A new hydraulic oscillator was designed, which was able to adjust pressure fluctuations through two sets of dynamic and fixed valves. Hydraulic oscillators can meet the frequency and axial force requirements of drilling at lower drilling fluid flow than general hydraulic oscillators. Oscillator structure was described in detail and over-flow area between the two sets of dynamic and fixed valves was calculated. Based on drilling fluid flow difference, the influence of different fluid flows on oscillator pressure drops was analyzed, its influence law was determined by the finite element software Fluent, and its effect was verified by numerical simulations. The related research is of great significance in the selection of dynamic and fixed valves and provides the theoretical basis for the optimization of the structural parameters of double-valve hydraulic oscillators.

In this study, a mechanical model suitable for deepwater test string was proposed. An analysis of the dynamic response of the test string under different frequencies, different water depths and different fluctuation amplitudes was carried out by using the finite element method based on the change in the internal pressure and axial force measured. The results of the analysis showed that the response parameters (maximum stress and maximum deformation) tended to be stable after one period of fluctuation in the axial force and half a period of fluctuation in the internal pressure, respectively. When a sine waveform fluctuation in the internal pressure and axial force occurred, the response parameters increased with an increase in the amplitude of the fluctuation and increased with an increase in the frequency of fluctuation, and the amplitude of variation decreased with an increase in the fluctuation period. Under fluctuation in the axial force, the response parameter decreased with an increase in the water depth. The response parameter decreased first and then increased with an increase in the water depth when the fluctuation in the internal pressure occurred with a sine waveform. The maximum deformation and stress of the test string always changed with a change in the load when the fluctuation in the internal pressure and axial force had a sine waveform, and the test string under a load with a sine waveform was prone to periodic fatigue failure. The relevant conclusions provide a basis for the analysis and prevention of fatigue failure in test strings.

A mechanical model for offshore casing cutting based on the field application of a mechanical cutting device in the South China is presented in this paper. The proposed model includes the calculation of the piston acting on the cutter and the calculation of the cutting torque and wellhead driving torque. The influence of structural parameters on cutting extension distance, cutting torque, wellhead driving torque, and the proportion of cutting torque to wellhead driving torque are analyzed. The required extension distance is related to piston displacement and cutter face angle, the cutter face angle and cutting depth (water depth) have obvious influence on the cutting torque and wellhead driving torque, and the drilling revolution affects the inertia torque and wellhead driving torque. Since the proportion of cutting torque to wellhead driving torque decreased with the increase in the cutting depth, we cannot determine whether the casing cutting is completed based on the sudden decrease in the wellhead driving torque with cutting depths greater than 800 m. The numerical simulation results of the cutting tool cutting the casing show that if the revolution speed is too high, the initial cutting melting of the casing may affect the cutting force, and it is recommended to increase the rake angle as much as possible within a certain range. The field example shows the limitation of judging casing cutting by a sudden drop in torque.

A new hydraulic oscillator was designed, which was able to adjust pressure fluctuations through two sets of dynamic and fixed valves. Hydraulic oscillators can meet the frequency and axial force requirements of drilling at lower drilling fluid flow than general hydraulic oscillators. Oscillator structure was described in detail and over-flow area between the two sets of dynamic and fixed valves was calculated. Based on drilling fluid flow difference, the influence of different fluid flows on oscillator pressure drops was analyzed, its influence law was determined by the finite element software Fluent, and its effect was verified by numerical simulations. The related research is of great significance in the selection of dynamic and fixed valves and provides the theoretical basis for the optimization of the structural parameters of double-valve hydraulic oscillators.