Threaded Connection Limit State Equations for Use in LRFD Tubular Design
Authors: Schwind B.E. , Chappell J.F. , Katsounas A.T. 
Published: October 2008

ABSTRACT
The purpose of the study was to identify and evaluate the variables that determine the leak resistance limit state functions of API 8-round and buttress [1] connections. This work will be incorporated into Load and Resistance Factor Design (LRFD) equations. Connectors are an integral part any well design program. Therefore, it is vital that they be included in the LRFD design approach. Makeup, tension, internal pressure and dimensional data were among the variables in the evaluation, which was based on finite element analysis, testing and structural mechanics. The leak resistance limit state for round thread connections is defined by contact pressure, stab flank engaged length and coupling yield, while for buttress is defined by contact pressure, stab flank clearance and coupling yield. Leak pressure, as defined by API Bul. 5C3 [2], is a function of makeup and dimensional data independent of thread type, tension, and pipe inside diameter and valid only in the elastic regime. Tension is detrimental in the leak resistance of 8-round connections, but does not compromise buttress leak resistance. Regression analysis was performed on structural mechanicsresults to produce the highest correlation to finite element results to account for end effects on round thread connections. It was determined. by testing that stab flank contact over a minimum length of engagement is not sufficient to prevent 8-round leak, despite sufficient contact pressure level. Teflon impregnated thread compound should be the choice for API buttress. The indications are that optimized makeup should be considered in the leak resistance capacity of API connections. Excessive makeup and/or tension or pressure could result in coupling yield causing leak upon re-pressurization. Further analytical and experimental work would improve the degree of accuracy with which the leak resistance capacity can be determined.

INTRODUCTION
The capacity of threaded connectors for use in LRFD design are defined by the structural and leak resistance limit states of the connector. The structural limit state was addressed by Lewis, et. al. [3]. This paper addresses the leak resistance limit state of API connections. The mechanics of API thread leakage is yet to be fully understood. Although, in a test or on the field we know a connector is leaking, it is not clear what has caused the leak. FEA analysis may give insight into the contact of individual threads, but the absence of thread compound and thread compound pressure complicate the determination of leak.
The API round thread is designed to provide a seal at both the stab flank and load flank, allowing only root and crest voids. Thread compound plugs those areas and prevents the spiral leak path. The geometry of the round thread makes it susceptible to leak when tension load is applied because of the tendency of the stab flank to open thus providing an additional leak path. On the other hand, the API buttress thread provides a seal at the root, crest and load flank. Thread compound is expected to plug the stab flank. Due to the steep load flank angle, tension does not compromise the leak resistance of the buttress connection.