As oil and gas well servicing operations involve increasingly larger coiled tubing (CT) sizes, the ability for extending their useful or working life becomes of increasingly greater economic importance. Not only do larger tubing sizes exhibit inherently lower fatigue lives, but also their replacement cost is considerably greater. Efforts are underway elsewhere for developing CT repair strategies for recovering some of the CT fatigue life that has been reduced due to dents and other damage or degradation of the tubing. Such local repair methods, however, have both material and practical limitations. The capability to remove finite section(s) of tubing that has been damaged or expended a greater percentage of its working life, would be of significant benefit rather than force the retirement of the complete tubing string. Removing sections of excessive ovality and subsequent replacement with new sections without discarding the complete tubing or the ability to extend the CT string length, are further examples of the benefits of performing reliable on-site girth-welded connection(s) in coiled tubing.

This paper describes the weld procedure specification (WPS) under development at BJ Services. The procedure utilizes an orbital tungsten inert gas (TIG) welding process equipped with a full-function "Magnatech D-Head". The welding sequence consists of a fine stringer bead technique for maximum grain refinement and a strategic control of heat input, preheat and interpass temperature control. Mechanized systems for tubing ovality removal, reaming of internal ERW seam weld flash and girth-weld edge preparation, are utilized to their fullest extent to maximize repeatability of the weldment integrity and minimize reliance on welder skill and quality of workmanship. Preliminary test results obtained indicate that the low-cycle plastic fatigue life of these mechanized girth welds can be increased by at least 100% compared with what is generally attainable from manual TIG girth welds in coiled tubing.

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