TH Hill Inc. - Standards & Technical Development

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Advanced Fatigue Mitigation Techniques

Standard DS-1

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Other Technology Development

Curvature Index

What is Curvature Index?

As defined in DS-1 Third Edition, Volume 2: Drill Stem Design & Operation, Curvature Index is a measure of the relative fatigue crack propagation life of a drill pipe tube rotating in a curved hole section, taking into account hole curvature, pipe diameter, grade, class and weight, and axial tension in the pipe. The Design Volume has graphical representations of Curvature Indices that take into account the variables described above that impact drill pipe fatigue performance.

How do you use Curvature Index?

Curvature Index is a quantitative design tool that allows the user to analyze different drill string and trajectory options and compare them for relative fatigue performance. It can be used in three primary ways:

To select the best design option for fatigue performance among competing alternatives
As a design constraint
To help set inspection frequency

The following is a sample CI graph taken directly from DS-1 Third Edition:

A lower Curvature Index number indicates better fatigue performance on a relative basis. Additionally, CI calculations can be compared quantitatively to gauge how much better one option is than its alternative. For example, a design with a CI of 4000 is two times better, relative to fatigue performance, than one with a CI of 8000. All else equal, the lower your Curvature Index the longer the fatigue life of your drill pipe.