drill collars

Aug 14th, 2016
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Drill CollarsIn the early years of rotary drilling , a typical bottomhole assembly (BHA) consisted of nothing more than a shortcrossover sub, or collar, between the bit and the drill pipe. Over time, drillers observed that penetration rates tendedto increase as more weight was applied to the bit,and so they began using BHAs with longer sections of heavy pipe(in keeping with the original terminology, these heavy joints of pipe have continued to be called drill collars).But even as these heavier BHAs brought improved bit performance, it became clear that applying more weight to thebit involved a trade-off: a greater potential for drill pipe failure due to increased compressive force and resultantbending or buckling.When drill pipe rotates in a compressed state, it experiences cyclic stress reversals. Each rotation of the stringcauses the drill pipe fibers to go from compression to tension to compression. This high cyclic stress bending causesmetal fatigue, which is cumulative and eventually leads to drill pipe failure ( Figure 1 ).Figure 1Buckling tendencies are greatest in the pipe body between the end upsets, which is the thinnest and most flexiblepart of the drill string. The abrupt change in cross-sectional area between the pipe body and the upsets sets up stressconcentrations just above and below the much stiffer tool joint.Thus, a fundamental objective in BHA design and drill collar weight selection is to "eliminate the potential for axial andtorsional b

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