Prediction of conditions of longterm operated drill pipes failure.
Oleg Yu. Vytyaz
Vol. 34, no. 1 (2017), s. 15-25, 
An experimental assessment of the force criterion for metal failure of longterm drill pipes operation wascarried out. Defined were the conditions under which during tripping the failure of longterm operated drillpipes is possible when they contain an external or internal circumferential transverse crack. The interrelationof critical external or internal circumferential transverse cracks in drill pipes to the drilling stringweight was established. It was shown that the internal cross cracks during drilling string tripping at depthsexceeding 3000 m are more dangerous than the external ones, whereas within the range of 1000–3000 m the external circumferential transverse cracks are more dangerous.
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