Use NC56 Connections on 8? Drill Collars and Cut 1? or3/4? Pin Stress Relief Grooves on Rotated BHA Connections NC38 and Larger

摘要

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.