Most drill pipe failures are caused by metal fatigue because of repeated or fluctuating stress. The cumulative damage rule and S-N curves are used in a tool to calculate the amount of fatigue life consumed in drill strings. Comparisons are performed between the stress methodologies proposed by Goodman and Soderberg. For the study, a stiff string torque and drag model is used to calculate stress such as axial force, torque, bending moment, and strains. S-N curves are applied to determine the number of alternating stress cycles needed to cause fatigue failure. The Miner’s Rule is used to calculate the cumulative damage, which results in the percentage of life spent during the drilling operations. A fatigue study of a vertical tortuous well, reaching a depth of 3115 meters is carried out. Three BHAs were used to model the drilling process, performing various operations including drilling, rotation-off-bottom, and backreaming operations. The overall fatigue life consumption in a drill collar connection was computed taking into account its reuse in all BHAs. The profiles of stress and fatigue are provided to help in the analysis of the issue. Comparing Goodman’s and Soderberg’s average stress rule models yielded the overall conclusion that the differences between them are very small for the case studied, Goodman’s responses being greater than Soderberg’s. The integration of the model into a torque and drag quantification tool allows analyses to be performed using both average stress techniques, allowing the drill string to be sized in a way that prevents excessive fatigue life consumption.

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