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Two-switch tri-state buck-boost power factor correction (PFC) converter operating in pseudo-continuous conduction mode is proposed and analysed in this study. Different from tri-state boost PFC converter, the proposed two-switch tri-state buckboost PFC converter does not need additional power switch to provide the additional degree of control freedom. Therefore it does not increase the complexity and affect the power conversion efficiency of the PFC converter as in the case of tri-state boost PFC converter. For the proposed two-switch tri-state buck-boost PFC converter, the current control loop and voltage control loop are decoupled, a simple voltage control loop with faster dynamic response can be designed to realise output voltage regulation and the additional degree of control freedom introduced by inductor current freewheeling stage helps to achieve unity power factor control. The operation principle, the input current, the inductor current ripple, the switch component stress and the smallsignal characteristics of the proposed two-switch tri-state buck-boost PFC converter are analysed. The simulation and experimental results show that the proposed two-switch tri-state buck-boost PFC converter benefits with fast dynamic response, high-efficiency and low-inductor current ripple over single-switch buck-boost PFC converter or two-switch buckboost PFC converter operating in continuous conduction mode and discontinuous conduction mode.

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Resonance drilling is a new technology, still at the laboratory stage. It has great potential to state mechanical vibration excited by harmonic vibration in rocks and an apparatus was built to achieve other parameters on the rate of penetration (ROP) in resonance drilling was analyzed. The results show that the rock drillability decreased with an increase in excitation frequency. When drilling with a large size drill bit, the ROP increased with excitation frequency. The ROP reached a maximum value at the resonant frequency of the rock. The ROP of the bit increased linearly with rotary speed when no vibration was applied on the rock and increased approximately exponentially when harmonic vibration was applied. In addition, the resonant frequency of the rock was changing during the process of rock fragmentation, so in order to achieve the desired resonance of the rock, it is necessary to determine an appropriate harmonic vibration excitation frequency.

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Effective cuttings transport is one of the most major concerns in designing hydraulic parameters during drilling complex structure wells. It is important and essential to study critical parameters how to influence cuttings bed development, especially the drillpipe rotation effects on cuttings transport behavior. In this study, cuttings transport simulations were carried out for inclinations from 45°to 90°, for pipe rotation speeds from 80 to 240 rpm, and for flow rates from 30 to 50 L/s. Annular cuttings volume, pressure drop, and cuttings distribution are recorded simultaneously within the simulation proceeding. The CFD simulations show that pipe rotation makes cuttings presented in asymmetric distribution along the wellbore and significantly improves the drag effects on drilling fluid in the tangential direction. At a low or medium flow rate, pipe rotation has a significant impact on annular cuttings volume and pressure drop. However, there is no additional contribution of pipe rotation after reaching critical speed at high flow rates. Buckingham-p theorem combined with least square method was applied to establish the empirical correlations for estimating cuttings concentration and annular pressure drop.

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