Stress analysis of drillstring threaded connections using the finite element method

Abstract

Stress analysis of drillstring threaded connections under axial, bending and torsion loadings has been carried out using the finite element method in order to determine the regions of highest stress concentration. This information is required for fatigue and fracture mechanics analysis. For axial loading, two-dimensional axisymmetric models of the threaded joints have been used for the connector types, in standard form and with bore back and stress relief, and also in standard form with some slight modification to thread root geometry. Full three-dimensional models of the connectors, but ignoring the helix angle, were studied for bending and torsion loading. The peak stress concentration factors in all cases were at the thread root of the first loaded tooth in the pin and the last loaded tooth in the box. In comparison with axial loading, bending showed lower stress concentration factor values, but for torsion the values were very small. The study showed clear advantages in the use of bore back and stress relief in reducing stress concentration factor. It was found that a further decrease in stress concentration factor is possible with minor changes to the thread geometry. Sensitivity analysis on the effect of thread profile on stress concentration factor showed that the fatigue life could be increased by an improved profile thread design.