Longitudinal fracture and water accumulation at 6 o'clock position of an API 5L X52 oil pipeline

Failure analysis of a 16-inch diameter microalloyed steel of grade API 5L X52 oil pipeline carrying different liquid hydrocarbons including kerosene, petrol, and gasoline was investigated. The failure was in the form of a longitudinal notch associated with a bulging at the place of opening, at 6 o'clock position. In order to identify the main cause of failure, chemical analysis of oil fluids and the corrosion deposits inside the pipeline were examined. Microstructural investigations were also performed at the location of the damage and the cutting edges. It was found that iron oxides, iron hydroxides, iron sulfide, and other corrosion products accumulated at the 6 o'clock position due to the partial separation of the water phase from the oil fluid phase. The water phase separation and accumulation were mainly attributed to the lower density and the low designed velocity of fluids inside the pipeline. The failure mechanism was further verified by computational fluid dynamics (CFD) simulation, considering the effect of water volume fraction and the flow rate on the flow pattern inside the pipe. The identified flow pattern revealed accumulation of water layers at the bottom of the pipe causing long time exposure of steel surface to the contaminated water, which eventually led to different corrosion reactions /pit formation. Coalescence of corrosion pits inside the thinned pipeline under the action of pressurized fluid finally caused the failure. The pitting corrosion was identified as the primary failure mechanism that occurred under the corrosion deposits.