Use NC56 Connections on 8" Drill Collars and Cut 1" or 3/4" Pin Stress Relief Grooves on Rotated BHA Connections NC38 and Larger
Authors: Ellis Sean , Reynolds Nicholas , Lee Kang 
Published: December 2007

When designing bottom-hole assemblies, the designer must specify both the type of connections and whether or not to use stress relief features. The relative fatigue lives of NC56 and 6–5/8 REG API connections were studied to compare the two connections in identical service. In addition, three common pin stress relief groove (SRG) widths were studied to determine the width that yields the best fatigue life. The relative magnitude of improvement in fatigue life was quantified using finite element analysis (FEA) and the Morrow strain-life model.

The combination of a more fatigue resistant thread form and increased box stiffness from a steeper taper gives the NC56 connection a dramatic fatigue advantage over the 6–5/8 REG connection. In addition, the results show the fatigue life of the pins modeled with 3/4" and 1" SRGs to be the best, with the 1? groove having a slight advantage. Pins cut with no groove can be expected to have the worst fatigue life, followed surprisingly closely by pins cut with the 1 1/4" groove.

Introduction

Because fatigue historically accounts for roughly 80% of all drill string mechanical failures and the cost of a failure can be quite substantial, it is in the interest of the oil and gas industry to utilize the most fatigue resistant connections. Operators will benefit from decreased costs associated with fishing and tool replacement, and drilling contractors and rental companies will benefit from increased tool life. In order to determine which designs will be most fatigue resistant, and to quantify the benefits of a particular design, analyses must be conducted that compare available options on a relative basis.

Finite element analysis has been used for many years to determine the stresses present in complex models under multiple loads. Advances in FEA software and computer technology have allowed problems that previously required expensive supercomputers and days of computer run time to be solved with a desktop workstation and only several hours of computer time. As a result of these advances, problems that were run in the past with simplifying assumptions (in the interest of time and money) can be revisited in the pursuit of more accurate results. In addition, new topics previously deemed too complex to analyze can now be investigated.

Given the multiple loads involved (e.g. make-up torque, externally applied tension, bending, etc.), accurately analyzing the stress and strain distribution in a rotary shouldered connection is a challenging matter. In general, the areas that frequently require the most attention are (1) the mesh, (2) the "application" of make-up torque (MUT), (3) the application of a bending load, (4) the stress and strain output, and (5) the fatigue life model. The mesh, application of MUT, and application of bending load are discussed in detail in Appendix A. The stress and strain output and fatigue life model are discussed below as they distinguish these analyses from those previously conducted.

Stress and Strain Output

The stress distribution and maximum stress have traditionally been the means by which the suitability and performance characteristics of a model have been rated. When evaluating the performance of a rotary shouldered connection, however, it is sometimes necessary to focus on the strain instead of the stress, or both. Strain is significant because the stresses in the critical areas of the connection (thread roots and shoulder transition region) often exceed the yield strength of the material. Referring to the stress versus strain curve in Figure 1, it is evident that once the yield strength of the material is exceeded, the stress changes only slightly until the ultimate strength is reached, but the strain continues to increase significantly. Thus, if the point strain is plastic, the strain and strain amplitude are important drivers that should be considered when evaluating connection performance, not just stress.