Analysis of the Effect of Whirl on Drillstring Fatigue

Abstract

There are many kinds of vibration forms of drillstring in petroleum drilling. Whirl is one of the main vibration modes when drilling tools are drilling, and it is also the main reason of fatigue failure of the drillstring. In this paper, the causes of drillstring whirl are analyzed. The results show that the speed of rotation (RPM) is a major factor that affects the severity of the whirl. The greater the speed of rotation, the more intense the whirl. The whirl of the drillstring will produce bending stress. The severity of the whirl of the drillstring is the main factor affecting the bending stress. The increase of RPM and weight on bit (WOB) will lead to the increase of bending stress. The RPM has a great influence on the bending stress. Given the complex stress condition of drillstring, the calculation model of safety factor is established. The bending stress and safety factor are calculated under the different WOB and RPM. The relationship between dynamic bending stress and the safety factor of drilling tools is analyzed. Bending stress has great influence on the safety factor of drillstring. The variation trend of safety factor is opposite to that of bending stress caused by whirl. It provides a theoretical reference for the optimization of drilling tools in drilling engineering construction.