As the use of coiled tubing (CT) in workover services, drilling services, coiled completions and coiled pipeline increases, the need for improved field welding techniques increases. This paper summarizes the results of a weld analysis research project in which failed welds were analyzed, welding procedures were reviewed and alternative welding techniques were considered.
A significant amount of research has been performed in recent years to improve the understanding of the CT pipe life. Pipe life prediction models are now being used to predict when the pipe is reaching the end of it's fatigue life. This understanding has improved the reliability of CT services.
The weakest points in a CT string are at the welds. Significant improvements have been made by the CT manufacturers to the welds made during the manufacturing process. Instead of welding tube-to-tube or "butt" welds on completed sections of CT pipe, the manufacturers are now welding the sections of strip material together before it is milled into a tube. The ends of the strips are cut at an angle or a "bias" for this type of weld, thus these welds are commonly known as "bias" or "CM" (continuously milled) welds. The bias causes the weld to be distributed helically along the axis of the tubing, and as a result increases the axial strength of the weld. This strip welding technique has significantly improved the reliability of CT pipe.