Coiled Tubing (CT) is subjected to plastic mechanical deformation as it is cycled on and off the reel and over the gooseneck. Repeated bending causes CT fatigue, and eventually failure. In the presence of an internal pressure, especially higher pressures (greater than 3,000 psi), CT fatigue is accompanied by diametral growth ("ballooning"). CT diametral growth can cause incompatibilities with deployment equipment and continuous use could thus cause severe mechanical damage to the CT. Therefore, an accurate predictive model for CT diametral growth is needed for both safety and reliability in CT operations.

A reliable CT diametral growth model has been developed based on test data and three-dimensional curve fitting. The developed model not only predicts CT diametral growth under various pressure and bending geometry conditions, but also evaluates the effects on CT growth of relatively small changes in tubing OD, wall thickness, material grade, internal pressure, and bending radius. These analyses will help operators detect early CT diametral growth before severe damage occurs, and eliminate or reduce operational factors that contribute to growth.

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