The pull out and well drop accident caused by serious thread galling damage of low alloy steel tubing connections were investigated in this paper. Representative samples were selected from five failure accidents, and the test and verification program were designed. Based on the experiment and analysis it is considered that the material used for tubing and the quality of thread processing meet the requirements of the standard. The field end of tubing connection pull out may be due to the serious thread galling damage, which reduced the joint strength. Using the torque-position control method to make-up the thread connections in oil field may ensure the connection quality and the connection strength, meanwhile, and hence to avoid the occurrence of tubing pulling out effectively.
Application of the concept of multifractal to control the quality of low-alloy steel