A Dipole Model for Magnetic Leakage Fields Created by a Circular Butt-Welded Joint

Abstract

Magnetic leakage fields created by circular butt-welded joints of a trunk pipeline are characterized by a variety of topographies differing in both shape and extension. Four sources of magnetic leakage fields around a circular welded joint are distinguished. Each source is simulated by a dipole moment with certain parameters. The developed mathematical model makes it possible to describe with high accuracy the entire variety of topographies of leakage fields around circular butt-welded joints of a trunk pipeline.When an oil or gas trunk pipeline (OGTP) is constructed, the pipes of which it consists are butt jointed by welding. The properties of the transverse welded joints differ significantly from those of longitudinal or spiral welded joints in the pipes they connect. The reason for these differences is that the longitudinal and spiral welded joints are created under strictly controlled manufacturing conditions, while butt-welded joints are created under field conditions where the manufacturing prescriptions can be observed only with great difficulty and the control over their implementation is even more problematic. For a pipe around 1.5 m in diameter and 10 to 15 m long, it is virtually impossible to ensure strict alignment of the joints of pipe walls 15 to 25 mm thick along the entire circumference; moreover, the welded pipes may have different thicknesses. For these reasons, to ensure the required strength and tightness of a welded joint, the gaps that inevitably appear should be filled with the material of an arc-welding electrode. As a result, in addition to reinforcement beads (collars) on the interior and outer sides of a welded joint [1], "steps" often appear, which are due to spatial mismatch of the segments of the pipes' ends ( Fig. 1).In the process of magnetic in-tube diagnostics of operating oil and gas pipelines, the magnetic leakage fields created by transverse butt-welded joints are recorded (along with fields of flaws) by an acoustic flaw detector to be further used both for referencing segments of an oil or gas pipeline to the path passed by a flaw detector (for a path length up to 100 km) and for timely forecasting of possible violations of the butt integrity and preventing emergencies.The in-tube testing is conducted in an applied magnetic field directed along the pipe's longitudinal axis. The strength of the magnetizing field is usually such that the material of the pipe is saturated (the induction is ~1.5 T). During testing, the tangential component of the magnetic field strength directed along the pipe's axis is measured at a distance of ~5 mm from the inner surface of the pipe's wall.Study [1] contains the results of a computer simulation of the magnetic field created by a transverse welded joint. We performed this simulation using the ELCUT 4.2 code (see, e.g., [2]). Since the diameter of a pipe is significantly larger than the thickness of its wall, the problem was simulated on a plane. Here, the main results of the simulation that were used in the cited article for developing a mathe...