Multiaxial Fatigue in Drill Pipes Under Non-Proportional Loading

The scope of this research is to estimate the fatigue life and crack initiation direction in components subjected to multiaxial, nonproportional loading. Two critical plane approaches — Fatemi-Socie and McDiarmid — were used to estimate low cycle and high cycle fatigue lives, respectively. A software program (Elrond) was constructed to estimate the fatigue life and crack initiation plane. The code was validated using experimental fatigue data sets from previous research, then was applied to a real-life drilling problem. Drill pipe operating in the “Build and Hold” drilling job condition was analyzed using the developed software. Results were compared with the API RP7G fatigue design guidelines. Fatigue life charts were prepared using fatigue lives estimated by Elrond at several axial loadings, dogleg severities and torque amplitudes. Also, maximum allowable torque under high cycle fatigue conditions was estimated.