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.
Introduction
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.
