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Abstract 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. Introduction 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 tubing. 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 dressing. 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. Design Features of LCF CT Connector 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.
- North America > United States > Texas > Harris County > Houston (0.28)
- North America > Canada > Alberta > Census Division No. 6 > Calgary Metropolitan Region > Calgary (0.24)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Tor Formation (0.99)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Ekofisk Formation (0.99)
Abstract The post-yield material performance of 70 and 80 ksi yield strength carbon steel and high-strength corrosion resistant alloy (CRA) coiled tubing (CT) has been investigated experimentally to evaluate its response to a sour environment following plastic bending straining induced during coiled tubing operations. Although this investigation was specifically targeted for under-balanced drilling of sour wells, the results obtained can also be interpreted with respect to CT work-over operations. The four different types of laboratory testing performed were low cycle corrosion fatigue (LCCF), NACE proof ring tests, double cantilever beam tests (DCB) and slow strain rate tests (SSRT). Full-body CT specimens were used for the SSRT and LCCF testing procedures and only SSRT test results are presented for a specific CRA material. The test environments consisted of aqueous H2S, H2S/CO2, varying pH and the standard solution specified by NACE for determining the relative propensity for HIC and SSC. Tests were also performed using the NACE solution treated with an H2S corrosion inhibitor. This paper presents the key findings from the carbon steel CT testing, the preliminary data for the experimental CRA CT and the basis on which these materials were evaluated for under-balanced drilling applications. Both 70 ksi and 80 ksi carbon steel grades were found to be fit-for-purpose for sour wells in accordance with certain qualifications discussed. A preference for the lower strength, 70 ksi CT has been identified when CRA materials are not being considered. For carbon steel, the low cycle fatigue (LCF) life in a noninhibited sour environment was reduced to between 15% to 20% of the life in air depending upon the pH level. With the application of an H2S corrosion inhibitor, the LCF can be increased significantly to about 2/3 of the life obtained in air. The results of this research are currently being implemented for a revised issue of Industry Recommended Practises for Critical Sour Under-balanced Drilling-Drill String (IRP 6.3). Introduction The requirement to deform coiled tubing beyond its yield strain during spooling operations is common knowledge in the coiled tubing industry. The flexure strains imposed are typically in the range of 2% to 3% plastic strain. Plastic deformation, even to a seemingly minor extent, is know to significantly reduce the resistance of carbon steels to sulphide stress cracking (SSC) and is therefore prohibited in conventional oilfield tubular goods. For coiled tubing, such repetitive cold working, has not impeded well work-over operations in sour environments that have been performed successfully for many years. For under-balanced drilling using coiled tubing (DUCT) in sour wells, however, the Alberta Energy and Utilities Board (AEUB) recognized a need to establish an industry recommended practice (IRP) that included the use of CT drilling in critical sour reservoirs. A preliminary IRP 6.3 has been published based on available information, publications and on limited static test data commissioned by the AEUB Drilling And Completions Committee (D.A.A.C). The material and operating constraints contained in this draft of the IRP was tantamount to eliminating the use of DUCT in sour reservoirs. It was subsequently recognized that for a more realistic and reliable fitness-for-purpose evaluation of material performance in sour conditions, additional test data, based on dynamic testing procedures and full body CT test specimens, was required. For this purpose, a joint industry project (JIP) was initiated by BJ Services. The complete results of this JIP, including the research performed independently by Shell Canada and BJ Services, were published recently in complete detail with the hope of improving the IRP guidelines and for the general benefit of the coiled tubing and oil and gas industries. The present paper summarizes the major results pertaining to the CT70 and CT80 carbon steel tubing and presents the complete test results for the CRA CT that were not published previously.
- North America > Canada (0.68)
- North America > United States > Texas (0.46)