3. Influence of Reel Back Tension

The 2.375-in x 0.188-in, 80 ksi case was run again, this time using the CTDef input file option. Three files were generated, one for each back tension force 0, 2000 and 4000 lbs. In each case, an internal pressure of 5000 psi was considered, and an axial force of 40,000 lbs with no tubing rotation. The results of the three cases are shown below.

Notice that a back tensions of only 2000 and 4000 lbs increase permanent elongation by 67% and 115%, respectively, and maximum elongation by 41% and 73%, respectively. These reel back tensions are only approximately 2% and 4% of the nominal body yield load. The reason this occurs is that bending and straightening over the spool and gooseneck are occurring simultaneously with the axial loading. Even small values of axial force trigger severe elongation mechanisms, when applied simultaneously with bending or straightening events. When the bending-straightening events occur with no external axial force, axial contraction can actually occur (Rolovic, R. and Tipton, S.M., “Multiaxial Cyclic Ratcheting in Coiled Tubing, Part II: Experimental Program and Modeling Evaluation,” Journal of Engineering Materials and Technology, American Society of Mechanical Engineers, Vol.122, 2000, pp. 162-167).

Keep in mind, that the extra elongation caused by back tension will depend in general on the internal pressure, as well as the down-hole axial force level.