STRESS-ANALYSIS OF DRILLSTRING THREADED CONNECTIONS USING THE FINITE-ELEMENT METHOD

Stress analysis of drillstring threaded connections under axial, bendi ng and torsion loadings has been carried out using the finite element method in order to determine the regions of highest stress concentrati on. This information is required for fatigue and fracture mechanics an alysis. For axial loading, two-dimensional axisymmetric models of the threaded joints have been used for the connector types, in standard fo rm and with bore back and stress relief, and also in standard form wit h some slight modification to thread root geometry. Full three-dimensi onal models of the connectors, but ignoring the helix angle, were stud ied for bending and torsion loading. The peak stress concentration fac tors 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 axia l loading, bending showed lower stress concentration factor values, bu t for torsion the values were very small. The study showed clear advan tages in the use of bore back and stress relief in reducing stress con centration factor. It was found that a further decrease in stress conc entration factor is possible with minor changes to the thread geometry . Sensitivity analysis on the effect of thread profile on stress conce ntration factor showed that the fatigue life could be increased by an improved profile thread design.