Elastic-plastic Fracture Mechanics Analyses of Surface Cracks in
Drill String Subjected to Combined Loading

doi:10.11885/j.issn.1674-5086.2013.12.09.01

Abstract

Abstract:

Abstract：The drill pipe near the surface stands the largest tension and torsion in the whole drillstring especially during drill
pipe sticking or bit bouncing. As an example，a 5 inch drill pipe made of 30CrMo that was near the well head of an ultradeep
straight well was analyzed here. Simplifying the crack of the drill pipe as circumferential semi-elliptical surface crack
and using the fitted Ramberg-Osgood equation as constitutional relation，we simulated elastic-plastic fracture feature of the
drill string under combined loading of axial force and torsion corresponding to its drilling behavior. The transition aspect
ratios are determined for the given combined loading and the crack shape could be determined approximately corresponding to
different crack depths. The contribution of the plastic J-integral is estimated and the effect of material nonlinearity on the crack
propagation is described. Variations of J-integral as a function of axial force and torsional moment respectively are compared，
and the dangers from tension and torque fluctuation are evaluated quantitatively. Compared with tension，fluctuation in torque
contributes much more to crack propagation. It is an important significance for increasing drill string′s service life to avoid the
drill string from severe torsion impact.

Key words: Key words：drill string, semi-elliptical surface crack, J-integral, elastic-plastic fracture, finite element

Zhao Guanghui, Wang Haohan, Shi Jian, Zhao Li. Elastic-plastic Fracture Mechanics Analyses of Surface Cracks in
Drill String Subjected to Combined Loading[J]. 西南石油大学学报(自然科学版), DOI: 10.11885/j.issn.1674-5086.2013.12.09.01.

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