Increasing Long-Term Fatigue Performance of an API Box Boreback Using an Enhanced Non-Truncated Thread Design and Revised Machining Methodology

摘要

Stress Relief Features (SRFs) are commonly added to Rotary-Shouldered Connections (RSCs) to increase the fatigue performance of the connection. An SRF aims to lower the critical stress at the last engaged thread and improve fatigue performance. The most common SRF prescribed by the American Petroleum Institute (API) for API box connections is the API Box Boreback. The API Box Boreback SRF is added to an API RSC by boring out both the box threads and ID to a specified cylinder diameter and depth from the box shoulder. The API boreback is then finished by profile turning a tapered transition of specified angles and lengths which connects the back of the boreback cylinder to the ID of the component. The traditional method of boring the cylinder diameter due to the tapered threads of API connections causes a removal of the thread crests at the back of the box. These truncations of the last engaged threads reduce the contact area of the thread flanks available to support the load. The repercussions of this include an increase in stress concentration, reducing the fatigue life, and an escalation in susceptibility to thread wear, which inevitably amplifies the effect of the thread truncation, further reducing the fatigue life and cancelling out the benefits of the SRF. Details of this phenomenon can be found in IADC/SPE 128931 (Gerdes & Lee, 2010). Despite its popularity, the API Box Boreback SRF can lose its effectiveness as the thread truncation at the back of the box can accelerate crack initiation and cause premature failures. To remedy this, an experimental study was undertaken to develop a machining method to cut a boreback with an enhanced non-truncated geometry on API box connections. The modified boreback geometry and machining methodology were perfected over multiple iterations, prototyped with a Computer Numerical Controlled (CNC) lathe using various routines and tooling selections. The dimensions were verified to conform to the requirements outlined in API Specification 7-2 and the machining process was validated on multiple API box connection types. This paper presents details of a modified design and a universal machining methodology developed to cut an API compliant Box Boreback which retains full-height threads at the back of the box, avoiding thread truncation, and preserving the increased fatigue performance a Box Boreback provides well into the life of the connection. The method meets all dimensional requirements outlined in API Specification 7-2,ensuring proper make-up. The machining process can be performed on either a CNC or manual lathe. The enhanced boreback design provides a drastic improvement in fatigue performance over the standard API Box Boreback design, while maintaining all other structural and dimensional design aspects, and without altering the make-up torque or Bending Strength Ratio of the connection.