Yatsyniak I. Prediction of the drill string elements life time with consideration of the fatigue failure similarity criterion

The dissertation research is oriented at prediction of the drill string elements life time with consideration of the fatigue failure similarity criterion based on the improved evaluation methods and techniques of the stress-strain state and fatigue strength. As a result of the research, the analytical evaluation method of stress-strain stress of the drill string elements on the well curves sections was improved; it makes it possible to determine the reaction spot of the wellbore wall with drill string elements and stress distribution along their length on the wellbore curved section. The method of normal stress redistribution in the pipe thread joints of the drill string elements was further developed using a thread pitch deviation. The device for stress measurement in the drill string was improved, its characteristic features is based on application of the strain-measuring operating principle of the strain transformation unit and electronic unit for their recording. The method of the fatigue failure similarity criterion application was improved; it used for the pipe thread joints of the drill string elements based on their finite elements analysis. The dependency of overloading ratio value Qol and the fatigue crack velocity for 40ХН steel grade was determined. The method was developed to evaluate the fatigue life of the drill string elements, considering their general and local stress-strain state.