Tube-to-tube joints in coiled tubing can be achieved by either fusion bonding or mechanical connections. A critical performance requirement of such joints is their ability to sustain plastic bending cycles that ideally, equal in number to that of the coiled tubing itself. Orbital girth welds in coiled tubing butt joints can typically achieve a low cycle bend fatigue (LCF) life in the range of 50% to 60% of the coiled tubing. Mechanical connectors of existing design and construction, have shown a considerably poorer LCF performance that is reflected by their infrequent use in coiled tubing operations.
A new spool-able mechanical connection with flush outer diameters for coiled tubing strings has been developed with comparable bend fatigue performance to that exhibited by orbital TIG girth joints. Current designs, that are based on strategic material selection and mechanical design details, range in size from 1–1/2" to 2–7/8". In addition to comparable or superior LCF lives, these connections overcome many of the disadvantages of welded butt joints. This paper describes the research and development, various mechanical and materials testing results and recent offshore well servicing applications undertaken with these new mechanical CT connectors.
Tube-to-tube butt joints have for many years been performed successfully for field repair or modifications of coiled tubing (CT) strings using manual or mechanized welding. Welded girth joints in coiled tubing can achieve a low cycle fatigue (LCF) life that is typically one half that of the parent tubing1. More consistent LCF life can be achieved with mechanized welding such as orbital TIG, however, the actual performance of each weldment remains strongly dependent on individual welder skill and quality of the edge preparation and finish dressing2. Offshore operations present additional difficulties in obtaining sound tube-to-tube welded joints, where for example, special and sturdy protective habitats are required to shield the welding operation and welder from severe weather conditions.
Alternative mechanical methods of joining coiled tubing strings, have been available for many years. Apparently, mechanical connections have not been utilized frequently due to their severely limited LCF life. Recently, BJ Service's Norway base initiated a joint investigation with StatOil into the LCF response of a commercially available spool-able connector installed in a laboratory test specimen assembly constructed from 2–7/8 × 0.156 QT1000 coiled tubing for bend fatigue testing at BJ's Coiled Tubing Research & Engineering (CTR&E) division in Calgary. Except for one significant difference, the test connectors were fabricated according to conventional designs used to date. The difference was that BJ selected and specified the material of construction in accordance with optimum plasticity, metallurgical, mechanical and other material properties.
The bend fatigue results obtained from these connector tests, showed improved yet unacceptable LCF performance. BJ decided therefore to develop its own spool-able connector design that would incorporate the same optimized material specified earlier for the commercial connector tests. The new "Composite LCF CT Connector"™ (patent pending) entails a composite construction in the sense that both super alloy steels and elastomer materials are utilized. Some of the design features, results obtained from mechanical and corrosion testing, progress made with the research and development and field applications for the new LCF CT connector, are described in the present paper.
A key requirement of any spool-able CT connector under bending loads is to deform uniformly in a continuous curve under elastic deformation and avoid the formation of any hinge points under subsequent plastic deformation. Plastic hinges are local sites of high bending strains and therefore hot spots for premature fatigue failure. Achieving continuous bending deflection curves under both elastic and plastic deformation requires a strategic combination of specific connector material mechanical and plasticity properties, unique profile distributions and other critical physical dimensions. To satisfy the requirement of avoiding large step changes in tubing and connector stiffness, a special "soft entry" or transition section was incorporated into the connector design where the coiled tubing first overlaps with the connector.