Inspection For Double-layered Steel Tube Using Transient Electromagnetic Method

Casing damage is increasing year by year in and abroad, that will directly exert an influence on the working life and production of oil and gas which bring huge loss to our national economy. At present, casing inspecting instruments can only detect single pipe because of the limit of their structures and measuring methods, tubing must be removed when inspecting casing, which leads to low work efficiency and high cost of inspection. To improve the convenience of detecting casing, in this paper, inspection for double-layered steel tube using transient electromagnetic method is present, casing be detected without removing tubing becomes possible.The paper is based on the theory of transient electromagnetic, the transient electromagnetic field in casing is calculated, then the formulas of equivalent electric parameters for pipes is present, the frequency spectrum of transmit waveform and the influence of parameters of waveform on detecting signal are analyzed; the equivalent circuit model of electromagnetic probe is built, step response in coil is analyzed, horizontal and vertical probes are designed and their parameters are calculated; the characteristic of transient electromagnetic signal is analyzed, and the influence of parameters of instrument on received signal is discussed; J55 steel pipes used in oil and water wells are used to do experiments, different defects combinations of double-layered steel tube are inspected by optimizing experimental parameters, good laboratorial detecting effects is achieved. The results of inspection show that corrosion and horizontal or vertical crack with certain length on double-layered steel tube can be detected by the testing method. The position and size of abnormal point can be seen clearly by using voltage section analysis for testing signals. Defects on casing and tubing are divided, and the kinds of defects on tubing are identified by the characteristics of abnormal curves in voltage section, the thicknesses of tubing and casing are estimated. The visualization of the inspection result is achieved, by which damages on double layered pipes can be seen directly. At last, repeat experiments on specimens are done; data analysis result implies the probes have good stability. This research subject makes possible tubing and casing be detected simultaneously.