Summary
Fatigue and corrosion fatigue account for the majority of drillstring
failures.1,2 The complexity of fatigue and the drillstring 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. Two
dimensionless indices, curvature index (CI) and stability index (SI), have been
developed to allow drillstring designers to quantitatively compare design
alternatives on the basis of normalized fatigue-crack-propagation life for the
purpose of selecting the one with the best fatigue performance. The two design
indices will not give meaningful values of absolute fatigue life, but they will
provide a quantitative comparison of the relative fatigue lives of components
operating under different sets of circumstances—unknown factors being
equal. This new approach normalizes many factors affecting fatigue that
are generally unknown to the designer, enabling the drilling engineer to
quantitatively compare the fatigue performance of available alternatives on the
basis of what the drilling engineer does know. The method has already
proven effective in field use.
Introduction
In designing a drillstring, the designer rarely decides the attributes of a
particular component. Instead, the drilling engineer forms a “string” by
screwing together up to several hundred off-the-ground items. In this
activity, the designer will try to choose components that balance a number of
often-conflicting needs, including loads, hydraulics, hole cleaning, rate of
penetration, steering, measurement, and, perhaps most critical, structural
soundness 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 drillstring
components are rented, a single component will be used and reused a number of
times by a number of designers. Furthermore, it is a market for which
components are specified and purchased primarily for strength under high,
relatively static loads. Resistance to overload failure, not resistance to
fatigue, is the principal focus of standards and specifications covering
drillstem components.3 Because of these two factors, the designer is often
left in the dark on issues that are very important for affecting a workable
fatigue design. This is best illustrated by looking at the fatigue
mechanism and at the approaches available to the designer for controlling
it.
© 2005. Society of Petroleum Engineers
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