INITIAL IMPERFECTIONS OF HIGH STRENGTH UOE MANUFACTURED STEEL PIPES AND THEIR EFFECTS ON PIPE BUCKLING

Abstract

The use of high strength steel pipes in the pipeline industry is increasing due to its cost reduction benefits. The development of high steel grades has enabled the manufacturing of line pipes with large diameter to thickness ratios. The high strength large diameter pipes are commonly manufactured by the UOE forming process, during which, thin steel plates are deformed using mechanical presses and expansion. The plastic deformation of pipe material induced during the UOE forming process is likely to result in initial geometric imperfections in the manufactured pipes. This research introduces a new imperfection measurement technique that employs a high resolution surface profiler for geometric data acquisition, and a reverse engineering software for the geometric analysis of line pipes. The measurements reveal that the geometric imperfections in UOE pipes follow certain patterns in terms of outside radii and pipe wall thickness deviations with respect to the location of the longitudinal seam weld. The outside radii are less than the nominal value at locations opposite to the longitudinal seam weld in a pipe cross section. The pipe wall thickness values are lower at locations near the longitudinal seam weld compared to the thickness at locations opposite to the longitudinal seam weld. These patterns can be expressed in a cylindrical coordinate system with respect to the longitudinal seam weld's location. The sources of the observed patterns can be traced back to the UOE manufacturing process itself. A finite element analysis further confirms that the thickness deviation imperfection observed in the scanned pipes has significant effects on the buckling capacity of line pipes. Critical buckling capacity was reduced up to 27% in unpressurized pipes modelled with the observed thickness deviation compared to the ideal case. This reduction occurred for the case when compressive strain develops in the regions near the location of the longitudinal seam weld. Similarly, for pressurized pipes, the thickness deviation imperfection observed in the scanned pipes reduced the peak moment compared to the ideal pipe by up to 20%. The dimensional tolerances for wall thickness specified in the ASTM codes may not be reliable in this regard. These tolerances need to be revisited for the cases where the critical strain capacity governs the design of pipeline. A higher quality control for UOE manufacturing process is required. The observed imperfection patterns will aid towards improving the quality control of UOE manufactured pipes.