Polycrystalline diamond compact bit has been used in more than 90% drilling length all over the world. Tooth wear is the main factor reducing the service life and performance of the polycrystalline diamond compact bit. In this article, the mechanical properties of polycrystalline diamond compact bit under wearing conditions are studied. The geometric model of the worn and unworn cutters is built. A numerical method to address the cutting parameters of cutters is first proposed. Then the effects of wearing degrees and the penetration per revolution of polycrystalline diamond compact bit on cutting parameters, forces, and wear conditions are discussed. The result shows that the numerical method can be utilized to obtain the force conditions of cutters, predict the wear trends of polycrystalline diamond compact bit, and optimize the cutter layout design. The effect of penetration per revolution on the mechanical properties of polycrystalline diamond compact bit and cutters is very limited. Under a constant penetration per revolution, the cutting section area almost keeps a constant with the increase of wearing degree; the direction of the transverse force keeps unchanged. The force conditions are closely related to cutting arc length and the wear degree.

Highlights1. A new comprehensive six degrees-of-freedom lump mass model of drill-string suitable for horizontal wells was proposed.2. The models aims to investigate instabilities caused by cutting rock formations and frictional effects within a Bottom Whole Assemble (BHA) and a drill-string. 3.We have observed all types nonlinear effects including stick-slip, bit bounce and whirling.

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Polycrystalline diamond compact bits have been widely used in the Oil and Gas drilling industry, despite the fact that they may introduce undesired vibration into the drilling process, for example, stick-slip and bit bounce, which accelerate the failure rate and lead to higher drilling costs. First, we develop an innovative ridge-ladder-shaped polycrystalline diamond compact cutter, which has ridge-shaped cutting faces and multiple cutting edges with stepped distribution, in the hope of reducing vibration and improving drilling speed. Then, the scrape tests of ridge-ladder-shaped and general polycrystalline diamond compact cutters are carried out in a laboratory, indicating that the cutting, lateral, and longitudinal forces on ridge-ladder-shaped polycrystalline diamond compact cutters are smaller and with minor fluctuations. Due to different rock-breaking mechanisms, ridge-ladder-shaped polycrystalline diamond compact cutters have higher cutting efficiency compared to general polycrystalline diamond compact cutters, which is also verified experimentally. Finally, the drilling characteristics of a new polycrystalline diamond compact bit fitted with some ridge-ladder-shaped polycrystalline diamond compact cutters are compared to those of a general polycrystalline diamond compact bit by means of finite element simulation. The results show that introducing ridge-ladder-shaped polycrystalline diamond compact cutters can not only reduce the stick-slip vibration, bit bounce, and backward rotation of drill bits effectively, but also improve their rate of penetration.

The hydraulic structure of conventional geophysical drill bit is designed for the general stratum. When conventional geophysical drill bit pierces into a limestone stratum, the shape of cuttings is large because of the high brittleness of limestone. The cuttings are ground repeatedly; this phenomenon can reduce drilling efficiency and increase drilling costs. According to the characteristics of limestone cuttings, the numerical simulation method is used to research downhole flow field characteristics of conventional geophysical drill bit. First, the influence of key hydraulic structure parameters on cuttings removal performance is found. Then, the hydraulic structure is optimized. The flow field characteristics of the hydraulic structure of the geophysical drill bit before and after optimization in the flow path is analyzed, at the bottom of the bit and the annulus area of the shaft lining. The optimized downhole crossflow area increased from 50% to 98%. No vortex was observed at the exit of the flow path and cuttings groove. The downhole pressure gradient increased from 0.12 Mpa to 0.15 Mpa. The cutting removal space in the annulus area of the shaft lining is fully utilized. Field tests show that the cutting removal and drilling performance of optimized geophysical drill bit has improved and the drilling speed increases by 20.6%.

Purpose The purpose of this paper is to propose a buoyancy-gravity adjustment device and a fuzzy intelligent controller for the depth control of a storage tank in-service inspection robot. Design/methodology/approach The structure of the robot is first designed based on the construction of the bottom of a crude oil tank and explosion-proof requirements. The buoyancy-gravity adjustment system is used to control the vertical movement of the robot. The motion analysis of the robot indicates that the diving or rising process is influenced by hydrodynamic force and umbilical cord tension. Considering the nonlinear model in-depth control, a fuzzy intelligent controller is proposed to address the depth control problem. The primary fuzzy controller is used to compensate for initial error with fast response. The secondary fuzzy controller is activated by an intelligent switch to eliminate the steady error. Findings The proposed fuzzy controller can better solve the complicated hydrodynamic problem of the coupling of umbilical cord and the robot during depth control by classifying the error values of depth, velocity and acceleration. Originality/value The buoyancy-gravity adjustment device and the depth control system of the robot can move through the heating coils by safe and accurate diving or rising.

Purpose This paper aims to introduce a new acoustic positioning method to solve the problem of space positioning for online inspection robots within the storage tank. Design/methodology/approach The proposed positioning system comprises two acoustic signal emitters and two receivers. Emitters are brought by the robot into the storage tank. Receivers are mounted on the external edge of the storage tank floor. The spatial coordinate values and motion directions of the robot in the storage tank are calculated by using the proposed acoustic positioning algorithm. Findings The experiment results and positioning error analysis indicate that the method can obtain the data of robotic space coordinates and motion orientation, while the positioning error of the method can be less than 20 cm. The accuracy reaches the positioning technology level of other tank online inspection robots. Originality/value This method not only expands the positioning of the inspection robots from 2D plane to 3D space but also significantly reduces the number of positioning sensors carried by a robot and improves the safety of a robot in the tank.

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