Mechanical defects, such as grinding marks, corrosion pitting, slip marks, kinking, etc., whether occurring in the factory or in the field, can be readily found on coiled tubing (CT) strings. Extensive studies have demonstrated the detrimental effects of mechanical defects, over the integrity of the coiled tubing, such as fatigue life, both in the lab and in the field. A few theoretical models have been proposed in the past, when it comes to the severity assessment of mechanical defects, particularly as it relates to the remaining fatigue life. For such models, the defect sizing is usually a prerequisite. In order to improve coiled tubing pipe management, it is important to assess the size of a defect, and its severity.

MFL (magnetic flux leakage) based devices are volumetric and non-contact technology that is now commercially available for inspection and monitoring of CT strings. These devices have been demonstrated to be sufficiently sensitive in identifying mechanical defects. However, defect characterization and evaluation based on the devices’ MFL signals is rarely seen. To fully utilize these MFL based inspection devices for coiled tubing pipe management, it is important to establish correlation between the MFL inspection signals and the size and severity of the underlying mechanical defects.

The current work developed a three dimensional FEA model to calculate magnetic flux leakage for mechanical defects on steel coiled tubing. The fidelity of the FEA model was verified against analytical and experimental results over a series of benchmark defects. By using the FEA model, parametric studies were performed to evaluate how the MFL signals are affected by various factors. The results are used to identify basic defect geometry features and to evaluate defect severity based on MFL signals. Physical limits behind these results were identified and analyzed. Ways to overcome such limits were proposed. Further, a novel approach was presented and validated to reconstruct three-dimensional MFL field based on a single MFL component measurement. The benefit of using multiple MFL components was demonstrated for defect characterization.