Drill String & Bits
From the drill pipe in tension at the top to the bit in compression at the bottom - the assembly that actually drills the hole.
The drill string is a long, carefully balanced column - pipe kept in tension at the top, collars kept in compression at the bit, and a cutter on the very end.
From the Top Down
Working from the surface down, the drill string begins with drill pipe - long, thin-walled steel tubes screwed together end to end. Drill pipe is strong in tension but not built to take compression, so it hangs from the hoisting system in tension most of the time. Below the drill pipe sits the bottom-hole assembly (BHA), the heavier, stiffer lower portion of the string that places weight on the bit and helps keep the hole on course. The BHA ends in the bit, the actual cutting tool at the bottom of the hole.
The BHA, Drill Collars, and the Neutral Point
Inside the BHA the key components are drill collars - thick-walled, heavy steel tubes - and stabilizers, which have full-gauge blades that centralize the string and help steer. Because the collars are heavy, they are used to put weight on bit (WOB): the collars push down on the bit so it drills rather than just spins. This creates a neutral point somewhere in the string - above it the pipe hangs in tension, below it the collars are in compression. Keeping the neutral point inside the collars - so the lighter drill pipe never goes into compression - protects the pipe from buckling and fatigue. This balancing act is treated here as a concept; the detailed stress analysis belongs to the Solid Mechanics course.
The bit cuts the rock, and two families dominate. Roller-cone bits have cones studded with cutting elements that roll and crush as the string rotates. Within this family, milled-tooth bits use steel teeth cut into the cone for softer rock, while TCI (tungsten-carbide-insert) bits use hard inserts for harder, more abrasive rock. The second family is the fixed-cutter PDC (polycrystalline diamond compact) bit: it has no moving parts and cuts with diamond-tipped blades that shear the rock. PDC bits drill fast and last long in many formations, which is why they dominate modern drilling, though roller-cone bits are still chosen for certain hard or interbedded rock. The industry uses the IADC bit classification as a reference code to label bits by their cutting structure and bearing type - an industry selection reference, not something this course grants.
Check your understanding
- The drill string runs from drill pipe (thin-wall, in tension) down to the bottom-hole assembly and the bit.
- Heavy drill collars in the BHA put weight on the bit; the neutral point separates the pipe in tension above from the collars in compression below.
- Bits are either roller-cone (milled-tooth or TCI insert, which crush) or fixed-cutter PDC (which shear with diamond blades); the IADC classification is an industry selection reference.