The existing API equation for internal leak predicts the internal pressure to overcome the pin-box contact pressure generated from the makeup interference plus the energizing effect of internal pressure which enhances the seal. For threaded connections, internal and external pressures close the connection and increase the leak resistance, whereas axial loads open the connection and decrease the leak resistance. These competing effects must be included to accurately assess the connection leak resistance under any combination of loads at any point in any string. Following the same approach used by API for internal leak, this paper obtains similar results for external leak. For API connections, the effects of combined axial force and backup pressure are then incorporated into the internal/external leak equations using results from the Mitchell and Goodman (2018) paper presented at the 2018 SPE-IADC Drilling Conference. Sensitivities of leak ratings to combined loads for API connections are presented for both tubing and casing sizes. An example design case shows the importance of considering combined loads.
Number of Pages
API Bull 5C3. (1994). API Bulletin on Formulas and Calculations for Casing, Tubing, Drill Pipe, and Line Pipe Properties. 6th edition. Washington, DC: API.
Drucker, D. C., and Prager, W. (1952). Soil mechanics and plastic analysis for limit design. Quarterly of Applied Mathematics, 10(2), 157–165.
Goodman, M. A. (2018, March). A Unified Approach to Yield, Buckling, and Leak in Well Tubulars. SPE Drilling and Completion, 33(01), 27-40. SPE-185855-PA. doi: 10.2118/185855-PA
ISO 10400. (2003). Petroleum and natural gas industries - Formulas and calculations for the properties of casing, tubing, drill pipe, and line pipe used as casing or tubing. geneva, Switzerland: ISO.
ISO 13679. (2002). Petroleum and Natural Gas Industries - Procedures for Testing Casing and Tubing Connections. First Edition, 2002-12-15.
Mitchell, R. F., and Goodman, M. A. (2018). Dilatancy and Pressure Dependence of Threaded Connection Performance. Presented at SPE/IADC Drilling Conference and Exhibition. Fort Worth, TX. 6-8 March. IADC/SPE-189712-MS.
Shigley, J. E. (1977). Mechanical Engineering Design. McGraw-Hill Series in Mechanical Engineering, McGraw-Hill, Inc.
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