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A component of the drill string that provides weight on bit for drilling, Drill Collars, are thick-walled tubular pieces machined from solid bars of steel, usually plain carbon steel but sometimes of nonmagnetic nickel-copper alloy or other nonmagnetic premium alloys.

The steel is drilled from end to end to provide a passage to pumping drilling fluids through the collars.

The outside diameter (OD) is machined to ensure roundness and give the desired profile, slick (plain surface) or spiral (machined with helical grooves). 

Threaded connections, Box (internal) on one end and Pin (external) on the other, are cut so multiple collars can be screwed together along with other downhole tools to make a bottom hole assembly (BHA).

Drill Collars can have handling profiles machined, generally at the Box End, such as Slip & Elevator grooves ("zipped")

With the enhanced knowledge of material fatigue, and its causes, it is generally considered good practice to machine slip recesses only and always lift drill collars with the appropriate lift subs or lift caps rather than apply undue stresses on an elevator shoulder which, if damaged, can cause the drill collar to be deemed scrap and unusable

Gravity acts on the large mass of the collars to provide the downward force needed for the bits to efficiently break rock. To accurately control the amount of force applied to the bit, the driller carefully monitors the surface weight measured while the bit is just off the bottom of the wellbore. The drill string (including the drill bit), is then slowly lowered until it touches bottom. After that point, as the driller continues to lower the top of the drill string, more and more weight is applied to the bit, and correspondingly less weight is measured as hanging at the surface. If the surface measurement shows 20,000 pounds [9080 kg] less weight than with the bit off bottom, then there should be 20,000 pounds force on the bit (in a vertical hole). 

​​Rental Drill Collars

PST Energy Ltd stock and supply various sizes and connections of rental Drill Collars from a variety of specialist Original Equipment Manufacturers

Drilling professionals don't rely on luck when choosing the Drill Collars for their specific requirements and therefore selecting the right Quantity and Size for the job is just as important as any other part of the planning, well design and drill string management

Choosing wisely may save you time and money.

Here are some questions you should take into consideration when choosing your drill pipe.

What kind of well are you drilling?

HPHT well / ordinary oil / gas well / geothermal well / onshore / offshore / fixed platform / floating rig / etc

What well profile are you drilling?
Straightforward / Vertical well / Directed well / Horizontal / S-shaped wellbore
What are you drilling through?
Some formations have concentrations of sour H2S gases, which may cause sulphide stress cracking failures or corrosion 
What drilling method are you using and how deep are you drilling?
The deeper and the more complex or demanding a well is the greater the risks, which can often be reduced or mitigated with the correct drill pipe

Don’t forget ….. 
Are the operating temperatures extreme or the formations particularly abrasive?

What capacity do you have to store pipe on the rig?

Which ranges can be handled by the pipe handling/racking system on the rig and do you have all the necessary lift subs, elevators, single joint elevators, slips, tongs, etc.

Although not a varied as Drill Pipe or HWDP, the possible variations in drill collars are still due consideration: 

  • Specification (API, DS, NS, etc.)
  • Manufacturer (NOV Grant Prideco, VAM Drilling, Tenaris, DP Masters, etc),
  • Grade of Steel (AISI 4137, 4142, 4145h Mod, etc)  
  • Type (Spiral / Slick / Non Magnetic)
  • Type of Connection (API NC, Premium Double Shoulder, High Torque, etc.),
  • Additional Specifications (Elevator, Elevator & Slip, Slip Grooves)