An Innovative Design Approach to Reduce Drillstring Fatigue

摘要

Fatigue and corrosion fatigue account for the majority of drillstring failures.1'2 The complexity of fatigue and the drillstring designer's inability to account for a great many factors affecting the mechanism make it impractical or impossible to accurately predict a component's fatigue life in cycles to failure. This paper describes the comparative-design approach. Two dimensionless indices, curvature index (CI) and stability index (SI), have been developed to allow drillstring designers to quantitatively compare design alternatives on the basis of normalized fatigue-crack-propagation life for the purpose of selecting the one with the best fatigue performance. The two design indices will not give meaningful values of absolute fatigue life, but they will provide a quantitative comparison of the relative fatigue lives of components operating under different sets of circumstances―unknown factors being equal. This new approach normalizes many factors affecting fatigue that are generally unknown to the designer, enabling the drilling engineer to quantitatively compare the fatigue performance of available alternatives on the basis of what the drilling engineer does know. The method has already proven effective in field use.