Fatigue life improvement of drill collars through control of bore eccentricity
✍ Scribed by M.J. Knight; F.P. Brennan
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 703 KB
- Volume
- 6
- Category
- Article
- ISSN
- 1350-6307
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✦ Synopsis
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 _nite 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 e}ect 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 signi_cant e}ect on its fatigue life[ However\ through improved quality control\ and industry standard speci_cations\ it is possible to enhance the drill collar fatigue life[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Keywords] Drill collar^Threaded connection^Bore eccentricity^Stress analysis^Fatigue life