Factors Affecting Ductile Fracture in Offshore Gas Pipelines

Abstract

tive stress is determined by the differential pressure across the pipe wall thickness. SUMMARYA series of fracture propagation experiments on 12.75 inch x 0.220 inch 5LX65 pipe were conducted using air as the pressuring medium. These 12-inch diameter experiments were conducted inside a 48inch diameter pressure vessel that was half filled with water and the remaining volume was pressured with air to simulate a water pressure corresponding to various water depths. {~e2yxperiments were designed based on earlier knowledge ' obtained on differentia pressure, decompression into pressured water and __ effects of water backfill. Experiments on land with sand backfill and the same or lower differential pressures readily propagated a ductile fracture in this lot of pipes. Five experiments were conductedunder water and in no instance was a long running ductile fracture obtained. Furthermore, the arre~ts that occurred were similar to arrests from a reflected decompression wave in that the fracture arrested while proceeding axially along the pipe rather than after turning into the helical direction, the usual direction of fracture arrest. It was not possible, in any of these experiments, for the reflected wave to have caused the arrest. It is currently believed that the over-pressure wave in the water from the exhausting gas sufficiently lowered the differential pressure locally and thus caused the arrest. The existence of the overpressure wave is not believed to be an artifact of this test. Some of the wave characteristics, velocity and shape for example, probably are dependent, however, on the test configuration. An overpressure wave should exist in the real offshore environment and should also lower the differential pressure across the pipe wall thickness. ThisThe objective os this research program has been to obtain an understanding of ductile fracture propagation in the offshore environment. The approach toward this objective has been to examine individually, the various factors that are known to affect ductile fracture of pipes in general. The mOfI)important of these factorscfye backfill effec:s ,gas d{Z~m pression effects ,and external p~pe pressure This paper presents the results of experimental research conducted during the past three years with Research was begun in 1980, and is continuing the objective of understanding ductile fracture propa-today to further examine the combined effects described gation in the offshore environment. Experiments above on long ductile fracture propagation insimuhave been conducted to examine decompression phenomenon lated deep offshore environments. These experiments inside the carrier pipe when the exhausting gas is were conducted on small diameter pipe pressured with into a simulated deep water environment. Ductile air and surrounded by pressured water inside a large fracture experiments of 12 inch pipe in a simulated diameter pressure vessel. This more recent research deep offshore environment have also been examined. and the conclusions are discussed below. The most current resear...

Fracture control assessment in the base metal and welded joint of a pipeline steel

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