The drill string connects the rig to the bit. It transmits rotation and torque, delivers weight to the bit, carries drilling fluid down to the bottom of the hole, and gives the driller a way to steer, measure and, when needed, free the bottom-hole assembly. It does all of this while being stretched, twisted, bent and vibrated. Understanding what each component does makes it easier to design a string that survives.
Drill Pipe
Drill pipe makes up most of the string's length. It is relatively thin-walled seamless pipe with heavier tool joints welded to each end, carrying the threaded connections. Drill pipe is designed to be run in tension: its job is to carry torque and fluid, not to push on the bit.
Drill pipe is specified by outside diameter, nominal weight, steel grade and connection type. Higher grades carry more tension and torque; high-torque connections allow more make-up torque, which matters in extended reach wells.
Drill Collars
Drill collars are thick-walled, heavy tubulars run just above the bit. They provide the weight that drives the bit into the rock and keep the lower string stiff and in compression, so the thinner drill pipe above stays in tension.
Spiral drill collars have grooves that reduce the contact area with the borehole wall, lowering the risk of differential sticking in permeable formations. Non-magnetic collars house survey instruments in directional wells.
Heavy Weight Drill Pipe
Heavy weight drill pipe (HWDP) sits between the collars and the drill pipe. It looks like drill pipe but has a much thicker wall, longer tool joints and often a central wear pad. Its main job is to smooth the transition in stiffness between the rigid collars and the flexible pipe. Sudden changes in stiffness concentrate bending stress, and that is where fatigue cracks tend to start.
In directional and horizontal wells, HWDP is also used to provide weight on bit, because it creates less drag and contact than collars in the curved part of the hole.
Subs
Subs are short components with a single job each:
- Crossover subs connect components with different thread types or sizes.
- Bit subs connect the bit to the collar above it.
- Saver subs take the wear of repeated make-up at the top drive, protecting a more expensive connection.
- Float subs hold a float valve that stops fluid flowing back up the string.
- Lifting subs give elevators a shoulder to latch onto on flush tubulars such as collars.
Stabilizers
Stabilizers have blades that touch the borehole wall and centre the bottom-hole assembly. By choosing where stabilizers sit relative to the bit, the drilling engineer can make the assembly build, hold or drop angle. Stabilizers also reduce lateral vibration, which protects the bit and measurement tools.
Putting It Together
A simple vertical-well string, from the bottom up, might be: bit, bit sub, near-bit stabilizer, drill collars with a string stabilizer, a jar, more collars, heavy weight drill pipe, then drill pipe to surface. Directional strings add a motor or rotary steerable system and measurement tools, and change the proportions of collars and HWDP.
Need Components for a New Program?
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