The fact that coiled tubing (CT) rotates when used is not currently included in CT fatigue models. The CT also experiences diametrical growth and elongation. Almost all fatigue testing is done without rotating the CT sample. Diameter growth models based on these fatigue tests have over predicted the amount of diametrical growth.
This ongoing work has already shown that rotation affects the fatigue life. Fatigue life calculations without rotation are usually conservative. It has also shown that rotation does not explain the over prediction of diametrical growth. It is currently believed that the axial force (weight) in the CT causes elongation and a diametrical decrease, which reduces the diametrical growth. It is also possible that the injector chains cause some reduction in diameter. The fatigue/plasticity model being developed will attempt to answer more of these questions. This paper presents fatigue testing with rotation and with varying pressures, which is being used to validate the model. Lab measurements made with a rotation measuring device are also presented.