Abstract It has long been known that CT rotates during its life because the orientation of the seam weld at the end of the CT varies. This rotation changes the bending plane and thus the fatigue life of the CT. None of the current CT fatigue and deformation (elongation and diametrical growth) prediction models considers this rotation. An experimental rotational-orientation measuring device (ROMD) was developed to measure the rotational orientation of the seam weld as the CT runs in and out of the hole. This device was used to record the rotation of CT strings in various applications. Results from these runs are presented. The affects of rotation on fatigue life are quantified using an existing fatigue model. Some fatigue test results are presented in which the CT was rotated between bends. Both the modeling results and the experimental results show that the fatigue life increases due to rotation.