 in. w.t., with rich natural gas as the pressurizing gas. A theoretical investigation which gives the crack velocity change in terms of the crack velocity curve and the gas decompression velocity curve is presented, and the theoretical predictions and the experimental results are in good agreement. The developed method can predict the required notch ductilities obtained from Charpy test and DWTT in order to arrest a propagating shear fracture according to the type of gas, design stress and acceptable fracture length in the pipeline.