Effect of line pressure on the performance of magnetic inspection tools for pipelines

Line pressure affects in-service inspection of oil and natural-gas pipelines by inspection tools which use magnetic flux-leakage detection. This pressure is a source of stress which alters the magnetic properties of steel. This effect of pressure can be important in the estimation of defect severity by comparison with signals from calibrated runs in test lines. In-service inspection is performed by an inspection tool, commonly referred to as a ''pig'', being pumped through the lines from one pumping or compressor station to the next. An array of defect sensors is mounted around the circumference of the inspection pig. The most commonly used sensors are magnetic flux-leakage anomaly detectors. These detectors use permanent or electromagnetic circuits to magnetize the pipe wall to near saturation. Anomalies or defect, such as corrosion pits, cause anomalies in the field distribution. Anomalies in the leakage fluxes near the pipewall are sensed by Hall probes or coils moving with the detector. The penetration of a corrosion pit is estimated from the amplitude of the anomalous leakage field and its diameter from the signal duration. This estimation is done by comparison with calibration signals obtained by the magnetic inspection tool being passed through a test pipe with knownmore » calibration defects.« less