Bends for Critical Line Pipe Projects: Advantages of the Off-Line Full Quenching and Tempering

Recent trends for linepipe projects reflect a sustained increasing complexity: Sour Service (SS), High Temperature & High Pressure (HT/HP) field conditions, deep and ultra-deep water oil and gas transportation, Artic and Artic “alike” areas. This reflects into stringent requirements for manufacturing and testing of the concerned products, which are not only the straight pipes but also a number of accessories, among which bends are the ones presenting the most complex combination of critical issues. As long as a whole chain reliability standpoint is assumed as the main concern, the design, production and supply of the bends have become a key stage within a linepipe project. Bends for linepipe projects are produced in general by hot induction bending (HIB). Two different fabrication routes can be clearly identified: HIB followed by Stress Relieving (SR) and HIB followed by off-line full quenching and tempering (QT X60 to X70 steel grades, different bending and post-bending heat treatments conditions and mother pipe chemistries. For each analysed item, the final bend, the corresponding mother pipe and samples taken in as-bent / as-quenched (TR / QT) conditions were fully characterised in terms of mechanical properties, hardness profiles and microstructure features. As a result, a much better performance was found for the bends produced by the off-line full Q&T method, principally due to the better quenching efficiency with respect to the in-line system. Production of bends through out the traditional method can be seen as a reliable option for bends which are not going to face hard conditions in the field, and therefore the corresponding specification requirements are no stringent as well (i.e. X52 grade or lower, Charpy tests required at 0°C or higher temperature, no corrosion tests required, etc.). If either stringent conditions are required or X60-X80 steel grades are involved, the off-line quenching tank route becomes the reliable option.Copyright © 2009 by ASME