COILED-TUBING DEFORMATION MECHANICS - DIAMETRAL GROWTH AND ELONGATION

Coiled tubing (CT) endures unique cyclic stress and strain histories. The loading imposed on pressurized CT can trigger deformation mechanis ms resulting in incremental plastic diametral growth and elongation. T his growth occurs despite the fact that both the hoop stress and net a xial stress from tension are well below the material yield stress. The most dominant factors controlling the deformation behavior of CT are the bending/straightening cycles associated with the spool and goosene ck. The interaction of the bending stresses and strains with those fro m axial loading and pressure result in plastic elongation and diametra l growth. Severe cyclic plasticity imposed by these events actually ch anges the structure of the CT material, causing a corresponding change in mechanical properties. The material properties in a section of CT along a string are thus dependent upon the localized service loading h istory. The operating parameters that control the loading on a section of CT are discussed in terms of the tubing geometry, the above-surfac e deployment equipment and the subsurface environment. With the load h istory characterized in terms of imposed stresses and strains, mechani sms that lead to diametral growth and elongation are demonstrated from the standpoint of simple material plasticity models. Refined models a re described, based on more sophisticated plasticity relations, capabl e of characterizing transient behavior and multiaxial effects. Implica tions from these results for depth calculation are discussed.