Drive string (rod string) fatigue failures may occur occasionally in Progressing Cavity Pump (PCP) applications. In a deviated well, the drive string is subjected to rotating bending while the pump is in operation. For each rotation of the string, each portion of the rod's surface undergoes a load cycle of compression and tension. This load cycle is overlaid on the approximately constant torque and axial load that are usually the primary loads considered in sizing a drive string. While the alternating load from rotating bending typically results in a small stress compared to the other loads, it can eventually lead to a fatigue failure.
This paper presents a model for estimating the life of a drive string before a fatigue failure occurs. This model considers the overall stress state in the drive string, the effects of curvature magnification (which occurs near the larger diameter connections in a coupled drive string), and environmental factors. The model has been developed only for solid rod bodies (whether coupled or continuous), and it is not applicable to hollow rods.
This paper also discusses several aspects of lab testing for determining fatigue life. Simple fatigue tests are typically performed under either pure rotating bending, or cyclic bending (without rotation). These tests may not give results that are representative of the actual fatigue life that may result from operation in a different stress state (i.e. when constant torsional and axial stresses are added to the cyclic bending stress).
This paper is aimed at assisting production engineers in designing drive strings for applications in which fatigue failures may occur.