Effect of Coiled Tubing (CT) Initial Configuration on Buckling/Bending Behavior in Straight Deviated Wells

Abstract

Abstract Current sinusoidal and helical buckling models are valid only for initially straight CT. This paper stresses the effect of the initial CT configuration (residual bending) on sinusoidal and helical buckling in straight, deviated wells. Using the conservation of energy and the principle of virtual work, new general equations are derived for predicting the sinusoidal and helical configurations of CT. Practical examples are provided to show the effect of the initial amplitude, hole inclination angle, and size on sinusoidal and helical buckling behavior of CT with residual bending. The effect of residual bending is important for predicting the axial force transfer, stresses along pipe, the maximum permissible axial force for sinusoidal configuration, and the least axial force to produce helical configuration. These new equations reduce to those previously published when the initial pipe configuration is straight. Introduction Current sinusoidal and helical buckling models1,2,4–9 assume that CT is initially (stress free) straight, lying on the low side of the wellbore. However, this assumption is not always satisfied in actual drilling operations. Minor initial curvature is present in all real CT. It is therefore desirable to investigate the buckling behavior of CT with an initial curvature. In our preliminary study1 we investigated the effect of this initial curvature on the imperfect column. Fig. 1 shows a free body diagram of a pipe with initial (stress free) curvature (1/R). The load-deflection curves for such a column are shown in Fig. 2 for pin-pin type end conditions. The deflection variation at the mid-span with a load ratio P/PE (actual axial load/critical load) is plotted for two different values of the initial radius of curvature (R=150 ft, and R=75 ft). Examination of these curves reveals that bending begins as soon as the load is applied. The deflection increases slowly at first and then more rapidly as the ratio of the applied load to the critical Euler load increases. Clearly, a pipe with a small radius of curvature deflects considerably at loads well below the Euler load whereas a pipe with a large initial radius of curvature does not bend appreciably until the load is fairly close to the critical Euler load. Therefore, it is reasonable to expect that the initial curvature of CT may affect the buckling/bending behavior. In this study, we focused on the effect of the CT residual bending on the buckling/bending behavior in straight, deviated wells. In order to attack this problem, we used the energy approach.