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

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 direc tly affects the static stress distribution within the connection-it al so affects the local mean stress levels about which the stresses arisi ng from fatigue loading oscillate. Considering a generic trapezoidal t hreadform, the relationship between the nominal applied load and the r esulting peak and local elastic stresses at the critical thread roots are established using the finite element method. The distribution of p eak 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 conver gence and model validation, a two-dimensional axisymmetric elastostati c modelling approach is used. In all cases, the roots of the first loa ded tooth in the pin and the last loaded tooth in the box are the site s of maximum peak stress as expected, the pin peak stress being the gr eater. However, considering the effects of fatigue loading by relating the individual preload and tensile load cases to local and peak stres s ranges and mean levels demonstrates that the box becomes the critica l component.