Abstract
A mathematical model of tubing forces is proposed which accounts for the bending moment due to stiffness of the coiled tubing (CT) string. The models commonly used in the design of CT operations have typically ignored tubing stiffness. Exclusion of tubular stiffness effects in such "soft string" models has not typically detracted greatly from the accuracy of predicted drag and torque loads. However, CT is being used increasingly in applications such as pipeline workover which may be characterized by unusually small bending radius. In these cases the exclusion of bending stiffness may be result in increased error. The calculation of CT bending moment for CT should also account for plastic bending and residual stresses incurred at the guide arch and reel. Example calculations are presented to evaluate the usefulness of the proposed "stiff-string" model for CT job design.
