基于可靠性理论的钻柱疲劳寿命预测

Research on fatigue life of drill string based on reliability theory

钻井过程中极易发生钻柱疲劳失效事故,不仅影响钻进速度,而且大大降低了钻井效益,因此准确预测钻柱的疲劳寿命,对于提高钻柱的安全可靠性、减少钻井事故的发生具有重要意义。根据钻柱疲劳裂纹的扩展与钻柱的应力状态有关,首先对钻柱进行了应力状态分析,并进行了等效应力合成。在应力分析的基础上,结合可靠性理论,确定了影响钻柱疲劳裂纹寿命的随机性参数:钻压、转盘转速以及疲劳裂纹初始长度。结合Form an模型建立了钻柱I、III复合型疲劳裂纹扩展速率的计算模型。算例分析表明,该模型能够计算相应可靠度下钻柱疲劳裂纹的循环寿命,且计算结果与现场钻柱的使用寿命比较接近,能够满足现场钻井工程的精度要求。该模型具有一定的实用性,为钻柱疲劳寿命的准确预测提供了理论依据。

Drill string fatigue failure often takes place during the course of drilling, which has not only slowed down the drilling speed but also decreased the drilling benefit, so accurate predicting of fatigue life of drill string is significant to increase reliability of drill string and decrease drilling accident. Since fatigue crack growth of drill string is related to stress of drill string, this paper at first analyzes stress of drill string, and then integrates it with equivalent stress. Based on stress analysis and reliability theory, random parameters that exert effect on fatigue crack life are determined, which consist of drill pressure, rotation speed, and length of fatigue crack. According to Forman model, the model used for calculating growth rate of Ⅰ -Ⅲ mixed mode fatigue crack extension crack is established. Case study analysis shows that the model can be used to calculate fatigue life of drill string under the condition of relevant fiduciary level. The result is the same as the drill string life on site and meets the accuracy requirement of drilling. It proves that the model renders great practicality and can be a theoretical reference to accurate prediction of fatigue life of drill string.