Research on fatigue life of drill string based on reliability theory

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.