Fatigue Analysis of 5-1/2 F.H. Threaded Connection

This paper examines the fatigue behavior of the 5-1/2 F.H. threaded connection. The finite element method was used to calculate the static and bending stresses in the pin and box for the purpose of estimating fatigue life of the connection. These stresses were compared with the material properties of the steel and a fatigue life was thereby predicted for a given dogleg. The Gerber criterion was selected as the theoretical basis for combining mean and fluctuating stresses. Mean stress was linearized from the static case associated with the preload from make-up torque. Alternating stress was obtained from applying a bending moment associated with a prescribed dogleg. The last engaged thread in the pin, closest to the shoulder, and the last engaged thread in the box, farthest from the shoulder, were seen to be the sites of maximum stress. The pin stresses govern fatigue life because mean stress is higher than for the box. In a parallel effort, a roll test was set up to examine fatigue life experimentally. A BHA was made up and installed horizontally in the roll machine. The deflection at the threaded connection was measured, and the assembly was rotated at a measured speed until the pin part of the threaded connection failed. Next, the finite element results were compared with roll test data and good agreement was obtained. The data was then extrapolated to generate a graph showing expected life as a function of dogleg severity for the 5-1/2 F.H. connection. Finally, the maximum dogleg for which the connection has a 10 million cycle life was predicted.