The MFL sensor system is used to convert the magnetic field signal to the electrical signal, which is the core of the magnetic flux leakage testing technology and determines the signal-to-noise ratio, sensitivity and stability and many other performance indicators of the magnetic flux leakage testing signal. The diameter of the drill pipe coupling varies greatly, and the magnetic flux leakage testing instrument must have good passability, that is, the sensor needs to maintain a certain avoidance distance from the surface of the rod body, and it is necessary to eliminate the wear of the sensor caused by drilling fluid and sand.
For this, under the premise of ensuring the sensitivity of magnetic flux leakage testing, it is necessary to develop a MFL sensor system suitable for large lift-off testing to improve the passability and stability of the magnetic flux leakage testing instrument. The array sensor method for increasing the magnetic flux leakage testing sensitivity obtains a MFL array sensor structure suitable for wellhead drill pipe testing with good consistency of testing signals through double-layer arrangement and adjacent series connection.
There are two types of objects picked up by the MFL sensor: the absolute value of the magnetic leakage magnetic field or the gradient of the magnetic leakage magnetic field, and the corresponding magnetic induction elements are mainly Hall elements and induction coils. Through a large number of experiments, various parameters such as the sensor's manufacturing process, signal-to-noise ratio, sensitivity and stability are comprehensively compared. The sensor system of the drill pipe adopts the induction coil as the magnetic field element. In addition, it was found in the experiment that using the coil to measure the normal gradient change of the leakage magnetic field has better resolution. Therefore, the testing system lays the induction coil flat to pick up the changing trend of the leakage magnetic field to the greatest extent.
In addition, in order to improve the passability of the magnetic flux leakage testing instrument and protect the sensor from friction by drilling fluid and sand, the commonly used close-contact testing method (lift-off value is 0.5mm). When the actual use requirements cannot be met, the sensor must achieve high-sensitivity testing at a large spatial distance from the surface of the drill pipe, that is, large lift-off testing (lift-off distance is 5.0mm). At this time, the sensitivity of ordinary induction coils can no longer meet the requirements. Therefore, the system uses an induction coil with a magnetic concentrating core as a testing sensor, and a magnetic concentrating core made of permalloy material is placed inside the induction coil to improve testing sensitivity.
After testing, the concentrating core has a significant effect of increasing the peak-to-peak value of the magnetic flux leakage testing signal. At the same time, from the perspective of testing cost, the MFL sensor has high cost performance, and the consistency of the sensor can also be guaranteed in the manufacturing process.
Consistency requirements for magnetic flux leakage testing of drill pipe: The same defect on the drill pipe must obtain the same testing signal amplitude through the MFL sensor system at any angle. Therefore, the arrangement of the sensor array must be reasonably designed to meet the full coverage and consistency requirements. To this end, a double-layer arrangement of adjacent series-connected magnetic flux leakage array sensors is proposed. In this way, the outputs of adjacent sensors are connected in series as a channel output, and the two-layer arrangement is arranged in the axial direction.