Patent GB-1513965-A (GB)
Title: Pipe-inspection apparatus for well bore piping
IPC: E21B47/09
Filed: 1975-05-14
Abstract: 1513965 Detecting electromagnetically defects SCHLUMBERGER Ltd 14 May 1975 [14 May 1974] 20280/75 Heading G1N Defects in a well pipe are detected by applying a magnetic flux to sections of the pipe and taking flux leakage and eddy current induction measurements. A pipe inspection tool 10 is lowered by cable 13 through a string of well casing 11 lining  the wall of well bore 12 and comprises an elongated body part 28 carrying a defect detection unit 31 and an induction type thickness measuring unit 35. The signals from the units are fed via cable 13 to memorizer circuit 27 and recorder 21 to which are also applied a depth signal from measuring wheel 25. Defect detection unit 31:-Magnetizer 34 An elongated paramagnetic or ferromagnetic annular core, Fig. 2 (not shown) forming an immediate part of tool body 28 has wound around it a potted magnetizing coil 42 surrounded by a non-magnetic casing and coupled to cable conductor 19 leading to power supply 22 whereby a longitudinally extending uni-directional magnetic field is produced which is coupled into the well casing by means of polepieces 45, 46. In accordance with the invention the magnetic reluctance of the core and polepieces 45, 46 is made sufficiently large in relation to the combined reluctances of the annular gaps between the outer faces of the polepieces and the casing 11 and the section of the casing that is being examined so that the average flux density is relatively constant in successive lengths of piping even when there are variations in the internal diameter of the casing and the condition of the wall surface thereof. This is achieved by making the core of low magnetic permeability, long length and small metal cross-sectional area. In order to detect any flux leakage due to defects and carry out eddy current induction measurements to detect defects in or near the internal surface of the casing 11 two sets of wall engaging inspection shoes 32, 33 are provided with the shoes of each set extending round the circumference of tool body 28 and the shoes of one set being angularly offset with respect to the shoes of the other set to assure a complete survey of casing 11. The outputs from the detectors in the shoes are processed in circuitry 23. Thickness measuring unit 35 Dependently suspended from tool body 28 is an inductive type thickness measuring unit 69 including an A.C. energized coil 68 and a receiver coil 67 coupled to a phase detection circuit 24. Due to the magnetizer 34 inducing a relatively low and constant flux density in the casing 11 unit 35 is responsive only to changes in the average wall thickness of the lengths of casing which are successively inspected and there will be little or no residual magnetism left in the casing after the magnetizer 34 has moved and thus permeability changes in the casing do not affect the measurements. Circuitry Fig. 3 Each inspection shoe 32, 33 includes a primary coil 47, all of which are energized from a common oscillator 50, and differentially connected secondary windings 48, 49, the windings 48, 49 of each shoe being connected to a respective detection circuit. Both a low frequency flux leakage signal and a high frequency eddy current signal are induced in windings 48, 49 and these are processed in channels 53, 54 respectively. The flux leakage signals are fed via a low pass filter 55, amplifier 57 and rectifier 58 to one input of a maximum signal selector 65 the remaining inputs of which are each coupled to a respective detector circuit of the remaining shoes in the set e.g. set 32. Similarly the eddy current channel 54 includes amplifier 59, detector 60 and a rectifier 61 for converting the signals to pulses and applying them to respective input of a maximum signal selector 66. Similar circuits and maximum signal selectors 63, 64 are provided for the shoes of the other set 33. The maximum signals from the selectors are fed via lines 14, 15, 16, 17 to the memorizer 27. In addition a thickness measuring signal is applied on line 18 from unit 69 wherein the phase difference between the current supplied to coil 68 from oscillator 70 and the signal induced in coil 67 is obtained from phase comparator 72 and which is a function of the average thickness of casing 11. By investigation of the four recordings on recorder 21 the type, extent and depth of a flow can be ascertained.