Three-dimensional nonlinear coupling vibration of drill string in deepwater riserless drilling and its influence on wellbore pressure field

Abstract In deepwater riserless drilling operations, the vibration behavior of drill string may have a significant impact on wellbore pressure, which leads to serious risks of drilling accidents such as well leakage and well collapse in shallow risk areas. In order to solve this problem, based on Hamilton's principle, a three-dimensional nonlinear coupling dynamics model of drill string in deepwater riserless drilling is established to simulate the dynamic behavior of drill string under offshore drilling conditions, considering the influence of heave and offset motion of offshore platform, ocean current load, drillstring-borehole contact and bit-rock interaction. The Newmark-β method is used to solve the nonlinear discrete equations of the system, and the effectiveness of the model and calculation program is verified by the field test data. Meanwhile, a wellbore pressure field model under axial, lateral and torsional motion modes of drill string in deepwater riserless drilling is established. The three-dimensional nonlinear coupling vibration characteristic of drill string and its influence on wellbore pressure fluctuation are investigated. The results indicate that: excitation and natural frequency play a leading role in axial vibration response; the collision between the drill string and the borehole wall mostly occurs in the middle part of the formation section, and the lateral vibration at the bit is the most intense; and the closer to the upper platform, the more high-frequency components of torsional vibration response and the stronger the chaos. The larger the axial vibration amplitude of drill string is, the greater the influence on wellbore pressure field is; for the lateral vibration, the middle and bit position have great influence on the wellbore pressure field; for the torsional vibration, the bottom position has the greatest influence on wellbore pressure field. Compared with the axial and torsional vibration, the lateral vibration of drill string has the greatest influence on the wellbore pressure field.