The coiled-tubing (CT) industry has evolved from using smaller pipes with lower-grade materials to bigger pipes and high- strength materials. However, the offshore installations and equipment still need to catch up with increased weight and handling demands. A tapered outer-diameter (TOD) string is one solution that provides the benefits of larger string while keeping size and weight at manageable levels. This paper presents a study and analysis of using a spoolable connector (SC) to join strings of two different outer diameters (ODs).

A TOD string requires an 8- or 10-ft long “transition section” so that the string can pass through specially retrofitted injectors. The transition section is milled, from one diameter size to another, using a cold-working process. This transition section needs to be welded to the two strings and requires a manual butt weld. As an alternative, a transitional spoolable connector is being developed and virtually tested using finite element analysis (FEA), thus eliminating the need for a specially milled, butt-welded transition section.

One challenge with TOD-connection systems is that they tend to fail at the butt-weld between transition and core end of string. A 10-ft long SC was designed to transfer this high-strain area away from the transition where it would be less detrimental to fatigue life. SCs for regular pipes have been designed and tested in variety of sizes from 1.5- to 2.875-in. OD CT. The test data provided a base to compare the performance of this new transitional connector. Results from a 2- to 1.75-in. tapered OD string are presented. One complete bending event of a 20-ft long assembly was simulated on a drum using FEA, and results were compared with actual and virtual test data from pipe with SC and a TOD string with transition section.

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