TH Hill Inc. - Standards & Technical Development

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Advanced Fatigue Mitigation Techniques

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Stability Index

What is Stability Index?

As defined in DS-1 Third Edition, Volume 2: Drill Stem Design & Operation, Stability Index is a measure of the relative fatigue crack propagation life of drill collars and HWDP sections that are simultaneously buckled and rotated. The Design Volume has graphical representations of Stability Indices that take into account hole size, connection type and equipment type.

How do you use Stability Index?

Stability Index is a quantitative design tool that allows the user to analyze different BHA 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

A lower Stability Index number indicates better fatigue performance on a relative basis. Additionally, SI calculations can be compared quantitatively to gauge how much better one option is than its alternative. For example, a design with a SI of 400 is two times better, relative to fatigue performance, than one with a SI of 800. All else equal, the lower your Stability Index the longer the fatigue life of your drill pipe. Stability Index values can only be compared across similar equipment types, so drill collar values are not comparable to HWDP values.