Drill Collar Weights, Size, Specs, Definition &Types
https://youtu.be/pZtXE-fUFD4

Drill-collar (D/C) are large diameter-small bore steel pipes that possess a great weight and great stiffness.  While designing BHA, drilling engineers place D/Cs above the  Drilling Bit as D/Cs are the predominant component of the Drilling BHA. To select the proper D/C you will have to have all information about Drill collar Weights, Size, Specs, Definition, lengths & Types.
https://www.drillingmanual.com/drill-collar/