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Prevent Rotary Shouldered Connection Failures
Jim Douglas, Gagemaker, LP, © 2011
Designers of the original rotary shouldered connection (RSC) would never have imagined that their designs would be taken to the extreme depths and subjected to the severity of directional drilling as they are today. Companies demand superior RSC performance as they run drilling tools longer and with higher forces to maintain schedules and reduce time on location. Unfortunately, galled shoulders, swollen boxes and fatigue stress cracking are costly connection failures that are fairly common and an expected expense by drilling contractors today.
The root cause of most RSC failures and damage is the lack of thread interference in the connection. The most important thread element of an RSC connection is the pitch diameter size, which is not addressed in the current API Specification 7-2 or API RP7G. Consequently, connectors are still manufactured using 92 year old inspection technology that produces connections that do not have full thread contact or proper radial interference when torqued together. In order for connections to achieve the performance level required today, a new approach to gauging and dimensional inspection must be pursued and implemented for not only new, but also used connections.
This paper explains the importance of isolating the connectors’ pitch diameters from the ring and plug gauge standoff and the other thread elements, making it a standalone attribute measurement. By measuring the pitch diameters, the operators can directly control the connector’s size during manufacturing as well as ensure that the thread cones go into interference properly when the connections are torqued together.