Subs are generally part of most drill strings and have two main functions:
To crossover connections
To extend the life of a more expensive drill stem item and/or as a disposable component
This means that subs have to be manufactured from selected bars of alloy steel, heat-treated to provide the strength and toughness required to carry the entire weight of the drill string or to withstand high torque differentials. Generally, subs exceed API specifications for drill pipe tool joint mechanical properties.
Subs are classified into four main categories:
Bit subs or crossover subs are used to connect the drill bit to the first piece of BHA equipment or to crossover connections in the drill string. Drill bits are manufactured with a pin, making make-up impossible without a bit sub.
Lift subs or handling subs are used to lift BHA components from the catwalk to the rig floor.
Top drive subs or saver subs serve as the sacrificial element between the drill string and the top drive, reducing repair and maintenance costs.
Workover subs or circulating subs are used to limit the allowable fluid-circulation rates.
Bit subs or crossover subs are manufactured from AISI 4145H-modified alloy, heat-treated to a Brinell Hardness range of 285-341 with a Charpy “V” notch minimum impact strength of 40 ft/lb at 70° F and one inch below the surface. Connections can be cold rolled after machining, if requested. All connections are phosphate coated to impede galling during initial make-up. They are available in standard lengths of 36” and 48” with other configurations upon request.
Lift subs and lift plugs are made of AISI 4145H-modified steel and manufactured to the same specifications used for drill collars. They are available in sizes from 3-1/8” to 11”.
Crossover subs come with a minimum yield strength of 110 KSI and are manufactured integral with the following connections:
COMMAND-SMFI offers a wide range drill string products and accessories with standard API or proprietary high-performance connections to meet the most demanding drilling requirements. Drill stem subs are available in any size or configuration required.