Line pipe is often coated prior to installation in order to achieve some protection against the environment. Many of the coatings used today require the pipe to not only be cleaned and degreased, but also to be preheated to a temperature of 200–240°C during application of the coating material. A typical coating thermal cycle involves rapid heating of the pipe using induction coils, application of the coating, and quenching to cool the pipe for handling purposes. It is generally understood that this thermal treatment on UOE line pipe, which can last from a couple to as many as ten minutes, has an effect on the pipe yield and, to a lesser extent, tensile strength. For ultra-deepwater offshore applications, where collapse is often the controlling design case, the increase in hoop-compressive yield strength is viewed as desirable because of the corresponding increase in collapse pressure. For onshore applications, however, bending due to differential ground movements can be the primary design consideration. In this case, an increase in tensile yield strength in the longitudinal direction may occur, and may result in a higher Y/T ratio. This increased Y/T can reduce the critical buckling strain of a pipeline designed to this limit state. In this paper, the effects of a coating thermal treatment on X70 grade UOE line pipe material properties are presented with particular attention being paid to the effect on the hoop-compressive and axial tensile yield as well as Y/T ratio. Both coating mill and coupon scale thermal treatment were investigated. Comparison was made between full thickness and round bar axial tensile samples. In addition, the influence of over-ageing or extended duration heating is investigated with respect to its impact on strength and ductility as well as the Charpy impact properties. Results of this study indicate increases between 13 and 16% for hoop-compressive yields while increases in the axial direction were approximately half that magnitude. Y/T ratio increases of around 5% were seen in the axial direction. The over ageing study did not demonstrate any detrimental effect of extended duration thermal treatment on the tested material properties.
Author: Hines, Josh R., Timms, Chris M, and DeGeer, Duane D.
Publisher: OMAE International Conference on Offshore Mechanics and Arctic Engineering, June 10-15, San Diego, California
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