Rotary Subs Information

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Rotary subs, also known as rotary substitutes, are short sections of threaded pipe certified by the American Petroleum Institute (API). They are used for crossover connections, tool handling, lifting, or other specialty applications related to the making and breaking of the drill string. As integral components of a drill string, rotary subs mate with downhole tools, drill pipe, and drill collars.

Rotary subs can be considered disposable parts and are often used for excessive handling and make-and-break cycles. Diverting wear action to the rotary sub preserves the more expensive rotary kelly, top drive, or drill string threads. (To this end, rotary subs are sometimes referred to as "kelly savers.")

Specifications

Rotary subs as specified by the type, the tool joints, and the nominal dimension.

Types

Lift subs are used to handle bottom hole assembly (BHA) components such as drilling motors, jars, and drill collars. They incorporate an elevator groove that facilitates lifting and handling between the catwalk and the rig floor.

Crossover subs are threaded adapters that allow connections between tubular elements of varying sizes and thread types. They may be specified by the size and tool joint types.

Tool Joint

Each end of the rotary sub incorporates a tool joint. The joint either consists of a male threaded connection referred to as the "pin" or a female threaded connection which is referred to as the "box". Rotary subs can be supplied as a box-box, pin-pin, or box-pin configuration. They may include standard API threads or proprietary threads. Each type of threaded connection is specified by threads per inch (TPI) and it's taper. Common API tool joints include regular (REG), full-hole (FH), and internal-flush (IF).

Size

Threaded connections are fabricated to nominal dimensions and are typically representative of the outside diameter of the drill pipe they mate with. The most common nominal sizes range from 2 3/8" to 6 5/8".

Standards

API SPEC 7-1 – Specifies requirements for rotary drill stem elements including kelly valves, square and hexagonal kellys, drill stem subs, drill collars, and drilling and coring bits.