Characteristic types of operational defects formed on the inner or outer surface of drill pipes of the S-135 strength group have been described. The results of technical diagnostics when drilling wells on the territory of the Dnipro-Donetsk gas and oil region are used. In 2018-2019, the Ukrburgaz Drilling Department (owned by JSC UkrGasVydobuvannya) rejected 81 drill pipes of the S-135 strength group when drilling wells to a depth of 4,000 to 6,000 m. Statistical evaluation of operational defects detected during deep drilling (4000-6000 m) of wells has been carried out. Potentially dangerous areas have been identified: in the area of drill pipe upsetting, and along the length of the drill string; the duration of drill pipeworks has been taken into account. While undergoing the defectoscopy of drill string pipes of the studied strength group, it is recommended to pay close attention to the sections of S-135 pipes from the end of the coupling or nipple in the range from 0.45 to 0.57 m. In addition, given the depth of drilling ( Lmax ), it is necessary to pay extreme attention to the segments with the most probable defect ( Lf ) along the length of the drill string when diagnosing pipes. For S-135 pipes, taking into account the relative length ( Lf 
Lmax ) of the drill string, a segment in the range from 0.34 to 0.47, on which there is the highest probability of an operational defect. The influence features of drilling operation duration for drill pipes of the S-135 strength group are established. In particular, during the long-term deepening of drill pipes of the strength group S-135, three stages of drilling were distinguished: Stage I – running-in (from start-up to 2 thousand hours); Stage II – stable work (from 2 to 7 thousand hours); and Stage III – accelerated destruction (from 7 thousand hours and more), and it was found that during defectoscopy of the pipe pay special attention to drill pipes, the service life of which in the first stage is from 602 to 998 hours, the second stage – from 3348 to 5344 hours, and the third stage – from 8942 to 10584 hours because during these periods there is the greatest probability of an inadmissible defect
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