Coiled-Tubing Surface Characteristics and Effects on Fatigue Behavior

Abstract

Summary The manufacturing procedure for coiled tubing induces different surface morphologies on the inner and outer surfaces. Since fatigue is a surface phenomenon, this can lead to different fatigue behavior at the two locations. This has been demonstrated in full-scale bending tests and in axial tests conducted on small coupon samples in closed-loop servohydraulic strain control. Furthermore, service environments can cause localized defects on the outer and inner surfaces of coiled tubing, such as grinding marks, corrosion pits, tool marks, or injector block damage, which can have a first order effect on fatigue strength. However, it is demonstrated in this article that there is an inherent tendency for cracks to initiate on the inner surface of coiled tubing. This tendency leads to the hypothesis that a "threshold severity" for external surface defects could exist, below which the fatigue strength of the tubing is not affected. Limited data confirm this hypothesis, and suggest that the severity could depend on the grade of the material.