As a result of full-scale and accelerated fatigue tests, a new data on coiled tubing (CT) fatigue life is now available. For a full-scale field test, operators set up an operational coiled tubing unit and induce stress/strain conditions identical to field conditions by moving pressurized CT from the reel to the well and back to the reel. Accelerated tests are performed with a laboratory bending machine that applies stress/strain cycles to the pressurized specimen. These cycles are similar to actual stress/strain cycles applied to CT over the reel or over the tubing guide. In this case, the specimen is 6 to 7 ft long. It is fixed at one end and simulates a cantilever with an alternating load at the other end.
Fatigue tests are performed on 1.25 to 3.50-in. OD CT sizes having the following minimum yield strengths:
70,000 psi (70k tubing)
80,000 psi (80k tubing)
100,000 psi (100k tubing)
An existing CT fatigue life model is refined to account for the tested CT sizes and materials. Main features of the model are described.