Magnetic characterization techniques for monitoring gas pipelines under severe loading

Abstract

The reliable operation of trunk pipelines requires efficient monitoring of their technical state. At present, the issues involving evaluation of stress and strain in the most dangerous black spots of pipelines are urgent for the oil-and-gas industry. The problem is especially pressing for operations in corrosion-active media containing hydrogen sulfide, since hydrogenation leads to the formation of pores and cracks in steel. This paper is concerned with the magnetic, mechanical and structural characteristics of two advanced steel grades which are widely used for the construction of high-duty gas pipelines. The study shows that the numerical values of magnetic parameters depend on the magnetization direction about the control-rolling axis and correlate with the strength of X70 steel grade. It is found that for estimating the operating tensile stress, one should measure the magnetic characteristics in different directions about the pipe axis. Exposure to the hydrogen-containing medium leads to a significant decrease in the metal plasticity, an increase in the ultimate stress and yield stress, and a slight increase in the coercivity. The duration of the hydrogen charging dramatically affects the deformation behaviour of coercivity, thus providing a tool for magnetic monitoring of mechanical stress in steel pipes for transporting aggressive types of natural gas. © 2014 WIT Press.