There are two active Coiled Tubing Mechanics Research projects underway:

1. Surface Defects and Fatigue Resistance of Coiled Tubing

This project is a continuation of the work begun by the Surface Defect and Fatigue Resistance JIP. Defects of controlled geometry are imposed in samples of coiled tubing for fatigue testing. The dimensions of the defects are carefully measured and recorded before each test. Samples are cycled to failure at constant internal pressure and the fatigue life is compared to baseline data collected using defect-free samples. The goals of the project include quantifying the degree to which a particular defect will accelerate fatigue damage development and observing and documenting how fatigue cracking develops in individual defects. Another goal of the project is to develop repair strategies and investigate their effectiveness.

A wide variety of defects have been and have been investigated. Defects are imposed using a number of techniques including NC milling, Electron Discharge Machining, and manually (by using hand tools). The majority of the defects are imposed on the outer surface, although inner surface defects are also being investigated. The defects are systematically varied from test, to test in order to quantify the influence of the individual parameters that define the defect. This can include its depth, width, length, and shape, but it has also been discovered that the means by which a defect is introduced can have a first order influence on how it affects fatigue strength.

In order to assist the field, samples with a variety of repaired defects have also been investigated as part of this study. It has been discovered that some techniques are effective under certain operating conditions, but can be less effective (or even damaging) under others. Techniques that work very well for some defects can be very damaging relative to others.

Based on the work done so far, a computer program has been delivered to the consortium members, making it possible to predict the influence of a defect on the remaining fatigue life of a sample in the field. It also enables the user to evaluate the effectiveness of repair techniques. Current and future testing are aimed at making the program more robust and reliable in terms of operating conditions and coiled tubing geometries and materials. The routine also makes more conservative predictions based on statistical analyses from the large data sets.

2. Coiled Tubing Inspection

The Surface Defect portion of this project is providing reliable techniques to predict the influence of a flaw of given dimensions. But in order to apply this these results in the field, flaws must be discovered.

Inspections are becoming more and more common in order to locate flaws in coiled tubing and avoid the problems they can cause. The most common NDE technology used to inspect coiled tubing is magnetic flux leakage (MFL). This technique is a reliable means of detecting flaws. Considerable effort was expended by CTMRC in an attempt to discern flaw characteristics from MFL signals.

After extensive experimental and numerical analyses, it was concluded that there are serious limitations on the ability of MFL to provide reliable information about the geometry of the flaws themselves. Finite Element Analyses was used to emulate the flux leakage and flux measurement phenomena very accurately, relative to experimental results. Using this technology, it was possible to modify flaw geometries analytically and simulate their resulting MFL signals. Among the conclusions from this study is the fact that MFL signals are not capable of identifying critical flaw dimensions accurately. This is especially for for small transverse flaw, which can be particularly damaging to coiled tubing fatigue life. The reason for this is the finite size of MFL sensors, which are large relative to the flaws they are detecting. This problem is exacerbated by the fact that the sensors cannot be placed into direct contact with the surface of the flaw.

Because of the inability of MFL to quantify flaw dimensions, a new approach is being investigated: the use of Laser Imaging Technology to fully measure the 3-dimensional geometry of an external defect.

The use of 3D laser scanning as an NDE tool is not new, but its application to coiled tubing inspection had never been attempted. An inexpensive laser scanning system was used to obtain promising results. A new, more accurate 3D laser measurement tool has been commissioned specifically for coiled tubing. This tool will initially be used in the lab to quantify the detailed geometry of defects used in the research. It can also be shipped to members for use in the field, to record and analyze flaw geometries. Software is being developed that will import flaw dimensions directly into the FlexorTU algorithm to directly assess the severity of a flaw.

Initially, the 3D Laser Scanning tool is being used as a measurement device. It will provide the flaw dimensions needed to asses its severity in terms of fatigue life prediction. Currently, this consists of depth, width, length and projected surface area. Eventually it is possible that more flaw details may be extracted from the surface map, including but not limited to the minimum radius of curvature at the notch root. Such information could be used to improve the accuracy of life estimation for defects in CT.

The longer term goals for the 3D laser scanning technology is to implement it with CT inspection technology. This could initially be done in conjunction with MFL technology (e.g., MFL finds the flaw and 3D laser scanners measure its geometry) or as a stand-alone approach where 3D lasers scan the surface of moving tubing, detecting as well as measuring the flaw geometry in real time.