Research On Mechanism And Preventive Measures Of Fatigue Fracture Of Tubing In Sucker Rod Pumping Wells

In the past a few years, the accidents of the tubing break in sucker rod pumping wells in Daqing oil field have been happening with increasing frequency. The accidents have caused enormous econimic losses. So far, the study to the tubing break has not been made in depth at home and abroad, so, in this thesis, the experimental study and the theoretical analysis are made systematically to the improper fracture failure of theJ-55 2% tubing in Daqing oil field.On the basis of fundamental fatigue concept, the observation and the analysis technique of the fatigue break face, looking into the macroscopic and the microscopic fatigue features on the tubing's break face, combining with the working load and the break position of the tubing, we conclude that the tubing break belongs to the fatigue failure.By means of the chemical composition analysis, the mechanical test and the metallurgical structure observation of the tubing produced by specialized factory and fractured in oil field, we conclude that the material quality of the tubing used in Daqing has some defects, but it accords with the API standard basically, in other words, the improper fatigue break of tubing is not mainly caused by the material quality defects.The process and the features and the main reasons of the improper fatigue fracture of tubing are found, through the macroscopic and the microscopic observation on the tubing's fractured face, and stress analysis in the root of tubing thread, that is, tightening up or loosening screw with heavy torque for many times and thread size not accurating to API standard make thread root of dangerous section (stress concentration position) high stress level. It cause heavy damage at root, which make fatigue strength lower fastly, so far as to produce congenital source of fatigue crack. The tubing with the damage in the oil well start origin of the fatigue crack quickly under cycle load, or congenital crackis extended and snaps eventually. Macroscope observation and energy spectrum analysis to corrosion on the origin and extension area of crack is made. It is concluded that corrosion can't cause fatigue crack directly, but it can accelerate crack.Finally, by means of the analysis mothed of fatigue strength and life, the actual fatigue strength at the root of thread danger section is estimated, and disscusses mechanism of origin and extension of crack deeply, and provide the measures to prevent fatigue break of the tubing.