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

摘要

When designing bottom-hole assemblies, the designer mustspecify both the type of connections and whether or not to usestress relief features. The relative fatigue lives of NC56 and6-5/8 REG API connections were studied to compare the twoconnections in identical service. In addition, three commonpin stress relief groove (SRG) widths were studied todetermine the width that yields the best fatigue life. Therelative magnitude of improvement in fatigue life wasquantified using finite element analysis (FEA) and theMorrow strain-life model.The combination of a more fatigue resistant thread formand increased box stiffness from a steeper taper gives theNC56 connection a dramatic fatigue advantage over the 6-5/8REG connection. In addition, the results show the fatigue lifeof the pins modeled with 3/4? and 1? SRGs to be the best, withthe 1? groove having a slight advantage. Pins cut with nogroove can be expected to have the worst fatigue life, followedsurprisingly closely by pins cut with the 1 1/4? groove.