When coiled tubing (CT) is plastically deformed by bending over a reel or guide arch, several important changes are known to occur. The tubing suffers irreversible and cumulative fatigue damage, and the diameter may increase in the presence of internal pressure. Fatigue has been the subject of extensive study, and several analytical and empirical models have been successfully developed. Diameter growth has received less attention, but recent industry trends are causing it to become more of a concern. These trends include:
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Use of tubing for a greater fraction of its fatigue life
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Repeated local cycling at higher pressures (e.g. drilling)
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Greater use of large diameter CT
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Operations at very high pressure >5000 psi)
It is worth noting that all of the above trends have been made possible in part by improved modeling, the effectiveness of which is undermined if an important parameter affecting the working life of the string is neglected, such as tubing diameter.
This paper presents an analysis of a new analytical model for predicting diameter growth, and compares the estimated working life of a tubing string when both fatigue and diameter growth are considered. Model predictions are compared with observed data from full-scale tests and fatigue test machines. Use of the model in real-time tubing monitoring software as a means to identify potential problem areas is discussed.
