ET09.pdf

From the point of view of magnetizing the pipe metal

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From the point of view of magnetizing the pipe metal in the longitudinal direction, the two applications are identical. The internal diameters and metal masses involved in the magnetic flux loop indicate that some form of active field excitation must be used. Internal diameters of typical production or transportation tubes range from about 100 mm (4 in.) to about 1.2 m (4 ft). If the material is generally horizontal, some form of drive mechanism is required. Because the test device (a robotic crawler) may move at differing speeds, the magnetic flux leakage probe should have a signal response independent of velocity. For devices that operate vertically, such as petroleum well casing test systems, coil probes can be used if the tool is pulled from the bottom of the well at a constant speed. In both types of instrument, the probes are mounted in pads pressed against the inner wall of the pipe. Because both line pipe and casing are manufactured to outside diameter size, there is a range of inside diameters for each pipe size. Such ranges may be found in specifications. To make the air gap as small as possible, soft iron attachments can be screwed to the pole pieces. For the pipeline crawler, a recorder package is added and the signals from discontinuities are tape recorded. When the tapes are retrieved and played back, the areas of damage are located. Pipe welds provide convenient magnetic markers. With the downhole tool, the magnetic flux leakage signals are sent up the wire line and processed in the logging truck at the wellhead. A common problem with this and other magnetic flux leakage equipment is the need to determine whether the signals originate from discontinuities on the inside or the outside surface of the pipe. Production and transmission companies require this information because it lets them determine which form of corrosion control to use. The test shoes sometimes contain a high frequency eddy current probe system that responds only to inside surface discontinuities. Thus, the occurrence of both magnetic flux leakage and eddy current signals indicates an inside surface discontinuity whereas the occurrence of a magnetic flux leakage signal indicates only an outside surface discontinuity. Problems with this form of testing include the following. 1. The magnetic flux leakage system cannot measure elongated changes in wall thickness, such as might occur with general erosion. 2. If there is a second string around the tested string, the additional metal contributes to the flux loop, especially in areas where the two strings touch. 3. A relatively large current must be sent down the wire line to raise the pipe wall to saturation. Temperatures in deep wells can exceed 200 °C (325 °F). 4. The tool may stick downhole or underground if external pressures cause the pipe to buckle.
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  • Fall '19
  • Magnetic Field, Magnetic flux leakage

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