In the oil and gas industry, the tubing string is used to transport oil or gas and operates under extreme operating conditions. Ensuring the reliability of threaded joints during operation is an urgent task.
The purpose of this work is to find out the causes of the destruction of the threaded joint of the pipe and coupling in the course of operation at the well.
For the study, separate fragments of the pipe and the coupling were provided. During the work, a complex of studies was carried out, including: 1) metallographic analysis; 2) evaluation of the contamination of the pipe metal with non-metallic inclusions; 3) mechanical test of the metal of the pipe and the coupling by tension; 4) hardness measurement of threads of pipe and coupling; 5) study of geometric parameters of thread and joint. The work was performed in the research laboratory of mechanical testing and metallographic analysis of materials at the Department of Materials Science and Metal Technology of Tomsk Polytechnic University.
The analysis of the destroyed threaded joint of the pipe and the coupling of grade N80 was carried out. In the course of the research it was found that there were neither significant deviations in the properties of the metal of the pipe and the coupling, nor other specific defects that would reduce the structural strength and provoke the accelerated destruction of the threaded joint. When analysing the relative location of the threads of the coupling and pipe, a discrepancy between GOST R 53366-2009 and the length of screwing was revealed. Macro analysis of the threads on the pipe and the coupling showed the presence of seizure and scoring areas. This led to a skew and wedging of the not tightened properly threaded joint; before the operation, there was no leak tightness in the joint. The combination of high pressure and a small flow area of the gap provided a high speed of abrasive flow of the driving fluid and intensive wear of the threaded surfaces of the pipe and the coupling at the point of flow. The appearance of through washouts led to a drop in pressure in the tubing string.
It was found that the failure was caused by the mechanical damage to the threaded surface during screwing in the absence or lack of lubricant.
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