The tendency for coiled tubing to grow in diameter due to internal pressureand elongate permanently due to axial loading has been the focus of recentre search. These permanent deformations occur in spite of the fact that nominal stresses due to internal pressure and axial loading are well within elastic limits in both the hoop and axial directions. Bending strains that are well into plastic regime are responsible for ratcheting mechanisms in coiled tubing. Progressive plastic deformation can occur under a combination of steady(internal pressure) and cyclic (bending and/or axial) loading. A portion of plastic strain, not recovered in each cycle, may lead to large accumulated plastic strains. A new analytical model capable of predicting ratcheting behavior of this type has been developed and applied to coiled tubing. The model predicted interesting trends, never before noted in the open literature, including incremental decreases in both diameter and length of a coiled tubing section during bending cycles under positive internal pressure. The accuracy of these predicted trends was validated experimentally. The success of the analytical model is based on a new incremental plasticity model that closely conforms to the physics of cyclic plastic deformation. Some elements of the new plasticity model are discussed and predictions are compared to experimental results from complex tests involving variable pressure and axial loading.
The mechanical behavior of coiled tubing has been the focus of several investigations. As documented in these references and others, coiled tubing has long been known to increase in diameter and length, in spite of the fact that circumferential stresses and net axial loading never exceed the yield strength of the tubing.