DOI: 10.14489/td.2014.011.pp.014-018 

Yakimov E.V., Goldstein A.E., Bulgakov V.F., Alkhimov Yu.V., Belyankov V.Yu.
MEASURING OF ELECTROCONDUCTIVE PIPE WALL THICKNESS USING EDDY-CURRENT METHOD IN PRESENCE OF LARGE GAP BETWEEN PIPE AND TRANSDUCER
(pp. 14-18)

Abstract. The paper describes a dual-frequency method of detuning from the effect of the change in the gap between the pipe and the eddy-current transducer on the readings of the eddy-current thickness gauge for electrically conductive pipes. Light-alloy drill pipes made of aluminum alloys are used to perform workover in structural, exploration and production drilling. Due to the fact that the pipe walls are worn during in-service use, the wall thickness of the main pipe section must be controlled. Eddy current method provides high efficiency control and sensitivity, and it can be used to perform non-contact measurement. However, the eddy-current transducer signals significantly depend on the gap size between the transducer and the pipe. Excitation of the eddy-current transducer with the signal of the same frequency does not allow extracting information on the gap size. A dual-frequency method is more appropriate for this purpose. The dependencies of the signal amplitude and phases on the wall thicknesses ranging from 5,1 to 11,9 mm and on the gap size ranging from 1,5 to 14,5 mm were obtained using the measuring apparatus developed to study the dual-frequency eddy-current technique. The signal, which is the sum of the low frequency (125 Hz) and high frequency (2 kHz) harmonic signals, was fed to the transducer excitation winding. The results of the research show that the dependence of the phase of a low-frequency signal on the pipe wall thickness can be approxi-mated with an error of less than 0,18 mm by the 4th order polynomials, which coefficients are also the polynomial functions of the gap. The signal amplitude can be used to determine the gap size since the amplitude slightly depends on the pipe thickness and it significantly decreases as the pipe gap increases. The approximation of the dependence with an error of less than 0,08 mm can be performed as the sum of two exponential functions. The application of the proposed algorithms of detuning can reduce the error by more than tenfold (from 4,1 mm to 0,2 mm) in case of the gap size varies by 5 mm.

Keywords: eddy-current thickness meter, light-alloy drill pipe, gap between pipe and transducer.

 

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