Experimental Assessment of Residual Fatigue Life of High Pressure Pipeline Section Containing Longitudinal Cracks

Non-destructive inspection of an operated section of DN 300 high pressure gas pipeline (internal diameter 300 mm) indicated a quite frequent occurrence of longitudinal cracks of depths up to 50 % of the pipe wall thickness. As such cracks would be very dangerous from the viewpoint of further pipeline operation, selected sections of the pipeline were taken from the ground for an experimental programme aimed at evaluation of residual fatigue life. The first part of the programme concerned (i) experimental calibration of direct current potential drop (DCPD) method of crack depth estimation, complicated by the longitudinal position of the cracks, (ii) estimation of actual depths of selected, most distinct cracks and (iii) metallographical evaluation of profiles and depths of selected cracks. The DCPD calibration confirmed that for the specific configuration of electrodes used and characteristic crack profile, analytical calibration using Johnson´s formula corresponds to the actual crack depth quite exactly provided that the cracks are of radial direction. However, metallographical analyses showed that analysed actual cracks, though quite long, even more than 2 mm in some cases, were quite inclined, in some cases almost parallel to the pipe surface. These cracks occurred obviously in production during rolling process, which was a good sign for a relative safety of the affected pipeline. Further theoretical considerations were made to enable estimations of the inclined crack depths using the DCPD method, necessary for crack growth measurement during internal pressure fatigue tests, which followed.