Fatigue and corrosion fatigue account for the majority of drill string failures.1,2 The complexity of fatigue and the drill string 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. This new approach normalizes many factors affecting fatigue that are generally unknown to the designer, allowing him or her to quantitatively compare the fatigue performance of available alternatives on the basis of what he or she does know. The method has already proven effective in field use.
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