Failure analysis of a steel tube joint perforated by corrosion in a well-drilling pipe

摘要

The corroded steel tube joint belonging to an offshore oil well-drilling pipeline was inves tigated. All of the pits and punctures distributed along the "steps" are formed by off-centre machining on the internal tube surface, and the pits have a horse's hoof and hemispherical contour. The metallurgical structure of some spots on non-penetrating corrosion pits and penetrating punctures looked markedly curved, i.e., curved deflections in the banded structure (or streamlines) were visible. The pits contour and metallurgical structure indicate that the pits and punctures are characterised by cavitation erosion. The chemical composition of the corroded steel tube joint is similar to 1320, 1330 and L80-1 steels specified in ASTM standards, and the content of non-metallic impurities in the failed steel tube was identified as Grade 1.0-1.5, which is within the permitted range. Corrosion pit surfaces and the tube joint showed a deposit layer and a clay-like substance that was rich in Fe, Mn, Ca, Ti, Cu, C, 0, S and Cl, and "crystal sugar-like" appearance observed on location of the penetrating puncture, indicating H_2S- and CO_2-assisted corrosion mechanisms. These investigations indicated that failure of the pipeline occurred by erosion corrosion, cavitations erosion and chemical corrosion, which is from both mechanical and chemical actions.