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Tubing corrosion is becoming more and more serious due to the harsh operating environment and corrosive medium transportation. It results in frequent seal failure of plugs used for tubing lift-off in snubbing operations. This paper develops a finite element model of a plug rubber cylinder sealing a defected tubing. The sealing performance of the rubber cylinder under the defected tubing is studied. The accuracy of model is tested experimentally. The related structural parameters affecting the sealing performance of rubber cylinder are pointed out by numerical derivation. The results show that when selecting the equivalent defected tubing size, smaller depth and radius and larger axial height can be selected. The expression of sealing performance of rubber cylinder in defected tubing environment is obtained. The influence of the key parameters on the sealing performance of the rubber cylinder is the axial height, depth and radius from high to low. The research work in this paper can provide theoretical guidance for the behavior of rubber cylinder seal corroding tubing in snubbing operation. • Performed a finite element analysis of a plug rubber cylinder sealing a defected tubing. • Got the prediction formula of sealing performance of rubber cylinder in the defected tubing. • The influence weights of the axial height, depth and radius of the defected area on the sealing performance of the rubber cylinder were obtained. [ABSTRACT FROM AUTHOR]

• A method for calculating the fatigue life of threaded joint with multi-axis fatigue. • A method for optimizing threaded joint. • Simulated the fatigue life analysis of a threaded joint subjected to 140MPa ultra-high pressure. • Vibration fatigue is the key factor affecting fatigue fracture of the union joint thread. Ultra-high pressure fracturing technology has become more and more important in China's deep Shale Gas extraction. In recent years, fracturing manifold failure accidents have happened frequently and service life has shortened drastically. There are more than 94% of failure accidents that are caused by vibration fatigue of the union joint. In this paper, it was established as a 3d finite element model of the union joint based on finite element theory and fatigue theory. The author proposed a safety evaluation method based on fatigue life, and compared it with that based on strength. The results show that vibration is the key factor affecting fatigue life, and the fatigue life-based safety evaluation method is more suitable to describe the union joint safety. Therefore, set fatigue life as optimization goal and the key parameters of the union joint thread are optimized by orthogonal optimization method. The optimal results are obtained from the study range, namely that the thread turns 8, the pitch is 10.47 mm, the bearing surface angle is 24.5°, and the guide surface angle is 24.5°. In comparison to the initial union joint, the fatigue life of the optimized union joint is increased by two orders of magnitude. These studies improve the fatigue performance of the union joint and have important guiding significance for improving the safety evaluation level of the fracturing manifold system in this paper. [ABSTRACT FROM AUTHOR]

Highlights: [•] Revealed that bending moment is the cause of thread gluing at the HDD exit side. [•] A 3D drill pipe thread FEM model was established which could load combining loads. [•] Put forward some improvement measures and successfully used in HDD. [•] Proposed the beveled shoulder thread which could strongly withstand bending load. [ABSTRACT FROM AUTHOR]