Stress analysis of drillstring threaded connections using the finite element method

Abstract

Stress analysis of the threaded connections in drillstrings and bottom hole assemblies has contributed to the successful resolution of some downhole failures. The preload applied from joint make-up torque directly affects the static stress distribution within the connection—it also affects the local mean stress levels about which the stresses arising from fatigue loading oscillate. Considering a generic trapezoidal threadform, the relationship between the nominal applied load and the resulting peak and local elastic stresses at the critical thread roots are established using the finite element method. The distribution of peak stress in the connection is determined based on the axial preload arising from make-up, and how this distribution is modified by tensile and compressive axial loads. Starting with a procedure of mesh convergence and model validation, a two-dimensional axisymmetric elastostatic modelling approach is used. In all cases, the roots of the first loaded tooth in the pin and the last loaded tooth in the box are the sites of maximum peak stress as expected, the pin peak stress being the greater. However, considering the effects of fatigue loading by relating the individual preload and tensile load cases to local and peak stress ranges and mean levels demonstrates that the box becomes the critical component.