| Shear rams of blowout preventer(BOP)are applied to cut off operating strings to ensure safe shut well in an emergency.Coiled tubing(CT),as a kind of working strings,is widely used in the oil and gas field of unconventional and unrecoverable reserves.However,new risks and uncertainties occurred in the well control safety due to the inherent high fracture toughness and not easy fracture characteristics of coiled tubing.Aiming at the new challenges and opportunities brought by the coiled tubing technology is applied to oil and gas drilling field for well control safety,this paper studies the working mechanism and evaluated methods of shearing performance of shear rams of the coiled tubing blowout preventer(CT-BOP),providing theoretical basis for optimum design of shear rams' structure.The mechanical properties of shear rams(impact blocks)are key factors in the ability to cut the work string.In order to explore the mechanical properties of shear rams in the shearing process,according to the working mechanism of the shear ram,the shearing strings process is divided into three stages of the shear point contact,cutting into pipes and enter the annulus regions penetrating pipes based on the position of the shear point.In each stage,the stress analysis model of the blade edge of shear rams are established by the wedge stress theory.The shearing capacity of shear rams to working strings is the core index to evaluate the working performance of shear rams.In order to predict the shearing force of the shear ram,based on the shearing motion equation,comprehensive considering the structural parameters of the shear ram,the performance parameters of working strings and shearing conditions,the shearing force prediction model were established by the Treace yield criterion,the theories of slip line field and wedge mechanics.The fracture sectional deformation degree,as a vital basis for evaluating the shear performance of shear rams,which is directly related to the recycling and efficiency of the drill pipe.For making a reasonable assessment of the fracture sectional raised height-an important aspect of the fracture sectional deformation degree,drill pipe is taken as the research object,based on the matrix form of the coordinate conversion formula and the theory of slip line,combined with the basic motion law of shear ram,the evaluation model of the fracture sectional raised height of the pipe is established.Further,the impact block of the critical structure parameters of shear rams and pipes,such as V-shape angle and edge chamfer,on the fracture sectional raised height of the sheared drill pipe is analyzed.The ABAQUS finite element analysis software was used to simulate the shearing process of shear rams shear CT90 tube,obtained the stress variation characteristics of shear rams and data of shearing force,which are basically consistent with the stress analysis model of the blade edge of shear rams and shearing force prediction model respect.The orthogonal test and range analysis method were used to study the influence of key structural parameters of shear rams,such as V-angle,edge chamfer,blade angle and blade mouth thickness,on the shearing force.The experiments result of shearing CT90 tube with shear rams of CT-BOP are highly consistent with the numerical simulation results in the shear rams' damage and the fracture morphology of the CT90 tube,indicating that the numerical simulation results are reliable.Further the reliability and applicability of the blade edge's stress analysis model and the shearing force theory prediction model were verified indirectly.And the relative error between the obtained CT90 pipe fracture data and the fracture height evaluation model of drill pipes is less than 8%,indicating that the evaluation model is reliability. |
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