Abstract
Manufacturing flaws are common on low frequency electric resistance welded pipe (LF-ERW), such as hook cracks and lack of fusion. One less commonly identified flaw is a lamination that terminates at the ERW weld. This interaction results in a unique flaw geometry, occurring mostly at the pipe’s mid-wall thickness but can vary in severity along the length of the affected pipe joint. This paper discusses the significance of this flaw geometry, the method of formation, and the impact of typical operations (fatigue cycling and hydrostatic testing) on the strains along the indication. First, a comprehensive set of metallurgical examinations were performed, including metallography along the flaw. Examination included light microscopy, scanning electron microscopy, electron dispersive x-ray spectroscopy, and sub-load hardness testing (HV0.5) along the flaw. This set of examinations helped identify the flaw and how it formed.
Second, several tensile straps were prepared across the indication to better understand how the feature behaves during the operation of the pipeline. Each tensile specimen was monitored using a digital image correlation (DIC) system with a focus on the ERW seam weld. Tensile straps were prepared and tested to understand the influence of a hydrotest on normal operations by completing the following three phases of simulated operations: (1) normal operations with fatigue cycling at stress ranges equivalent to operating pressures between 6% SMYS and 56% SMYS (MOP), (2) a spike hydrostatic test at stresses equivalent to an operating pressure of 80% SMYS, and (3) normal operations with fatigue cycling at stress ranges equivalent to operating pressures between 6% SMYS and 56% SMYS (MOP). This sequence of testing permitted comparison of the strains at the flaw before and after the simulated hydrotest conditions. The comparison between the strains recorded via the DIC system showed an increase in strain of approximately 40% across the feature in the sample subjected to the spike test, which included an irrecoverable strain of approximately 0.06%. The evaluation process described in this paper provided a valuable understanding of the impacts of a hydrostatic test on manufacturing flaws. This type of evaluation can be used to better understand the effect of spike hydrostatic testing on other common planar flaws including ERW hook cracks, ERW lack of fusions, and stress corrosion cracking.
