As coiled tubing is deployed into and out of a well, it is wrapped and un-wrapped over its spool, and over a tubing guide arch, commonly referred to as a "gooseneck." The bending and straightening action imposed on coiled tubing is extremely severe. Cyclic bending strains can be as great as 2-3%. Coupled with internal pressure, this can result in fatigue lives less than 100 cycles. The picture below shows how the tensile strain in a particular section of tubing varies as that section moves into and out of a well.

View Cyclic Bending Demonstration

Professor Steve Tipton has developed coiled tubing fatigue life prediction routines used by companies to monitor the fatigue life of their tubing in the field. These include the CoilLIFE routine, used by Schlumberger Dowell, and the Achilles model used by CTES.

Diametral Growth and Wall Thinning

In addition to fatigue, another important characteristic of coiled tubing is its tendency to increase in diameter as it is cycled with internal pressure. In test fixtures, diameters have been observed to increase as much as 30%, a phenomenon sometimes referred to as "ballooning." This permanent (plastic) growth occurs despite the fact that maximum internal pressure levels only cause circumferential stresses less than 50% of the material yield strength! In field applications, the tubing cannot pass through deployment equipment if diametral growth on the order of 6-10% occurs.

For tubing that does not rotate, such that the neutral axis remains fixed, non-uniform wall thinning accompanies diameter growth. This is depicted below. The material furthest from the neutral axis becomes significantly thinner, relative to the material at the neutral axis, which maintains its initial thickness. Loading at the neutral axis is essentially elastic.

View "Ballooning" Demonstration

The phenomenon of diametral growth is well known throughout the industry, but another interesting discovery was made here at The University of Tulsa: diametral shrinkage. Certain operating conditions can cause the tubing to actually decrease in diameter, while it is cycling under positive internal pressure!

Elongation

In addition to fatigue, another important characteristic of coiled tubing is its tendency to increase in length as it is used, even though axial loading on the tubing remains well within the nominal elastic limit. Reports of up to 10 feet of permanent elongation per trip have been reported from the field, which sparked a long-standing debate over whether this can actually occur. That question has been answered by a joint industry project, conducted here at The University of Tulsa.

A typical section of tubing along a string in the field is bent and straightened several times, prior to entering the hole. Before the axial load is picked up by a particular section of tubing, the injector (the tractor drive mechanism used to deploy the tubing into and out of the well) straightens that section. At this point, there are residual stresses of yield strength magnitude in both tension and compression on opposite sides of the tubing. When the down-hole axial load is applied, a redistribution of the residual stress field occurs, such that permanent plasticity occurs at ANY axial load level.

An extremely robust program was developed as part of the University of Tulsa joint industry project to study elongation and diametral growth. The routine predicts changes in diameter, wall thickness and length for any simple or complex combination of bending, axial and pressure loading.