Crosspoint Localization of Spiral and Girth Welds of Spiral Steel Pipelines

Crosspoint of spiral and girth welds of spiral steel pipelines transporting high-pressure oil and gas is easy to crack due to welding defects and corrosion, so routine excavation and careful inspection are required. In order to reduce the workload and cost of excavation, it is necessary to determine the crosspoint's angular position on the girth weld. This article proposes a low-cost and high-efficiency method for locating the spiral-girth weld crosspoint based on the inner Spherical Detector (SD). Theoretical analysis and experiments demonstrate that the magnetometer with a smaller lift-off could detect more noticeable magnetic valley of the spiral welds at the pipe bottom, so a smaller SD should be used; the spiral direction can be determined by the phase difference of the dual-channel magnetic signals, when dual magnetometers are symmetrically fixed about the median vertical plane on the rotation axis of the SD. When the girth weld, at least one spiral weld and the spiral direction are detected and determined, the crosspoint's angular position can be calculated. The maximum localization error of the crosspoint is less than 30 degrees and the average error is less than 20 degrees, that is, no more than one hour's angle, whose accuracy is sufficient for excavation and maintenance in the field, because it will reduce the amount of excavation by nearly half.