Research on Burst Tests of Pipeline With Spiral Weld Defects

On some old long distance transmission pipelines of PetroChina some severe spiral weld defects existed. These weld defects were found to be from a lack of penetration and lack of fusion during pipe manufacturing. To avoid possible failure accidents, tri-axis magnetic flux leakage (MFL) in-line inspections were performed on some certain pipelines. A large quantity of spiral weld defects were detected in the inspections and their sizes were reported, some of them with large depth and large length along the spiral weld seam. However, there is no applicable engineering critical assessment (ECA) method for this kind of defects up to now. Assessment methods provided in BS7910:2005 and other codes are only applicable to assess flaws in cylinder oriented axially or circumferentially. Projection processing must be used to utilize these methods for spiral flaws. Three other different assessment methods for spiral defects in linepipe were proposed by Mok et al., Fu and Jones, and Bai et al. separately. But these methods were originally developed for spiral corrosion and have not been proved to be applicable for spiral weld defects on the old pipelines of PetroChina. In this paper’s work, several burst tests of pipeline with spiral weld defects were carried out. Different projection assessment methods based on BS7910:2005 were conducted as well as the spiral corrosion assessment methods proposed in previous studies. Predicted results were compared with burst tests and analyzed in this paper. As a conclusion of comparison, the axial projection method based on BS7910:2005 was suggested to assess this kind of spiral weld defects despite of its conservatism.