The bending stress magnification influence on drillpipe fatigue

Fatigue phenomena were identified as the main cause of most of drillstring failures. The alternating stresses that result from cyclic loading are usually most damaging at abrupt changes in drill string cross-section. This work presents the analytical solution for maximum drill pipe curvature when an axial compressive or tensile load, in the practical range, is applied to the drill string in a dog leg. Contact (point and wrap) between the drill pipe and the hole wall as well as tool joint contact load is taken into account. The results of Lubinski are rederived and the case of axial compression of the drill pipe is included. Hence, the appropriate equations are derived here for both tension and compression. The results given complement calculations based on torque and drag models of drillstring. Finally one design application is presented for drill pipe, to show the bending stress magnification (BSM) effect is significant in practice and it must be considered in drillstring design.