When CT with internal pressure is bent around the reel or guide arch, and straightened in the injector chains, it experiences both plastic and elastic deformation. This results in internal residual stresses which cause bending moments and residual curvature. A “straightener” can be used to reverse bend the CT, which causes it to be fairly straight (no significant residual bend) but it still has significant residual stresses. Residual curvature and stresses decrease the buckling load (the CT is more prone to buckling) and cause safety issues if the CT is suddenly released. They can affect the fatigue life, but they are not in themselves an indicator of the amount of fatigue damage (as some have mistakenly thought). This paper presents the concepts and equations used to determine the residual curvature and residual stresses for a sequence of bending events, axial loadings, with internal pressure. These concepts and equations are then used to demonstrate the behavior of CT for several practical situations such as when it is run in and out of a well, and when it passes through a straightener.

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