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We present measurements of the azimuthal orientation θ(t) and thermal amplitude δ(t) of the large-scale circulation (LSC) of turbulent rotating convection within an unprecedented large Rossby number range 1≤Ro≤314. Results of θ(t) reveal persistent rotation of the LSC flow in the retrograde direction over the entire Ro range. The rotation speed ratio remains a constant of 0.13 ± 0.01 for 10≤Ro≤70, but starts to increase with increasing Ro when Ro>70. We identify the mechanism through which the mean retrograde rotation speed can be enhanced by stochastic cessations in the presence of weak Coriolis force, and show that a low-dimensional, stochastic model provides predictions of the observed large-scale flow dynamics and interprets its retrograde rotation.

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A brand new impact hammer–Axial & Torsional impact hammer (producing high-frequency axial and torsional impact forces simultaneously)–has been developed to improve the penetration rates and eliminate stick-slip vibrations in deep and ultra-deep wells. This paper presents its rock-breaking mechanism and evaluates its field application effects. Based on the finite element method, a numerical model was first proposed to investigate the rock-breaking processes of a single PDC tooth under axial, torsional and axial & torsional impact forces respectively. The simulation results show that the axial & torsional impact drilling method can break rock with least mechanical specific energy and increase the rate of penetration (ROP) significantly. The mechanism is that the axial component of the compound impact force can increase the penetration depth of PDC teeth into rock and enlarge the rock-breaking volume, and that the torsional component facilitates to reduce stick-slip vibrations and prohibit premature failures of bits. An Axial & Torsional impact hammer was then developed according to the principle of modulating hydraulic pulses. The hammer has been used in 20 deep and ultra-deep wells in western China, achieving 105.1%~163.4% higher ROP compared with the average ROP in the same block. Besides, the drilling process became more stable and the lives of PDC bits were greatly increased. This paper presents a comprehensive comparison of the penetration rate, torque fluctuation, bit wearing degrees and cuttings shape between this impact tool and other tools used in offset wells. Finally, the structure of the new hammer is simple and reliable, and it is also convenient to maintain.

Axial excitation tools (AETs) have the ability to improve slide-drilling efficiency by reducing the friction between the drillstring and the wellbore wall. However, drag-reduction effects are not always satisfactory, and excessive vibration may cause failures of downhole tools in some cases. Thus, a mathematical model was proposed to simulate the vibration responses of a drillstring. In the model, velocity-dependent friction is adopted to calculate the friction-reduction effect. The effect of drillstring joints on the weight on bit (WOB) was first investigated. The simulation results indicate that the joints intensify the stick-slip motion of the drillstring system. The effect of the location of an AET was then examined. The results show that it is better to place an AET near the drill bit rather than near the rear of a build section. Because the frictional drag acting on the lower portion of the drillstring dominates the axial stick-slip motion of a drill bit. Finally, the resonance responses were examined in terms of the drillstring system acceleration. The results show that resonance moderately increases the accelerations of a long horizontal drillstring system in a heavy-damping environment but that the growth of the exciting force can profoundly increase the accelerations.