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Abstract 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.
Because pin and box are offset, applied axial tension (or compression) creates axial shear across engaged threads that opens a connection and reduces leak resistance. Less understood is the pin-end effect which does the same thing. Internal pressure acting on the pin nose pushes the pin out of the box, creating an additive axial force that contributes to the thread shear and decreases leak resistance. This paper addresses these axial loads for threads with non-symmetric flank angles.
Multiple new concepts are presented. First, the thread shear equation, which depends on the pin-end effect, is developed for Buttress type threads in terms of the two flank angles. Second, the leak ratings for specific Buttress connections are presented in terms of a new leak criterion with dependence on backup pressure and axial force. Third, the leak dependence of Buttress type threads on backup pressure is shown to be significant and suggests that qualification testing of any connection for leak, without backup pressure, is not adequate.
Abstract The API equation for internal leak of API connections is uniaxial since it ignores axial force and external backup pressure. ISO 13679 for qualification of premium connections is biaxial at best. It includes tension/compression but ignores backup pressure for both internal and external leak tests. For tubular design, this paper introduces a new fully triaxial safety factor for threaded connections with dependence on thread shear and hydrostatic pressure. Hydrostatic behavior is modelled with the Mean Normal Stress, and thread shear behavior is modelled with the shear component of the von Mises Stress. A Leak Line for use like the pipe body ellipse is proposed for quick leak assessment. Leak ratings are presented for an example case of 7-in. 35-ppf N80 LTC. The new triaxial safety factor with two connection constants applies to all types of threaded connections, including tubing, casing, and drill pipe, so long as the two constants are evaluated with appropriate but simple physical tests.
This paper presents stress and leak-resistance equations based on the theoryof elasticity for API 8-round connectors in tension and compares results ofthese equations to those of the finite-element method (FEM) and full-scalephysical testing. The new equations identify significant nonconservativeaspects inherent in current API methodologies.
Interest concerning leak resistance of API 8-round connectors prompted theAPI to fund research that identified and assessed many parameters affectingleakage. This research, based on the FEM, provided details that mark a turningpoint in the definition of leak resistance. Although trends in leak resistanceare clearly identified, quantitative application of these data to 8-roundconnectors other than those analyzed is not possible.
Consequently, equations were developed that evaluate API 8-round connectorstress and leak resistance on the basis of the theory of elasticity. Theseequations consider such parameters as coupling OD, pitch diameter, pipe wallthickness, engaged thread length, taper mismatch, number of turns duringmakeup, tension, and internal pressure in determining connector stress stateand leak resistance. The equations provide insight into the interaction ofthese parameters as well as a basis for specifying load capacity.
API 8-round Connections
Fig. 1 shows a cross section of an API 8-round connector. The threadedregion of the connector performs a dual function: it transfers axial loadbetween lengths of pipe and seals formation and internal fluid pressures. Roundthreads form sealing surfaces if the threads are free of foreign substancesthat prevent surface contact and if root and crest voids are sufficiently"plugged" with thread compound. Surface contact is established duringmakeup. Relative axial advancement in the tapered thread geometry results in aninterference fit between pin and coupling. The amount of interference willgenerally vary along the threaded region and depends on pin and box taper andthe number of turns during makeup. During makeup, root and crest voids arefilled and thread flanks are plated by the metal particles suspended in thethread compound.
Fig. 2a shows a thread with the contact pressures resulting from makeup,indicated by the resultant loads on the stab and load flanks, Fcsf and Fclf,respectively. Because of Poisson's effect (sit), these individual thread forceswill generally not be equal; however, force equilibrium is maintained throughthe threaded region. A greater number of makeup turns causes greaterinterference and a higher thread-flank contact pressure. e.g. 2b shows the samethread after tension was applied. Stabflank contact pressure is reduced by anamount proportional to the applied tension, delta y, and Poisson's effect.Load-flank contact in-creases because of the axial load and is slightly reducedbecause of Poisson's effect.
Fig. 2c shows the same thread with sufficient tensile load applied toeliminate stab-flank contact. The axial component of load-flank force is nowequal to the applied tension load. The thread has no leak resistance because ofstab-flank separation. The thread moves closer to a jumpout failure as tensionincreases.
Fig. 2d shows a thread with makeup and pressure loading and no axial load.Internal pressure loading increases contact on thread flanks, as shown.
Fig. 2 also shows that the thread-flank loads necessary to maintain leakresistance vary with makeup, tension, and internal pressure. Increasing thenumber of makeup turns causes higher thread-flank loads, which increase leakresistance. Stab-flank loads decrease with tension, which reduces leakresistance. Internal pressure increases flank loads and leak resistance. Thefollowing analysis concerns leak resistance in tension and examines stab-flankcontact pressures.
The value of contact pressure required to seat a specific fluid pressure isan empirically based number and the subject of ongoing re search. Because somesolid particles in the thread compound plate thread flanks while the vehicle(silicon- or petroleum-based) is extruded with time and load, the sealingmechanism of the thread compound can be considered analogous to that of agasket.
Research on the leak resistance of gaskets uses a sealing factor, A, toevaluate the effectiveness of a gasket:
A = Pcr/Pi.
A lower value of A relates to a more effective seal. Yield strength ofsurface material, contact pressure, thread compound filler material. andsurface roughness all affect the A values.
Because contact pressure is one parameter that affects the value of A andbecause contact pressure varies with the number of turns during makeup,connector diameter, weight, and grade, the sealing factor for 8-roundconnectors is a variable. An assessment of the sealing factor's effect onequation accuracy is made when it is compared with test data.
This paper covers research funded by the API Production Research Advisory Committee (PRAC) on leak resistance of API 8-round connectors. It details the sensitivity of leak resistance to variations in makeup turns, pipe diameter, grade, and applied ten-sion. Findings show that the leak resistance of the connector relative to pipe-body ratings increases with the number of makeup turnand decreases as pipe-body ratings increases with the number of makeup turnand decreases as diameter and yield strength increase. Finally, tension is found to lower leak resistance in a manner that renders hydratesting insufficient for defining leak resistance in typical service conditions.
Interest concerning leakage in API 8-round connectors prompted API to fund research projects that have identified and assessed many parameters affecting leak resistance. The objective of PRAC Projects parameters affecting leak resistance. The objective of PRAC Projects 84-53 and 85-53 was to establish reliable leak data to support internal pressure-resistance ratings, which were lowered in 1983. Leak data are developed with a proprietary finite-element computer code. The analyses considered typical field loading sequence and levels in evaluating the significance of diameter. makeup, tension, and grade on leak.
API 8-Round Connections
The cross section of an API 8-round connection is shown in Fig. 1. The threaded region of the connector performs the dual function of transferring axial load between lengths of pipe and seating formation pressures and internal fluid pressures. Round threads form sealing surfaces if threads are free of foreign substances that prevent surface contact and if root and crest voids are sufficiently prevent surface contact and if root and crest voids are sufficiently "plugged" with thread compound. Surface contact is established during makeup. Relative axial advancement in a tapered-thread geometry results in an interference fit between pin and coupling. During makeup, root and crest voids are filled, and thread flanks are plated by the metal particles suspended in the thread compound. plated by the metal particles suspended in the thread compound. Fig. 2a shows a thread with the contact pressures resulting from makeup indicated by the resultant loads on the stab flank and load flank. Because of Poisson's effects, these individual thread forces will generally not be equal, however, force equilibrium is maintained through the threaded region. The greater the number of makeup turns, the greater the interference and the higher the threadflank contact pressure. Fig. 2b shows the same thread after tension has been applied. Stab-flank contact pressure is reduced by an amount equal to the applied tension, T. and Poisson's effect. Load-flank contact increases from the axial load and is slightly reduced because of Poisson's effects. Poisson's effects. Fig. 2c shows the same thread in which a sufficient tensile load has been applied to eliminate stab-flank contact. The axial component of load-flank force now equals the applied tension load. As a result of stab-flank separation. the thread has no leak resistance and will move closer to a jump-out failure as tension is increased. Fig. 2d shows a thread with makeup and pressure loading and no axial load. Internal pressure loading increases contact on thread flanks, as shown. As Fig. 2 illustrates, the thread-flank loads necessary to maintain leak resistance vary with makeup, tension, and internal pressure. Increasing the number of makeup turns causes higher pressure. Increasing the number of makeup turns causes higher thread-flank loads, which increases leak resistance. Stab-flank, loads decrease with tension and reduce leak resistance, and internal pressure increases both flank loads and leak resistance. The following pressure increases both flank loads and leak resistance. The following investigation concerns leak resistance in tension and is pursued through examination of stab-flank contact pressures.
API's decision to investigate leak resistance using the finite-element method (FEM) was founded primarily on considerations of accuracy, benefit, and cost. The accuracy of the method was verified both by comparison to test results and by sensitivity studies. Benefits of the method concern the detail of information and the ability to isolate single performance parameters. In contrast to physical testing, the FEM provides a complete picture of connector subsurface stress state and surface interaction. The significance of single parameter variations-such as mechanical properties, dimensions, and parameter variations-such as mechanical properties, dimensions, and load-on leak can be evaluated directly by observation of surface contact. Because fully isolating a single parameter during physical testing is not possible, a number of test specimens are required to evaluate performance parameters. The ability of the FEM to identify to isolate, and to assess performance parameters cost-effectively can greatly reduce or eliminate the costs of subsequent testing. The enhancements of the finite-element codes used address the nonlinear connector-specific phenomenon of surface contact, plasticity, and fluid loading of threads. The analysis of API 8-round plasticity, and fluid loading of threads. The analysis of API 8-round connectors uses an axisymmetric model, like that shown in Fig. 3. Special elements used to model surface contact between pin and coupling account for incremental makeup and local surface deflections during loading. Although most structural analyses are limited to the elastic regime, accurate connector analysis usually requires a plastic solution because of stress concentrations at sealing surfaces and high membrane stresses caused by makeup, tension, and pressure loads. It is generally accepted that an elastic analysis with local stress levels above yield is acceptable as long as membrane stresses remain low. Analysis of thread-flank contact, however, requires special consideration because the region of interest occurs in the stressconcentration zone and because plasticity directly affects both the magnitude and resolution of forces transferred from pin to coupling. These compound effects directly impact the interpretation of leak resistance and identify nonconservative aspects of an plastic analysis. Fluid loading of threads resulting from internal pressure is accounted for with an automated method. This method first compares the sealing thread-flank contact pressure with the internal fluid pressure and then refers to the active leak criteria to determine whether pressure and then refers to the active leak criteria to determine whether pressure has penetrated (see Figs. 4 and 5). If pressure penetration pressure has penetrated (see Figs. 4 and 5). If pressure penetration is indicated, the program automatically loads the appropriate thread (or threads) with the applied internal pressure. This procedure is repeated until either full penetration has occurred or equilibrium is achieved with partial penetration. The leak criterion assumes effective plugging of the root and crest voids. Solid particles in the thread compound plate the thread flanks, while the vehicle (silicon- or petroleum-based) is extruded with time and load. This plating of metal particles is analogous to a gasket. Research on the leak resistance of gaskets uses a sealing factor in evaluating the effectiveness of a gasket. The sealing factor, Fs, is defined as Fs =pc/pi, where fi, is the average contact pressure at which leak occurs when the internal pressure is increased pressure at which leak occurs when the internal pressure is increased to a value pi. A lower value of F, indicates a more effective seal. Values of F, have been measured for various surfaces. Yield strengths of surface material, thread-compound filler material, and surface roughness all affect the F, values. Sealing factors range from 7 to 100 for copper and 0.8 to 3.0 for lead for contact pressures of interest. Sliding. heating, and burnishing of thread flanks pressures of interest. Sliding. heating, and burnishing of thread flanks during makeup probably lower the required sealing factor 7 for a threaded connector compared with ASME sealing factor.