4. Influence of Reverse Bending

Straighteners are being used increasingly for field applications where extended reach is needed. Straightening is accomplished by imposing a reverse bend prior to the tubing entering the injector. To impose a reverse bend using CTDef, a negative bending radius must be imposed. A set of 16 runs for the 2.375-in × 0.188-in, 80 ksi case were made using reverse bending radius, Rrev, values of 75, 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 450 and 500 inches. A sample input file is presented on the following two pages for a reverse bending radius of 75 inches. This value is simply replaced to investigate another radius.

The maximum and permanent elongation after 10 trips are plotted versus 1/Rrev for all 16 cases, and for the no-reverse-bending case (1/Rrev = 0). Notice that reverse bending actually decreases elongation significantly, up to an optimum value. This is probably associated with the radius that actually causes straightening, but this was not confirmed. Also notice that the points do not form a “smooth” trend. This is due to the numerical nature of CTDef, and the fact that it must compute the axial force over a cross section through iteration. A tolerance of ± 0.5% is used for axial forces, unless zero is the target value, then the convergence tolerance is ± 50 lbs. Tightening these values would result in a smoother trend, but would increase run times significantly.