Local strain approach for fatigue life prediction of coiled tubing with surface defects

Fatigue failure is the primary limiting factor affecting the service life of coiled tubing (CT). The ultra low cycle fatigue behavior of coiled tubing is affected by the severe cyclic bending strain history, internal pressure and presence of any surface defects. A new local strain approach is proposed for predicting the fatigue life of coiled tubing with a variety of mechanical surface defect geometries.;The approach is first developed using baseline coiled tubing fatigue test data for the bulk plasticity state of the tubing and later extended to the local strain state at the root of a defect. Two commonly used coiled tubing materials with various tube and defect geometries are subjected to fatigue loading, consisting of bending-straightening cycles with varying levels of internal pressures, using specialized testing machines. The approach separately maps the influence of hoop stress due to internal pressure using a pressure concentration factor and surface defects using a strain concentration factor. The empirical factors account for localized multiaxial stress-strain states too small to be measured directly. The strain concentration factor is correlated with a surface defect severity parameter.;The approach is demonstrated to make conservative life predictions for low and high pressure loading conditions, with uniform variation about the 50% survival prediction across the entire CT life regime.