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应力集中是钻具疲劳破坏的一个关键因素 ,定量计算钻具在井下所承受的应力水平及其分布规律显得尤为重要。建立了带扶正器下部钻具组合在垂直井和斜直井眼中的有限元力学模型 ,用编制的软件求解并得到了下部钻具组合的形变和弯曲应力。揭示了下部钻具组合失效的力学机理。下部钻柱组合在长跨距内变形挠度最大。钻柱变形后 ,危险点不在挠度最大处 ,而在钻柱变形转角大的位置。在扶正器附近的转角大 ,因而弯矩也大。扶正器附近是应力集中区 ,而远离扶正器的两跨之间应力水平相对较低。从钻具的结构上看 ,钻具的连接螺纹正处在这些最大应力集中区 ,这是连接螺纹疲劳失效的根本原因 ,而扶正器或配合接头的失效多发生在连接螺纹部位。
Stress concentration is a key factor in the fatigue damage of drilling tools. It is very important to quantitatively calculate the stress level and distribution of the drilling tools under the well. The finite element mechanics model of vertical well and vertical wellbore with lower centralizer assembly was established, and the deformation and bending stress of lower underbore assembly were obtained by software. The mechanical mechanism of failure of the lower drilling assembly is revealed. The lower part of the drill string has the greatest deformation deflection in the long span. After the deformation of the drill string, the dangerous point is not at the maximum deflection, but in the drill string deformation corner large position. The corner near the centralizer is large, so the bending moment is also large. Near the centralizer is the stress concentration zone, while the stress level between the two spans away from the centralizer is relatively low. From the point of view of the structure of the drilling tool, the connecting thread of the drilling tool is at these maximum stress concentration areas, which is the root cause of fatigue failure of the connecting thread, and the failure of the centralizer or the mating joint occurs in the connecting thread portion.