Equations for Leak Resistance of API 8-Round Connectors in Tension

Abstract

Summary 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. Introduction 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. and surface 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.

Leakage Resistance Envelopes of API 8 Round Casing Connections using FE Analysis

“…Figure 4 illustrates two proposed criteria to determine if fluid penetration occurs: 1) fluid penetration occurs if fluid pressure exceeds SFCP; 2) fluid penetration occurs if SFCP is zero [13]. Schwind et al [14,15] suggested two parameters to quantify sealability: 1) average pressure on the stab flank and 2) average pressure on stab flank normalized by the internal pressure. Assanelli et al [10] use those criteria to predict sealability, reproducing via FE analysis results of capped-end tests.…”

Flow in Microchannels between Sealing Surfaces of Casing Connections: LBM Simulation

“…Focusing on the seal failure of threaded connections, several researches have been carried out. Schwind discussed various factors that affect the sealing performance of API round threaded connections, and the equations for stress and leak resistance were proposed based on the elastic theory [9]. Gong studied the effect of the tolerance of thread height, thread angle, and thread taper on the sealing performance of standard API casing threaded connections [10].…”

Envoltórias probabilísticas da estanqueidade de conexões de revestimento de poços

“…Fonte: Adaptado de SCHWIND e WOOLEY (1989). SCHWIND et al (SCHWIND, 1987;SCHWIND et al, 1995) sugerem dois parâmetros para quantificar a estanqueidade:…”