An Innovative Design Approach To Reduce Drill String Fatigue
- Tom H. Hill (T.H. Hill Associates Inc.) | Sean E. Ellis (T.H. Hill Associates Inc.) | Kang Lee (T.H. Hill Associates Inc.) | Nicholas M. Reynolds (T.H. Hill Associates Inc.) | Nanjiu Zheng (T.H. Hill Associates Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2005
- Document Type
- Journal Paper
- 94 - 100
- 2005. Society of Petroleum Engineers
- 1.6 Drilling Operations, 4.1.5 Processing Equipment, 1.10 Drilling Equipment, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.4 Drillstring Design, 1.6.1 Drilling Operation Management, 2 Well Completion, 4.2.3 Materials and Corrosion, 4.1.2 Separation and Treating
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Fatigue and corrosion fatigue account for the majority of drillstringfailures.1,2 The complexity of fatigue and the drillstring designer's inabilityto account for a great many factors affecting the mechanism make it impracticalor impossible to accurately predict a component's fatigue life in cycles tofailure. This paper describes the comparative-design approach. Twodimensionless indices, curvature index (CI) and stability index (SI), have beendeveloped to allow drillstring designers to quantitatively compare designalternatives on the basis of normalized fatigue-crack-propagation life for thepurpose of selecting the one with the best fatigue performance. The two designindices will not give meaningful values of absolute fatigue life, but they willprovide a quantitative comparison of the relative fatigue lives of componentsoperating under different sets of circumstances—unknown factors beingequal.This new approach normalizes many factors affecting fatigue thatare generally unknown to the designer, enabling the drilling engineer toquantitatively compare the fatigue performance of available alternatives on thebasis of what the drilling engineer does know.The method has alreadyproven effective in field use.
In designing a drillstring, the designer rarely decides the attributes of aparticular component.Instead, the drilling engineer forms a "string" byscrewing together up to several hundred off-the-ground items.In thisactivity, the designer will try to choose components that balance a number ofoften-conflicting needs, including loads, hydraulics, hole cleaning, rate ofpenetration, steering, measurement, and, perhaps most critical, structuralsoundness of the drillstring itself.In maintaining structural soundness,the designer faces two separate challenges: first, preventing overload failure,and second, preventing fatigue failure. In a market in which many drillstringcomponents are rented, a single component will be used and reused a number oftimes by a number of designers.Furthermore, it is a market for whichcomponents are specified and purchased primarily for strength under high,relatively static loads.Resistance to overload failure, not resistance tofatigue, is the principal focus of standards and specifications coveringdrillstem components.3Because of these two factors, the designer is oftenleft in the dark on issues that are very important for affecting a workablefatigue design.This is best illustrated by looking at the fatiguemechanism and at the approaches available to the designer for controllingit.
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