Failure analysis of a gas pipeline

A 15-km section of a gas pipeline presented leakage during field hydrostatic testing. Preliminary investigation indicated the presence of a cavity, which is roughly composed of a cylindrical surface (region 2) and a plane surface (region 1), both oblique and forming a V-shaped cavity, presenting thickness of 200 mum in the region adjacent to the bore. The surface of the region 1 showed more intense surface plastic deformation (approximately 60 mum deep) than region 2 (approximately 20 mum deep), which presented the formation of metallic chips. The presence of encrusted SiO2 particles associated with intense superficial plastic deformation lines of the ferritic microstructure-following the orientation of SiO2 particle/ferrite grains interface and resembling hardness indentation marks-was also observed in the region 1. The results indicated that the formation of the cavity was promoted by the action of an erosion mechanism due to an error during the sand blasting procedure. The local thinning and formation of a stress raiser led to the localised transversal rupture of the tube by a ductile mechanism during the pipeline field hydrostatic testing. (C) 2003 Elsevier Ltd. All rights reserved.