The Surface Defect and Fatigue Resistance Joint Industry Project was completed in June, 2000. The goals of this project were: to gain an increased understanding of the influence of surface defects on fatigue resistance and to evaluate the effectiveness of various repair strategies.

A new coiled tubing fatigue test machine was constructed. The new machine is located and operated at Sherry Laboratories-Midstates, in Broken Arrow, Oklahoma. The new machine was used with an existing coiled tubing test fixture at The University of Tulsa to conduct experiments on tubing samples with a variety of defects and repairs.

In total, over 825 samples were tested. All tests were conducted with constant internal pressure on 80 ksi samples, with a diameter of 2.375-in and wall thicknesses of 0.156-in and 0.204-in. The bending radius for all tests was 72-in. Data were thoroughly analyzed and correlated with defect geometric details and loading parameters.

It was discovered that the means by which a defect is imposed could be as important as the geometry of the defect itself. Repair techniques were also investigated. Their advantages and limitations are outlined. Attempts were made to identify valid analytical techniques for predicting the influence of defects on the fatigue resistance of coiled tubing.

Based on experimental results, a semi-empirical approach was developed which is valid only over the range of geometries and conditions tested in this study. The approach is applicable to any coiled tubing life prediction approach and a routine is included with the deliverables from this project to investigate the influence of defects and the effectiveness of repair techniques.

A Masters thesis was generated with detailed elastic plastic FEA results for dimpled coiled tubing.

The experiments conducted during this project are summarized below.
 

Experiments Conducted during Year 1 Baseline Milled balls, conical & cylindrical defects Milled spherical slots Weld repair of “fresh” defects Grind Repairs

Experiments Conducted during Year 2 Baseline Milled transverse (ball-slot) Pressed balls & 90° chisels Pressed hex-nuts & deep balls Transverse saw-cuts / "peen" repairs HT-pressed balls Weld repairs (pinholes) Grind repairs Gouges

Important conclusions from the project are listed below.

1. Milled defects (balls, cylinders, cones, and transverse cuts) decrease life considerably. The depth, width, length and shape (projected defect area on tubing cross section) of the defect appear to be the most influential geometric parameters. The root radius of the defect was not influential for the geometries studied.

2. Impressed ball defects are considerably less damaging than comparable milled defects. Thus, the means by which a defect is imposed is a characteristic of the defect that is equally important as geometry when assessing influence on fatigue strength.

3. Defects caused by larger impressed objects (fully impressed balls and transverse chisels) were generally less damaging at higher pressures than at lower pressures. The same trend was found smaller impressions. This could result because circumferential pressure stress act to reduce the driving force for plastic deformation at the root of the defect.

4. It was suspected that residual compressive stress at the root of an impressed dimple was partially responsible for the less-damaging nature of impressed dimples. To test this theory, samples were heat treated for stress-relief before and after impressed dimples were imposed. However, these tests were inconclusive.

5. A semi-empirical parameter, based on defect size and shape and tubing dimensions, does a reasonable job of assessing fatigue strength reduction for milled and pressed defects. A computer model was written to implement the approach. The approach is applicable to any valid coiled tubing life prediction routine.

6. A Masters thesis explains that the strain state at the root of a partially spherical dimple correlates well with the strain concentration estimated from fatigue life data generated during Year 1 of this project.

7. Transverse defects that involve material removal (milled slots or saw-cuts) are extremely damaging.

8. Grinding away a defect (such as would be accomplished with a file and emery cloth) can be an effective repair technique. For high-pressure use, de-rating the tubing for reduced wall thickness appears to be overly conservative. An empirical approach is suggested. The surface-removal (or grind repair) technique was shown to be effective for milled defects (where material was removed), and for impressed chisel defects, but not for impressed ball defects.

9. Weld repairing was effective for “fresh” defects (repaired immediately after their imposition). On the other extreme, samples with defects cycled to failure could not be successfully weld-repaired.

10. Gouges (longitudinal and circumferential) examined in this study did not appear to be damaging, up to a depth of 0.032-in in depth and 0.75-in in length. More work is needed with more substantial fixturing for imposing gouges with repeatability.

This research answered many questions about the influence of surface defects on fatigue resistance. However, as is usually the case with good research, many other questions have been raised.

The results of this project shall serve as the basis for the establishment of The University of Tulsa Coiled Tubing Mechanics Research Consortium. The TU-CTMRC is an ongoing research effort to serve the coiled tubing industry.