In this study, the fatigue strength of polished drill pipe specimens was investigated and compared with previous test results of corroded and not-corroded pipes. The resonant fatigue test rig, which was designed and implemented by the University of Pisa, is initially presented by providing a detailed description of the set-up of the machine, the calibration of the strain gauges, the control system, and the correct identification of the vibrational node locations. A polishing rig was also designed and put into operation to remove the corrosion pits from the outer surface of almost the entire length of the drill pipe specimens. After the fatigue tests with the resonant rig, and the observation of the fatigue fracture of the specimens, a few samples were extracted from different zones (corroded and not corroded) of the failed drill pipe specimens. This allowed for investigations to be carried out using a scanning electronic microscope. The obtained results were analyzed using the Murakami model, and a discussion is presented about the effect of the corrosion pits on the fatigue strength.

In this study, a resonant bending fatigue test rig, designed and implemented by the University of Pisa, is presented, providing a detailed description of the set-up of the machine, the strain gauges calibrations, and the control system used with the main electronic devices. Several geometries of drill pipe connections and pipe samples made of different materials were tested, and all the obtained experimental fatigue results are presented in the paper and compared to previous experimental data. Fractographic images are provided to clearly show that, in two kinds of drill pipe connections, the crack initiation was found at the connection zone, whereas for another connection geometry, it was found at the pipe body. In order to interpret these latter results, a discussion about the section modulus of bending of the various sections of the drill pipes was provided, along with an FE model of a specific zone of one of these connections.

Understanding drill string dynamics is essential to improving drilling efficiency and preventing accidents. Previous studies have mainly focused on self-excited oscillations due to stick–slip and bit-bounce. However, in deepwater drilling, large weight-on-bit (WOB) fluctuations due to heave motion are a significant concern that can affect torsional dynamics. This article reports analytical, numerical, and field data investigations on torsional vibrations of a drill string in deepwater drilling. First, we analyzed the field data from a deep-sea scientific drilling vessel Chikyu. The field data showed that heave, WOB, and torque oscillated at similar frequencies. This result indicated that the drill string could have forced torsional vibrations due to heave instead of self-excited vibrations due to stick–slip. Second, we analytically and numerically investigated drill string torsional dynamics. The torsional vibration of the drill string can be described only by self-excited vibration due to stick–slip if WOB is constant. However, when the WOB fluctuates, the forced vibration of the WOB must be considered. The results showed that vibration at the heave frequency had a more significant effect on torsional vibration than the axial self-excited frequency for the same amplitude of WOB variation. In addition, large WOB fluctuations increased the risk of stick–slip and reverse rotation of the drill bit. Numerical experiments with the field data showed that the forced torsional vibration caused by heave motion was predominant in deepwater drilling. These results show the importance of capturing forced torsional vibrations and reducing WOB fluctuations.

Improving operation efficiency including avoidance of problem leading to non-productive time is a priority issue. JAMSTEC operates the scientific drillship Chikyu for scientific research. As one of primary aims of scientific drilling is to recover core samples from sediment layers under the seabed, we should focus on improving core recovery rate as improving operation efficiency.