Fatigue life improvement of drill collars through control of bore eccentricity

Abstract

The majority of drillstem failures are attributed to fatigue, particularly in BHA connections where there is a high level of stress concentration in the thread roots. Any stress concentration combined with a modest amount of bore eccentricity in the drill collar may result in a notable reduction in the fatigue life of the component under bending loads. This paper examines the relationship between drill collar bore eccentricity and fatigue life improvement. A detailed two-dimensional axisymmetric finite element analysis has been performed on a standard API connector exhibiting varying amounts of bore eccentricity, as so to identify stress concentration factors. From these it is possible, using well-established strain life equations, to predict what effect bore eccentricity has on the fatigue life. This approach allows the study of material sensitivity in fatigue instead of a simple consideration of stress concentrations. The results are presented in terms of percentage reduction in relative fatigue life as a function of stress level and bore eccentricity. Drill collar bore eccentricity is an unavoidable defect of mass production, and can be seen to have a significant effect on its fatigue life. However, through improved quality control, and industry standard specifications, it is possible to enhance the drill collar fatigue life.