Fracture Propagation in Underwater Gas Pipelines

Two full-scale ductile fracture propagation experiments on segments of line pipe pressurized with nitrogen gas have been conducted underwater at a depth of 40 ft (12 m) to evaluate the ductile fracture phenomenon in underwater pipelines. The pipes were 22-in. (559-mm) diameter and 42-in. (1067-mm) diameter. Fracture velocities were measured and arrest conditions were observed. The overpressure in the water surrounding the pipe resulting from the release of the compressed nitrogen gas contained in the pipe was measured in both experiments. The overpressure in the water reduces the stress in the pipe wall and thus slows down the fracture. In addition, the water surrounding the pipe appears to be more effective than soil backfill in producing a slower fracture velocity. Both of these effects suggest a greater tendency toward arrest for a pipeline underwater than would be the case for the same pipeline buried in soil onshore. Further verification of this effect is planned and a modified version of the existing model for predicting ductile fracture in buried pipelines will be developed for underwater pipelines.