Fatigue life analysis of rotary drill pipe

In this paper, the crack problem of a certain type of rotary drill pipe in use is studied, and a simplified calculation method for the fatigue life of rotary drill pipe is proposed. The bench test device built can be used for the fatigue life test of all tubular structures by analogy. The calculation method of fatigue life of rotary drill pipe is studied through field test and bench test of drill pipe. Firstly, a simplified model of the drill pipe was established with the outer rod of the rotary drill pipe as the research object, and the model was verified. Then, the difference between the hot spot stress extrapolation method recommended by the IIW standard and the DNV standard was compared, and the hot spot stress of the drill pipe was calculated by the two-point linear extrapolation method recommended by the IIW standard, and the equivalent fatigue limit of the drill pipe was obtained by the Von Mises criterion. Finally, the equivalent load was deduced according to the stress level of the actual work test, and the constant amplitude load test was carried out on the bench, and the fatigue life of the drill pipe under the constant amplitude load was obtained. The results show that both the simplified model of the drill pipe and the bench test drill pipe can well reflect the stress level of the field test. The rotary drill pipe mainly bears torsional load, and the crack position of the bench test corresponds to the actual drill pipe crack position. The fatigue life of drill pipe is calculated by applying the Von Mises criterion and Basquin's SN curve power function. The calculated fatigue life is consistent with the results of the bench test. The calculation method can be well applied to the life analysis problem of multiaxial fatigue of large structural parts.