Quantitative study on far-field magnetic signal response of steel pipe girth welds with weak magnetic excitation

Abstract

Unequal wall thickness welds are weaknesses in pipelines, which require stress monitoring. The magnetic-based overground stress testing method avoids excavation costs and risks during strain gauge installation. However, magnetic signals excited by the geomagnetic field lack sufficient strength and stability to meet online stress monitoring requirements. Enhancing signal strength and stability via the excitation of magnetic fields is proposed as a solution to this problem. The study of response law of magnetic signals to stress in girth welds with nonlinear material and structures under excitation magnetic fields and the effect of hysteresis on the sensitivity of magnetic signal response is meaningful. In this paper, a full-size pipeline multi-scale experimental system with unequal wall thickness girth weld is established to reveal the enhancement mechanism of excitation magnetic field on the magnetic signal and the effect of hysteresis on the sensitivity of magnetic signal. Drawing on finite element theory, a forward model for the multi-zone far-field magnetic field of girth welds is developed and validated. The results indicate that under excitation magnetic fields, magnetic signals respond rapidly to stress with excellent repeatability. The stress sensitivity of magnetic signals in X80 steel with unequal wall thickness is 6.8 (nT/m)/MPa. The forward model accurately quantifies the magnetic signal response to stress, and on-site applications confirm the feasibility of technology.