In recent years, the performance envelope of coiled tubing (CT) has expanded because of improvements in tubing manufacture and the prediction of low-cycle fatigue life. More accurate life predictions have been achieved with data from laboratory-scale and full-scale CT fatigue tests. To date, most testing has evaluated only tubing pressures below 10,000 psi. Fatigue models at higher pressures remain unvalidated.
This paper presents results from new, full-scale fatigue tests performed at 7,500 to 15,000 psi. CT with 90,000-psi minimum yield strength was cycled to failure at internal pressures of 7,500, 10,000, 12,500, and 15,000 psi. The measured fatigue-life values helped validate the fatigue model used for CT job design, and allowed the model™s further refinement.
Additional data are presented for full-scale fatigue tests performed with extended tubing-stroke lengths. This testing verifies the correlation between the tubing life observed in controlled, full-scale, short-stroke fatigue tests and the fatigue life observed in field applications with significantly longer stroke lengths. Measurements and analyses of tubing-diameter growth during high-pressure cycling are included.