Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
1998-09-21
2001-05-29
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S220000, C324S225000, C324S209000, C702S038000
Reexamination Certificate
active
06239593
ABSTRACT:
BACKGROUND
The invention relates generally to nondestructive methods for inspecting steel pipelines for plastically deformed regions caused by mechanical damage to the pipeline. More particularly, the invention is a method and system that uses nonlinear harmonic detection methods to detect mechanical damage in pipelines. The invention uses a time-varying magnetic field to sense magnetic properties of the pipeline. The odd-numbered harmonic frequencies are detected and their amplitudes are related to the magnetic condition of the material under test to determine mechanical damage.
Detection and characterization of plastically deformed regions caused by mechanical damage in pipelines, particularly gas transmission pipelines is important because of the danger to pipeline integrity posed by this type of defect. A plastically deformed region is a region in a metal that has been strained beyond the elastic limit and is now permanently deformed. The detection of this type of mechanical damage becomes even more critical because most of the reportable incidents of problems related to defects on operating pipelines are caused by third party contact with the pipeline that results in mechanical damage. Mechanical damage can be in the form of dents or gouges, or both.
Residual stresses and plastic deformation indicating mechanical damage in materials can be nondestructively measured by a variety of methods, including x-ray diffraction, ultrasonic and electromagnetic techniques. Because of the limited penetration depth of x-rays in metals, x-ray diffraction is restricted to measurements of surface stresses, generally in a laboratory environment. Ultrasonic techniques measure the velocity of ultrasonic waves in the metal and relate those measurements to stress. However, there are difficulties in differentiating stress effects from the texture of the material. With electromagnetic techniques, one or more of the magnetic properties (such as permeability, magnetostriction, hysteresis, coercive force, or magnetic domain wall motion during magnetization) of a ferromagnetic material are sensed and correlated to stress. These techniques rely on detecting the change in magnetic properties of the material caused by stress which is known as the magnetoelastic effect. Many of these techniques are difficult to implement without a controlled sample available while the measurement is being done. In addition, these techniques are less sensitive to changes in the magnetic properties of ferromagnetic materials caused by stress than nonlinear harmonics techniques. They are also not generally appropriate for rapidly survey stress states within pipelines, especially when the pipeline is being tested with a pigging device moving through the pipeline at a relatively high rate of speed.
SUMMARY
The present invention detects mechanical damage in ferromagnetic material using nonlinear harmonics (NLH) electromagnetic techniques. The NLH method is applicable to mechanical damage detection because it is sensitive to the changes in magnetic properties of the steel caused by stress and plastic deformation associated with mechanical damage to pipelines. The method and system of the present invention uses nonlinear harmonics techniques to detect and characterize mechanical damage in pipelines. The nonlinear harmonics approach uses a time varying magnetic field to sense the magnetic properties (i.e. permeability) of a component. This method is based on applying an alternating sinusoidal magnetic field at a given excitation frequency. The frequency used may be typically about 10 kHz, but the excitation frequency can range between about 100 Hz to about 100 kHz. Because of magnetic hysteresis and nonlinear permeability of ferromagnetic material, the magnetic induction in the material becomes distorted. The distorted magnetic induction waveform contains odd numbered harmonic frequencies of the applied magnetic field. With the nonlinear harmonics method, one or more of these harmonic frequencies are detected and their amplitudes are related to the magnetic properties of the material under test. Because of the magnetoelastic affect, stress and plastic deformation affect the magnetic properties and thus the harmonic signals. Areas of mechanical damage are then identified and characterized by the NLH response. This technique can be used for rapidly surveying stress states in pipelines where the NLH sensors are attached to a pigging device moving through a pipeline at a relatively high rate of speed (about 10 m/s, or 30 ft/s).
The present invention comprises a system for nondestructive testing utilizing nonlinear harmonics techniques to determine mechanical damage within the ferromagnetic material of a pipeline, comprising a means for supplying a time varying current at a fundamental frequency to a nonlinear harmonic sensor and for outputting a phase reference signal contained within a pigging device for passing through a pipeline, means for amplifying and selecting a portion of the signal that represents a harmonic frequency component of the signal and generating an output signal using the phase reference signal, means connected to the pigging device for converting the output signal to a digital harmonic signal; means connected to the pigging device for storing the digital harmonic signal, and computer means for analyzing the digital harmonic signal to detect areas of mechanical damage within the pipeline. The nonlinear harmonic sensor attached to a pigging device for passage through a pipeline comprises an excitation coil for generating a magnetic field within a pipeline when supplied with the time varying current as the pigging device passes through the pipeline and a sensing coil for detecting a signal caused by induced magnetic field in the pipeline. The computer means for analyzing the digital harmonic signal may be contained within the pigging device. In the preferred embodiment, the computer means for analyzing the digital harmonic signal is external to the pigging device. In the preferred embodiment, the harmonic signal is at a third harmonic frequency of the fundamental frequency. The means for supplying the time varying current at the fundamental frequency to a nonlinear harmonic sensor and for outputting the phase reference signal may be a signal generator and power amplifier.
The digital harmonic signal stored for analysis comprises an in-phase signal component and a quadrature signal component. The phase reference signal component is output to an amplifier which generates an in-phase and quadrature reference signal component. The fundamental frequency may be a selected frequency in the range of between about 100 Hz to about 100 kHz.
A means for amplifying and selecting the portion of the signal that represents the harmonic frequency component of the signal and generating a harmonic signal using the phase reference signal further comprises a filter means for filtering the output signal to remove frequencies other than harmonic frequencies and means for frequency multiplying the phase reference signal and passing the multiplied phase reference signal to a lock-in amplifier that uses the multiplied phase reference signal and filtered harmonic signal to generate a complex harmonic signal with in-phase and quadrature signal components.
The pigging device may comprise a pressure vessel and the nonlinear harmonic sensor may comprise a plurality of nonlinear harmonics sensing devices attached to the pressure vessel to form sensor arrays. The nonlinear harmonics sensing devices may extend outward from spring-loaded suspensions attached to the pressure vessel and the sensing devices may rest against the inner surface of the pipeline. The nonlinear harmonic sensors may be attached to the pressure vessel and oriented with the magnetization direction parallel with a pipeline axis or may be oriented with the magnetization direction perpendicular to a pipeline axis. The pigging device may be capable of traveling through the pipeline at a speed of about ten meters per second.
The present invention comprises a method of nondestructive
Burkhardt Gary L.
Crouch Alfred E.
Snow Walter E.
Southwest Research Institute
Taylor Russell & Russell P.C.
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