Study On Failure Of Drilling String In Dynamic Fracture Mechanic

Drill pipes often work in such abominable condition as alternating stress and the friction collision between drill pipe and borehole a wall, which puts drill pipes in the weakest component in the whole drilling equipment and tools. Accidents that crack propagation results in perforation or part often emerge, rendering serious hazards to development of oil fields.This paper, based on study and analysis on the accidents of drill string washout, cavity in the field, under high rotation speed, puts forward the failure mechanism that collision between drill pipe joints and case hole makes the upset area of drill pipe crack produce failure of mode â…¡ crack and mode â…¢ crack of facture mechanics interpretation. At the meantime, the relativity between mode â…¡ crack and mode â…¢ crack and drill string movement is also put forward. Then the top stress wave strength of mode â…¡ crack and mode â…¢ crack is studied under drill string lateral collision and torsional loading. The study provides a new theoretical method for the drill string failure study.In this paper, according to crack propagation energy criterion and J integral project-criterion, the crack propagation characteristic is analyzed and discussed. The stress intensity factor not only describes the stress, strain and the fracture in crack top, but also provides a convincing testimony to prevent drill string crack from propagating. Additionally, through the analysis on the security of drill string components, it is shown that the requirement of toughness of drill string components changes in accordance with its using condition: if the stress is different, the range of permitted defect is different, and the requirement is also different.When drill string crack propagates in high speed under dynamic loading, it becomes much difficult to use the static fracture mechanics theory to interpret and analyze for the affect of inertial effect. This paper tries to use the dynamic fracture mechanics theory to evaluate the propagation conditions on drill pipe crack under dynamic loading, and changing range of the dynamic fracture toughness of the material is also studied in this paper.This paper briefly introduces the study method and development of current criterion fracture toughness. Referring to the Charpy impact test, the impact experiment instrument has been designed by myself. At the meantime, the experiment equipments also have been equipped relevantly. After studying the experiments of drill pipe crack propagation condition under the impact loading, this paper, through experiments, draws some conclusions which can provide important reference for the evaluation and analysis of dynamic fracture tougliness of oil drill string components, and also provides reliable theoretical references and project methods for the settlement of the hazard brought by drill pipe crack.