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To study the working mechanism and failure causes of semi-sealed ram blowout preventer, the full-size simulation model of semi-sealed ram is established in this paper. The failure analysis and strength evaluation of semi-sealing ram are carried out. The simulation of the blowout preventer ram in the process of well sealing under different well pressure impacts is studied, and the stress and strain distribution law is found out, and the causes of rubber core seal failure are analyzed. The analysis results show that the groove root of the ram body just enters the yield deformation when the oil cylinder pressure exceeds a certain value, and the maximum von Mises stress occurs at the groove root of the ram body, and the stress concentration and fluid impact pressure lead to the premature fracture failure of the ram. It is also found that the stress concentration in the contact area between the rubber core and the upper end of the ram is the most obvious, indicating that the upper part of the rubber core is severely deformed due to extrusion because of well pressure, which will cause fatigue damage to the rubber core over time. Finally, the extrusion behavior of the contact part between the ram and the drill pipe is highly consistent with the shape of the high-stress zone in the simulation analysis, which proves that the simulation result is reliable. [ABSTRACT FROM AUTHOR]

• The optimum constitutive relation data of rubber core were obtained. • Obtain real calculation results through the simulation calculation of rubber core. • The secondary optimization design of the rubber core structure is studied. • The superiority of the improved structure is verified by the test and application. In order to reduce the failure of the spherical blowout preventer, the basic calculation formula of the sealing geometric parameters of the spherical blowout preventer are deduced combined with the existing structure and working principle of the spherical blowout preventer in this paper. Based on the test data of four constitutive relationships of rubber core material, the constitutive relationship of rubber core material is obtained by optimizing the four kinds of test data. And this paper studies the deformation law, stress distribution and sealing performance of the rubber core by using finite element software, and finds out the internal of failure causes of the spherical rubber core. The calculation results show that the stress concentration is obvious at the bottom of the rubber core, which is one of the main reasons for the falling off of the rubber rings. The stress values of the neck and back of the support rib are high, which is easy to cause the support rib to bend and break along the neck and back. The indoor test shows that the designed rubber core structure meets the pressure test requirements, and the appearance of the annular rubber core is intact after used in the field. It shows that the optimized structure is helpful to improve the service life of the spherical rubber core. [ABSTRACT FROM AUTHOR]

• The fatigue optimization of the rubber core should meet its sealing requirements. • Yeoh model fits well with the uniaxial compression test data of the rubber. • Taguchi method combined with numerical simulation is used to optimize rubber cores. • The applicable working conditions are selected to reduce the risk of blowout. • The performance of the optimized rubber cores is evaluated in field applications. In the process of underbalanced pressure drilling, the drill pipe body and its joint repeatedly pass though the sealing rubber core of the rotating blowout preventer (RBOP), making the rubber core susceptible to alternating cyclic load, and prone to fatigue failure. Under the premise of effective sealing realized by the rubber core, to increase its service life, first the Yeoh constitutive model and constitutive parameters describing the material of rubber core were determined by uniaxial tensile and compression tests, and the finite element governing equation of the nonlinear large deformation of the rubber core was obtained according to the principle of virtual work; then a three-dimensional finite element model of the rubber core was established by ABAQUS software to simulate the dynamic sealing process, contact pressure and Mises stress distribution and variation on the sealing surface were analyzed, and the accuracy of the finite element analysis results was verified by comparing the existing test results; subsequently, the influence of structural parameters such as inner angle, outer angle and inner diameter on the sealing surface stress was studied, and these parameters were optimized through Taguchi experiment. The optimal combination of structural parameters was obtained as inner angle 29°, outer angle 69° and inner diameter 84 mm. The maximum Mises stress was reduced by 2.48 MPa and the Mises stress amplitude by 42% after optimization. Finally, the suitable conditions of the optimized rubber core were selected based on the applicability analysis to conduct the field application and result shows that the rubber core has good sealing performance and significantly increased fatigue life after optimization. [ABSTRACT FROM AUTHOR]

• Rotary blowout preventer is the key equipment of underbalanced drilling. • The rubber core is prone to failure when subjected to alternating cycle load. • Simulate the rubber core sealing process based on finite element method. • Optimize the structure of rubber core and safe lower speed of drill pipe. • The optimized rubber core was verified in laboratory and field experiments. In the process of under-balanced drilling, the rubber core of rotating blowout preventer (RBOP) subject to the alternating cycle load was prone to fatigue failure. Firstly, the sealing principle of RBOP was analyzed and the stress of rubber core in dynamic sealing process was obtained. Then, the Yeoh constitutive model in the deformation process was established by the tensile test of the rubber core material. Then we used ABAQUS software to establish the finite element model of the dynamic sealing of the rubber core, and the simulation of the sealing process of the rubber core during the drilling and tripping operations and the optimization analysis of the factors affecting the fatigue life of the sealing surface of the rubber core were carried out. In this paper, the structure of the rubber core was optimized, and the optimal outer angle of the rubber core was 155°, the inner angle was 153°, and the base angle was 125°. Then the inner diameter size of rubber core that was matched with different outer diameter drill pipes was optimized and the best combination of rubber core was obtained. The paper also optimized the lowering and lifting speed of drill pipe under different wellhead pressures. The higher the well pressure, the more likely the rubber core fatigue failure would be. The safe lowering and lifting speed of drill pipe under different pressures was obtained. At last, according to the Chinese standard, indoor seal test and field application test were carried out in the Well Control Equipment Quality Supervision and Inspection Center of China national petroleum industry to confirm the reliability of finite element optimization, which was of great practical significance to improve the service life of rubber core and oilfield operation safety. [ABSTRACT FROM AUTHOR]