The kelly pipe is located at the upper end of the drill string, and its main role is to transfer the torque and withstand the total amount of the drill string. The end face of the drive part of the kelly pipe is divided into square and hexagon, the more square is used in oil drilling, and the water eye is a regular hexagon, because the wall thickness is about three times that of the drill pipe, and is made of high-strength alloy steel, so it has high tensile strength and torsional strength.

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The drill pipe is the hub connecting the surface and the underground. In rotary drilling, the drill string is used to transfer the energy needed to break rock, apply weight on bit to the bottom of the well, and transport drilling fluid in the well. When drilling with downhole power drilling tools, it withstands the reverse torque of downhole power drilling tools and delivers liquid energy to downhole power drilling tools at the same time.

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(1) Apply weight on bit; (2) Ensure the necessary strength under compression conditions; (3) reduce the vibration, swing, and beat of the drill to make the drill work smoothly; (4) Control well inclination.

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The drill collar is located lower on the drill string and is the main component of the BHA. Its main characteristics are large wall thickness (generally 38~53mm, equivalent to 4~6 times the thickness of the drill pipe wall), with large gravity and stiffness. In order to facilitate the trip work, the hanger groove and slip groove can be machined on the outer surface of the inside thread of the drill collar.

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Drill pipe is divided into three categories: kelly pipe, drill pipe and weighted drill pipe. The connection sequence is kelly pipe (1 piece)+ drill pipe (n pieces, determined by the well depth)+ weighted drill pipe (n pieces, determined by the drill assembly design).

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