Fatigue in BHA Connections: How to Cope with It and Extend Product’s Life

Abstract

Bottom-hole assembly (BHA) drill stem components and tools are heavy and have increased wall thickness compared to drill pipe. These components are typically run in compression, which can result in buckling, subjecting the BHA components to bending. Rotation under bending creates cyclic stress, fatigue, and can lead to possible failure. Changes in cross section and the presence of sharp corners and geometric discontinuities in the BHA thread/connection profile produce a higher localized stress state, a stress concentration, in comparison with the tube body of the BHA component. As a result, the connection is almost always the point of fatigue failure in drill collars (DC) and the point of failure in heavy weight drill pipe (HWDP) about half of the time. Therefore, increasing the fatigue life of the connection is beneficial. To increase BHA connection fatigue life, stress reducing features such as pin stress relief grooves, box borebacks, and a cold-rolling of the thread roots (creating a compressive stress state) have been incorporated into BHA connection design. Additional connection designs have been developed that provide additional stress reduction and extend the connection fatigue life. Besides improving the resistance to fatigue crack initiation, these designs provide an added benefit of reducing the material loss when repairing (recutting) the connection. Three different field proven connection technologies are presented in this paper. The connection features that contribute to improved fatigue performance are described. The first is a modification to an API rotary shouldered connection. The second is a proprietary double-shoulder connection developed specifically for BHA tools. The benefits of both are discussed, including the results of full-scale comparative fatigue testing. The amount of material loss from repairing the connection and the impact on product life are also examined. Third, the use of streamline proprietary double-shouldered connections on heavy weight drill pipe (HWDP) is discussed. Recommendations are presented.