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Coiled tubing (CT) is used in the oil field for a wide variety of drilling, servicing, and completion applications. CT is fabricated from steel in diameters from 1.25 to 3.5. Upon fabrication, it is wound onto a large spool in lengths as great as 25,000 feet. In service, it is deployed and re-spooled routinely, imposing severe cyclic bending while simultaneously carrying fluids at high pressure. This can lead to fatigue cracking and to failure in a very low number of cycles, similar to what a paper clip exhibits when bent by hand. This subject has been studied in depth and fatigue life prediction models are used in the field to monitor the life of CT. In the harsh oil filed environment, it is typical for tubing to incur mechanical defects such as nicks, dents, gouges and scratches, which can adversely affect the fatigue life of CT. A joint industry research project is underway with the following major objectives: 1) to understand and quantify the influence of surface defects on the fatigue resistance of CT, and 2) to investigate repair techniques and quantify the improvement they provide. To date, over 375 full scale fatigue tests have been conducted on pressurized CT samples on specialized testing fixtures. Data are used to validate analytical prediction models for use in the field. The experimental procedures undertaken for this project are presented and discussed. The data reveal that not only is the geometry of the defect important, but also that its means of imposition is significant as well.
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