Drill string

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A drill string on an oil rig is a column, or string, of drill pipe that transmits drilling fluid (via the mud pumps) and rotational power (via the kelly drive or top drive) to the drill bit. The term is loosely applied as the assembled collection of the drill pipe, drill collars, tools and drill bit. The drill string is hollow so that Drilling fluid can be pumped down through it and circulated back up the annulus (void between the drill string and the formation).

1 Drill string components
2 Running a drill string
3 Stuck drill string

[edit] Drill string components

The drill string is typically made up of 4 sections:

Bottom hole assembly (BHA)
Transition pipe (Often HWDP - Heavy Weight Drill Pipe)
Drill Pipe
Drill Stem subs

Each section is made up of several components and joined together using special threaded connections known as tool joints.

[edit] Bottom hole assembly (BHA)

The BHA is made up of a drill bit which is used to break-up the rock formations, drill collars which are heavy, thick-walled tubulars used to apply weight to the drill bit, and stabilizers which keep the drilling assembly centered in the hole. The BHA may also contain other components such as a downhole motor, Rotary Steerable System, measurement while drilling (MWD), and logging while drilling (LWD) tools.

[edit] Transition pipe

Heavyweight drill pipe (HWDP) is used to make the transition between the drill collars and drill pipe. The function of the HWDP is to provide a flexible transition between the drill collars and the drill pipe. This helps to reduce the number of fatigue failures seen directly above the BHA. A secondary use of HWDP is to add additional weight to the drill bit.

[edit] Drill pipe

Drill pipe makes up the majority of a drill string. A drill string is typically about 15,000 feet in length for an oil or gas well vertically drilled onshore in the United States and may extend to over 30,000 feet for an offshore deviated well.

[edit] Drill Stem Subs

Drill stem subs are used to connect drill string elements.

[edit] Running a drill string

Most components in a drill string are manufactured in 31 foot lengths (range 2) although they can also be manufactured in 45 foot lengths (range 3). Each 31 foot component is referred to as a joint. Typically 2 or 3 joints are joined together to make a stand.

Pulling the drill string out of or running the drill string into the hole is referred to as tripping. Drill pipe, HWDP and collars are typically tripped in stands to save time.

[edit] Stuck drill string

A stuck drill string can be caused by many situations.

Packing-off due to cuttings settling back into the wellbore when circulation is stopped.
Differentially when the formation pressure is too low, the wellbore pressure is too high or both, essentially pushing the pipe onto the wall of the wellbore.
Keyhole sticking occurs mechanically as a result of pulling up into doglegs when tripping.
Adhesion due to not moving it for a significant amount of time.