Electromagnetic inspection tool for ferromagnetic casings
First Claim
1. In an electromagnetic thickness measuring tool having means for generating a thickness signal indicative of the thickness of a ferromagnetic casing, and means for generating a permeability signal indicative of the permeability of the casing the improvement comprisng:
- means for generating a corrected permeability signal corrected for the variations of inside diameter of the casing andmeans for generating a corrected thickness signal corrected for the variations of permeability of the casing.
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Abstract
An electromagnetic thickness tool is disclosed to produce a measurement of the wall-thickness of a ferromagnetic casing corrected for the effects of casing permeability. First apparatus provides an uncorrected thickness signal proportional to the thickness of the casing uncorrected for the effects of casing permeability. A second apparatus provides a permeability signal representative of the permeability of the casing. A signal processor uses the mathematical square root of the permeability signal as a correction factor for the uncorrected thickness signal for generating the corrected value. The second apparatus, responsive to a signal proportional to the magnitude of the inside diameter of the casing, corrects the permeability signal for the effects of casing inside diameter.
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Citations
22 Claims
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1. In an electromagnetic thickness measuring tool having means for generating a thickness signal indicative of the thickness of a ferromagnetic casing, and means for generating a permeability signal indicative of the permeability of the casing the improvement comprisng:
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means for generating a corrected permeability signal corrected for the variations of inside diameter of the casing and means for generating a corrected thickness signal corrected for the variations of permeability of the casing.
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2. A method of electromagnetically measuring the thickness of a ferromagnetic casing using coaxial coils along the center of the casing, comprising the steps of:
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(a) energizing a thickness transmitter coil with a low frequency thickness signal thereby creating a flux pattern passing outside the casing and returning through the casing to the thickness transmitter coil; (b) measuring a receiver signal proportional to the time rate of change of said flux with a thickness receiver coil spaced along the axis from said coil; (c) generating an uncorrected thickness signal, Δ
φ
, proportional to the phase difference between said receiver signal and said energizing signal;(d) energizing a permeability transmitter coil with a permeability measuring signal at a frequency higher than said thickness signal, thereby creating a second flux pattern passing primarily through the casing and returning to the coil; (e) measuring a permeability signal, μ
, proportional to the time rate of change of said second flux pattern in a permeability receiver coil spaced along the axis from said permeability transmitter coil;(f) energizing a caliper transmitter coil at a high frequency, thereby creating a third flux pattern passing along the inside skin depth of the casing and returning to the caliper transmitter coil; (g) measuring a caliper signal proportional to the time rate of change of said third flux pattern in a caliper receiver coil spaced along the axis from said caliper transmitter coil; (h) using said caliper signal and said permeability signal, μ
, to generate a corrected permeability signal, μ
c, corrected for the effects of variation of casing inside diameter on said permeability signal μ
; and(i) generating a corrected thickness signal proportional to Δ
φ
/√
μ
c.
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3. An electromagnetic casing thickness tool for generating a thickness signal corrected for effects of varying casing permeability, the tool comprising:
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(a) first means adapted to be positioned within the casing for generating an uncorrected thickness signal representative of the thickness of the casing uncorrected for the effects of the casing permeability; (b) second means adapted to be positioned within the casing for generating a permeability signal representative of the permeability of the casing; (c) said first means having a pair of thickness coils, including a transmitter thickness coil adapted to be energized at a first frequency for inducing eddy currents in said casing, and a receiver thickness coil adapted to be responsive to voltage induced by flux from said thickness coil and by said eddy currents; (d) said second means having a pair of permeability coils, including a transmitter permeability coil adapted to be energized at a second frequency higher than said first frequency, the receiver permeability coil adapted to be responsive to the voltage induced by the flux generated by said transmitter permeability coil after traveling in the casing wall, said thickness coils being spaced along the axis of the casing, and said permeability coils being spaced along said axis at coaxial locations between said thickness coils; and (e) a signal processor responsive to the permeability signal and to the uncorrected thickness signal, including circuitry for applying a signal proportional to the inverse square root of said permeability signal to said uncorrected thickness signal, for generating a corrected thickness signal for recording, said signal processor further including means for detecting the relative locations of said set of thickness coils and said set of permeability coils and for generating an adjustment signal correlating said uncorrected thickness signal and said permeability signal to a common point on said casing.
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4. A method of electromagnetically determining the thickness of ferromagnetic casing using sets of electrically energized coils spaced along an axis of the casing, comprising the steps of:
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(a) generating an uncorrected thickness signal representative of the thickness of the casing at a given point, said uncorrected thickness signal being uncorrected for the effect of permeability of the casing; (b) generating a caliper signal having a value representative of the inside diameter of the casing; (c) generating an uncorrected permeability signal representative of the permeability of said casing uncorrected for the effect of the particular inside diameter of the casing; (d) generating, in response to said caliper signal and to said uncorrected permeability signal, a corrected permeability signal representative of the permeability of said casing corrected for the effects of the inside diameter of the casing; and (e) generating, in response to said corrected permeability signal, and to said uncorrected thickness signal, a corrected casing thickness signal at said given point, said corrected casing thickness signal being corrected for the effects of the permeability of the casing thereon. - View Dependent Claims (5, 6, 7)
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8. An electromagnetic casing thickness tool for generating a thickness signal corrected for effects of varying casing permeability, the tool comprising:
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(a) first means adapted to be positioned within the casing for generating an uncorrected thickness signal representative of the thickness of the casing uncorrected for the effects of the casing permeability; (b) second means adapted to be positioned within the casing for generating a permeability signal representative of the permeability of the casing; (c) third means for generating a caliper signal representative of the inside diameter of the casing; (d) permeability correcting means responsive to said caliper signal and to said permeability signal for generating a corrected permeability signal corrected for the effects of the inside diameter of said casing; and (e) a signal processor responsive to the corrected permeability signal and to the uncorrected thickness signal, including circuitry for applying a signal proportional to the inverse square root of said permeability signal to said uncorrected thickness signal, for generating a corrected thickness signal for recording. - View Dependent Claims (9, 10, 11, 12, 13, 14, 15)
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16. A thickness tool for determining the thickness of a ferromagnetic casing, comprising:
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(a) thickness coil means, adapted to be positioned within the casing and energized by a first signal, for generating an uncorrected thickness signal having a phase shifted from that of the first signal by an amount representative in part of the thickness of the casing, said uncorrected thickness signal being uncorrected for the effects of the permeability of the casing; (b) permeability coil means, adapted to be positioned within the casing, for generating an uncorrected permeability signal representative in part of the permeability of said casing, said uncorrected thickness signal being uncorrected for the effects of the inside diameter of the casing; (c) caliper coil means, adapted to be positioned within the casing, for generating a caliper signal representative of the inside diameter of the casing, said thickness coil means, said permeability coil means and said caliper coil means being disposed coaxially along the axis of said casing and positioned therealong such that said thickness coil means are disposed at first spaced locations, said permeability coil means being positioned at second spaced locations within said first spaced locations, and said caliper coil means being positioned at third spaced locations adjacent said first spaced locations; (d) means for generating a corrected permeability signal in response to said uncorrected permeability signal and to the caliper signal, said corrected permeability signal being corrected for the effect of the inside diameter of the casing on the value of permeability as measured by said permeability coils; and (e) a signal processor responsive to the corrected permeability signal and to the uncorrected thickness signal for generating a corrected thickness signal having a value corrected for the effects of permeability of said casing. - View Dependent Claims (17, 18, 19, 20, 21, 22)
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Specification