The Use of Finite-Element-Calculated MFL Signals To Study Characterization ofDefects in Steel Coiled Tubing

摘要

A primary consideration with coiled tubing (CT) is that it isconsumed by fatigue loading during routine operations. Also,rugged oilfield conditions routinely lead to corrosion and othermechanical surface damage. Since fatigue is a surfacephenomenon, the presence of a surface imperfection has asignificant influence on fatigue damage mechanisms. Thispaper describes the study of magnetic flux leakage (MFL)inspection signals caused by surface defects in the form ofmilled circumferential grooves in steel CT. The focus of theinvestigation is to identify and estimate the size of surfacedefects based on characteristic MFL signal features. It isdemonstrated that this effort is greatly enhanced by finiteelement analysis (FEA). The ultimate objective is toaccurately extract surface flaw dimensions from conventionalMFL signals. These dimensions are used in computer CT lifeprediction models.An axisymmetric FEA model is developed and used tocalculate leakage flux density solutions for milled circular andrectangular shaped grooves in 1.75' O.D. x 0.156' W.T., 90-ksi CT samples. FEA results are compared to axial and radialMFL signals measured with an experimental inspection unit.Favorable agreement is observed between experimental andFEA data. Furthermore, signal features are correlated with theknown slot geometries to identify basic geometry recognitionpatterns for different circumferential grooves. Signal featuresreveal qualitative and quantitative trends relative to surfaceflaw dimensional characteristics.The need persists to make the operator's stringmanagement decision-making process more reliable andautomatic with respect to determining fatigue life expectancy.The obstacle here is that due to the inherent inaccuracies incommonly used MFL inspection techniques, there is limitedreliable real-time flaw evaluation and characterizationcapability.