Fatigue and corrosion fatigue account for the majority of drillstring failures.1,2 The complexity of fatigue and the drill stringdesigner’s inability to account for a great many factorsaffecting the mechanism make it impractical or impossible toaccurately predict a component’s fatigue life in cycles tofailure. This paper describes the comparative design approach.This new approach normalizes many factors affecting fatiguethat are generally unknown to the designer, allowing him orher to quantitatively compare the fatigue performance ofavailable alternatives on the basis of what he or she doesknow. The method has already proven effective in field use.
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