This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 194180, “An Analytical Model for Double-Shouldered Connection Strengths,” by Grant Pettit, SPE, Bureau Veritas, prepared for the 2019 SPE/IADC International Drilling Conference and Exhibition, The Hague, 5–7 March. The paper has not been peer reviewed.
The extra torsional capacity and clearance available when proprietary, double-shouldered connections are used instead of public-domain, single-shouldered connections enables well teams to drill farther, faster, and with less damage to the drillstring. No analytical model as yet can accurately calculate the tensile and torsional capacities of double-shouldered connections. This paper presents a set of equations that extends the approach of the original single-shouldered equation to account for a second shoulder, and helps to understand connection strengths better.
Single-shouldered connections such as API drillstem connections use a straightforward analytical equation 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 made the industry confident in the equation’s strengths and aware of its shortcomings.
Because no analytical model is available to calculate these capacities in proprietary double-shouldered connections, the connection designer or manufacturer typically creates an empirical formula that is calibrated through laboratory and field testing. Although this approach is acceptable, it hinders tool designers who 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.
The complete paper states that what is needed is to extend the approach of the original single-shouldered equation to account for a second shoulder. According to the author, although the mathematical complexity increases, the assumptions are the same, lending confidence to the extended equation.
The complete paper presents the derivation of a set of equations that use the same basic assumptions as the original single-shouldered connection torque equations, in the same way as do the API equations, to advance understanding of the torsion and tension capacities for double-shouldered equations. The implicit assumptions present in the original equations are discussed; then the same ideas are applied to a double-shouldered connection. The full set of new equations is developed and described, including tips on their practical use gained from tool-design experience.
Current Single-Shouldered Connection Model
Connection strength formulas contained in API RP 7G Appendix A are the basis for the connection torsion and tensile capacities in that standard and in many other oilfield references. These equations and resulting numbers have been in place for decades, creating industry confidence in their usefulness and awareness of their weaknesses.