Drive String Fatigue in PCP Applications

Abstract

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.

Introduction

The Progressing Cavity Pump (PCP) has been used increasingly for artificial lift in heavy oil applications since the early 1980s.  Its use in other applications, such as coal gas, medium and light oil reservoirs, high watercut, and more recently in thermal applications, has also increased over the past three decades.  The downhole PCP has two main components: a stator, usually installed at the bottom of the production tubing string; and a rotor, installed at the bottom of a rod string, also known as a drive string.  This drive string is rotated from surface and delivers torque and power to the pump.  The rod string is also subject to axial load, both from the pump and from its own weight.

Any deviations in a wellbore mean that the tubing and rod strings must bend to follow the profile of the well.  This bending of the rod results in bending stress in the rod, in addition to the stress from torque and axial load.  The bending stress in a PCP application is usually small.  However, the bending stress changes direction as the rod rotates; and over millions of rotations, and this cyclic loading can result in fatigue damage to the drive string, and its possible eventual failure.

 

Author: Skoczylas, P.

Publisher: SPE Artificial Lift Conference & Exhibition-North America, 6-8 October, Houston, Texas, USA

Year: 2014

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