An Analytical Model for Double-Shouldered Connection Strengths

Abstract

Abstract The drilling industry has become enamored with the extra torsional capacity and clearance that are available when double-shouldered connections are used in the place of the single-shouldered connections that are in the public domain. These proprietary connections allow us to drill farther, faster, and with less damage to our drill string. To this point, however, no analytical model has been introduced that can accurately calculate the tensile and torsional capacities of these useful connections. Typically, the connection designer and/or manufacturer creates an empirical formula that is calibrated through laboratory and field testing. This approach is perfectly acceptable, but it hinders those tool designers that need something different from what is offered on the public market. It is rarely cost-efficient to perform laboratory and field testing for a one-off connection design. Single-shouldered connections, such as API drill stem connections, have a straightforward analytical equation that can be used to determine the capacities of any connection. This equation may not be perfect—it relies on linear assumptions that are probably not descriptive of the connection loading—but several decades of use have allowed the industry to be confident in its strengths and aware of its shortcomings. Thus, what is needed is to simply extend the approach of the original single-shouldered equation to account for a second shoulder. Though the mathematical complexity increases a bit, the assumptions are the same, lending the same confidence to the extended equation that the industry has for the original. This paper presents the derivation of just such an equation. The implicit assumptions present in the original equations are discussed, and then the same ideas are applied to a double-shouldered connection. The full set of new equations are developed and described, including tips on their practical use gained from tool-design experience.