The three cone bit is the most widely used bit in petroleum drilling. This is because the three cone bit has the functions of impacting, crushing, and shearing rocks when rotating. The contact area between the teeth and the bottom of the well is small, with high specific pressure, low working torque, and a large total length of the working edge. Therefore, the three cone bit can be applied to various types of rocks, making it the most widely used bit.
Three cone bits can be classified into single three cone bits, double three cone bits, three cone bits, and multi-three cone bits based on the number of cones. Currently, the three cone bit is the most commonly used.
The three cone bit can be divided into five parts: the bit body, the arm, the cone, the bearing, and the water hole. In addition to these basic parts, sealed jet bits also have an oil storage compensation system.
The upper part of the bit body is threaded to connect to the drill string, and the lower part is equipped with arms. The bit body is fitted with nozzles. Three cone bits can be divided into two types: those with a body and those without a body.
The arm is connected to the cone shaft and has a journal to support the cone.
The cone is a cone-shaped structure with teeth on the outside and a rolling body raceway (or sliding friction surface) machined inside to match the journal. It can be divided into single-cone and multi-cone structures.
The bearing structure of the three cone bit includes rolling bearings and sliding bearings. Currently, the widely used structure is a large bearing sliding pair, a ball bearing, a second thrust bearing, and a small shaft sliding pair in a sealed sliding bearing structure.
The three cone bit is designed with water holes between the cones. Depending on the requirements, nozzles of different types, diameters, and quantities can be installed in the water holes.
The three cone bit relies on the impact crushing and sliding shearing actions generated by the rotation of the cone around the bit axis and the rotation of the cone around its own axis to break rocks.
When the three cone bit works at the bottom of the well, the impact crushing action of the teeth on the rock, caused by the longitudinal vibration of the bit, is the main way the three cone bit breaks rocks. When the bit rotates, the teeth impact and press into the rock at a certain speed. This breaking method is similar to the characteristics of static indentation breaking tests. Sufficient specific pressure and contact time are required for the teeth to press into the rock. Therefore, the axial load of the teeth on the rock includes both static pressure and impact load. The static part is the weight on the bit, while the impact load is generated by the longitudinal vibration of the bit during rotation, causing the bit-drill string system to compress and stretch continuously. The lower drill string transmits this periodic elastic deformation energy to the cone, which is the source of the impact pressure of the teeth when breaking rocks. Increasing the impact pressure of the bit teeth on the formation can improve the efficiency of rock breaking.
In highly plastic rocks, in addition to requiring the teeth to have an impact crushing action on the rock at the bottom of the well, it is also necessary for the teeth to have a certain sliding action on the formation to shear the rock and enhance the rock-breaking effect. This is particularly important when drilling in soft formations. Measures to make the cone produce slippage during rolling at the bottom of the well include overhang, multi-cone, and offset axes.
When the teeth penetrate the rock under axial pressure (weight on bit), the wedge-shaped surface of the teeth generates a horizontal shear force on the rock in contact with it. The rock undergoes shear failure along the shear plane, and the broken rock is removed as the teeth move. This is the shearing action of the teeth themselves on the rock.
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