Advanced Assessment of Drillpipe Fatigue and Application to Critical Well Engineering

摘要

This paper addresses the nature and assessment of drillpipefatigue. A detailed examination and statistical analysis of thethree published sources of drillpipe S-N curve datasupplemented by principles of machine design, provides aconsistent basis to determine S-N curves for drillpipe ofarbitrary size and grade. The machine design approachtranslates fatigue life characteristics of a small sample to thatof drillpipe as a larger component. This translation accountsfor size, manufacturing process and influences such asenvironment and vibrations. The resulting fatigue behavioragrees with previous data on grades D and E collected on fullscale pipe specimens. Further, the current approach isdemonstrated to be successful in predicting the fatiguebehavior of higher drillpipe grades (G, X, S, V) for whichscant data is currently available. Finally, the work is shown tobe consistent with Hansford and Lubinski’s 1964 analysiswhich informs the fatigue guidelines in API RP7G.Given the S-N curve and magnitude of variable loads,fatigue assessment requires tracking the number andmagnitude of stress cycles. Since wellbore tortuosity andchanges of drilling parameters with depth render thecalculations cumbersome, a computer simulation is necessary.The simulation enables rapid and efficient evaluation of holesection, drillstring, trajectory and drilling parameterpermutations. This is particularly useful in developmentswhere a number of wells are drilled from a single structure.Application of the program is illustrated with a case study.The effect of a large shallow dogleg on accumulated fatigue inthe drillpipe is discussed, enabling informed decisions ondrilling subsequent hole sections.