Coiled-tubing surface characteristics and effects on fatigue behavior

The manufacturing procedure for coiled tubing induces different surface mor phologies on the inner and outer surfaces. Since fatigue is a surface pheno menon, this can lead to different fatigue behavior at the two locations. Th is has been demonstrated in full-scale bending tests and in axial tests con ducted on small coupon samples in closed-loop servohydraulic strain control . Furthermore, service environments can cause localized defects on the oute r and inner surfaces of coiled tubing, such as grinding marks, corrosion pi ts, tool marks, or injector block damage, which can have a first order effe ct on fatigue strength. However, it is demonstrated in this article that th ere is an inherent tendency for cracks to initiate on the inner surface of coiled tubing. This tendency leads to the hypothesis that a ''threshold sev erity'' for external surface defects could exist, below which the fatigue s trength of the tubing is not affected. Limited data confirm this hypothesis , and suggest that the severity could depend on the grade of the material.