Junk Sub – contains a narrow trough which exists in the sub to collect small pieces of metal.

Shock Sub – dampens the drill bit vibration. Sacrificially prolongs the life of the drill string, bit and rig. They are not stiff when compared to drill collars. They tend to be limited to straight hole drilling. Sometimes larger drill collars may be more effective in controlling vibration.

Roller Reamer – they cut the wellbore to gauge and to keep the drill string centered. The rollers are usually impregnated with tungsten carbide inserts. They usually have 3 to 6 rollers. They are used in hard rock where bladed reamers would cause excessive torque.

Drill Collars – they provide a form of centralization; they maximize the stiffness of the BHA as they increased resistance to bending. They maximize torsional dampening and minimize axial vibrations. They are expensive to buy and maintain and can generate high torque. They are also very difficult to fish.

Spiral Drill Collars – the same function as the straight drill collars however the surface of the collar has spiral patterns machined into it. This reduces the rick of differential sticking of the BHA.

Non- Magnetic Drill Collars (NMDC) – also known as monel. They are used to reduce the magnetic interference caused by the magnetic fields caused by the BHA. It both moves and isolates the compass from the BHA’s magnetic field. Four factors to consider are;

1. Their lengths
2. Location of the compass with respect to the monel
3. The metal composition
4. Distinguishing “hot spots”. These are zones of high magnetic field strengths within the material of the collars. Such zones can disrupt the compass by as much as 4° (2° is common). In a cold collar the disruption may be only 0.25°.

They are usually made of stainless steel, however an alloy of 60:30 percent Nickel : Copper is used.

Medium and Large Round Collars – provides weight and stiffness to the drill bit. They also prevent flexure. Can be used for jarring weight.

Heavy Weight Drill pipe – serves the same purpose as medium drill collars. Prevents flexure.

Keyseat Wipers – these are run in the BHA or the drill pipe. They are used to wipe “key” areas of the wellbore. The wipers tend to be 0.125” to 0.5” larger than the collars or tool joints. Such wipers may be single or double acting allowing them to ream only on the up or down stroke or both.

Bumper Sub – these are used on floating drilling vessels to counter the heave caused by the ocean currents. They permit the extension and contraction of the drill string. They are located between the BHA and the drill string. Can be used in fishing in a manner similar to jars.

Safety joint – rarely used; but gives the ability to easily release the BHA.

(Mitchell 1995)

Bibliography

Mitchell, Bill. Advanced Oilwell Driling Engineering. Richardson, Texas: Society of Petroleum Engineers of the AIME, 1995.