The results of the testing have shown that debris-induced lift-off, in all its forms, has a detrimental effect on the acquired MFL data. Since there is no reliable and accurate way to determine that lift-off has occurred, it will always be a potential problem. Deeper, wider, and axially aligned slot defects appear to experience the greatest reduction in peak MFL amplitude when under the effect of debris-induced lift-off. This will result in severely underestimated or missed defects. The data indicates that it is very easy for smaller defects to be totally missed. Perhaps the most interesting observation is that a large percentage of the signal degradation occurs in the first 0.125 inch of lift-off. This observation highlights the importance of pre-inspection pipeline cleaning. This essentially indicates that low levels of sensor lift-off and simultaneous sensor and magnetizer lift-off produce significant degradation in the acquired MFL data. If pipeline defects are located in an area where lift-off has occurred, the potential is much higher for them to be missed or underestimated.
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