An in-service failure was associated with an NPS 12, 6.35 nominal wall API 5L X42 grade pipe. This segment of the pipeline had been in service since 1957 and had a tar coating. The pipe was manufactured by piercing followed by rolling/extrusion to produce a seamless pipe. The pipeline has always carried refined petroleum products. The rupture event was captured by pressure history records that recorded an overpressure experienced during a sudden valve closure event. The section of pipe removed included portions of the upstream and downstream pipe sections to carryout a failure investigation. The paper presents the findings from the investigation. The paper presents the observations from inspection and experimental testing carried out on the removed pipe section. These included NDE, fracture surface examinations and destructive examination such as metallography that helped in obtaining information on the reduction in the resistance of the material to withstand the in-service pressure conditions. The above observations together with results from mechanical testing was used to estimate failure pressures based on a “deterministic” primary flaw size, using assessment methods (ECA) applicable to the failure case under consideration. The investigation of the failure indicated that the major contributing factors were: (a) the corrosion feature at the failure site that reduced the remaining wall thickness below 2mm, and the overpressure caused by the valve closure down stream in the vicinity of the failure site. Pipe specification checks from material removed from the section of the failed pipe, indicate that the pipe meets the API 5L X42 specification requirements.

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