Coiled tubing endures unique cyclic stress and strain histories. The loading imposed on pressurized coiled tubing can trigger deformation mechanisms resulting in incremental plastic diametral growth and elongation. This growth occurs in spite of the fact that both the hoop stress and net axial stress from tension are well below the material yield stress. The most dominant factors controlling the deformation behavior of coiled tubing are the bending-straightening cycles associated with the spool and gooseneck. It is the interaction of the bending stresses and strains with those from axial loading and pressure that result in plastic elongation and diametral growth. Severe cyclic plasticity imposed by these events actually changes the structure of the coiled tubing material, causing a corresponding change in mechanical properties. The material properties in a section of coiled tubing along a string are thus dependent upon the localized service loading history.
The operating parameters that control the loading on a section of coiled tubing are discussed in terms of the tubing geometry, the above-surface deployment equipment and the sub-surface environment. With the load history characterized in terms of imposed stresses and strains, mechanisms that lead to diametral growth and elongation are demonstrated from the standpoint of simple material plasticity models. Refined models are described, based on more sophisticated plasticity relations, capable of characterizing transient behavior and multilateral effects. Implications from these results for depth calculation are discussed.