Intensely Spontaneous Magnetization of a Drill Stem for Oil and Gas Field

The objective of this study was a theoretical explanation of the reason causing a new phenomenon in NDT. When some drill stems for oil and gas field operated through a long period of time were maintained, NDT personnel discovered that the magnetic particle inspection could be carried out on them without magnetization, and only had to apply suspension upon them. An item analysis was conducted on this concrete condition, on the basis of geomagnetic induction effect and the unsymmetrical result of the magnetization demagnetization process of the ferromagnetic materials. The principal finding of this item analysis was that a ferromagnetic object rotating in the geomagnetic field had to be magnetized intensely. On the basis of this finding, a drill stem for oil and gas field after its operation for a long time must be magnetized intensely and spontaneously. In May, 2002, Mr. David Yan, at The Forging & Casting Factory for Heavy-Duty, Chong-Qing, China, asked by telephone the author the reason causing a new phenomenon (When the drill stem for oil and gas field are maintained after a long period service, all one has to do is to apply the magnetic suspension directly upon the stems, the magnetic particle testing on them will be carried on, because they are already magnetized intensely). Present paper is specially written for answering this difficult problem. 1. The rotating of an object made of ferromagnetic material perpendicular to the geomagnetic field A rotating object with any shape of transversal section is composed of countless thin pieces through the rotating axis (Figure 1.a). And each thin piece may be considered as a composition of infinite closed coils in frame shape (Figure 1.b). When an object with electric conductivity is whirling around axis ω perpendicular to the geomagnetic field He (Figure 1.c, for simplicity suppose the object is in the shape of a cylinder, and its central axis coincides with axis ω), these closed coils all cut the magnetic lines of force of He. According to Faraday s Law on electromagnetic induction [1-3] , the electro-motive force E and induced electric current i must be induced in these closed coils, and E, i change directions twice in one cycle of the object rotation (Figure 1.c). In other words, each closed coil is a alternating current generator, but the generated electric current flows only within themselves, and these are Foucault (eddy) currents [1-3] . If the rotating object is made of ferromagnetic material, it must be magnetized and demagnetized respectively once in every cycle of rotation by the magnetic field H yielded from current i, that is, at an arbitrary point of a ferromagnetic rotating object there will be magnetization and demagnetization once in every cycle of whirl. Figure 1 Rotating object in geomagnetic field