In the coiled tubing (CT) business, it is a common practice to track the fatigue-life of CT strings to help avoid costly failures during operations. Existing fatigue-life tracking models account for parameters such as differential pressure, bending radius, tubing diameter, wall thickness, and materials grade. However, these models do not take into account the effect of localized damage, which is the leading factor contributing to premature failure. In the past years, models have been developed to account for the fatigue effects of mechanical defects. A key input for such models is the geometrical severity parameter, which is defined as a function of the defect geometries (length, width, depth, and shape). Normally, these inputs must come from manual measurements and prove-up. Most recently, a new technique was developed which does not rely on the manual measurements. The new technique utilizes inline magnetic flux leakage (MFL) technology to automatically determine the defect severity in terms of fatigue-life reduction.

The current paper presents new development and results of enhanced fatigue evaluation by virtue of MFL measurements. A hybrid approach is proposed and demonstrated, in which the results of conventional fatigue tracking are coupled with the fatigue-life reduction determined based on MFL measurements on physical defects, to come up with a more comprehensive and accurate fatigue-life assessment. Experimental results are presented to support the validity of this hybrid approach. Also, the new approach is applied on a number of case studies to demonstrate its applicability in real-world CT pipe management.

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