Rotary Subs, short for Rotating Substitutes, consist of a large group of tools that run in the drill string. There are Subs for Lifting, Subs that house a Float Valve (used when controlling the mud flow is necessary). There are subs that connect the bottom tool in the drill string to the drill bit, and most importantly Subs that connect different parts of the drill string to each other.
Below are just a few of the different types of Rotary Subs that Houston Downhole builds.
Flow Thru Lift Subs have the same function with one additional capability. They are longer than a standard Lift Sub so a Box connection can be threaded in the top upset. By threading a Circulating Sub into this connection, you can circulate thru the Sub. A standard Lift Sub would have to removed from the string to perform this same operation.
Top Drive Saver Subs Connect the Top Drive System to the top of the Drill String. The OEM of the Top Drive Assembly determines if the Upper Connection is a Box or a Pin. It also indicates what the OD, ID and Shoulder to Shoulder length a TDS Sub will be. The Lower thread on a TDS Sub is always a Pin that matches what is on the Drill Pipe. These tools are built as short as possible to help with spacing at the top of the String. The material requirements for TDS Subs are stringent because of their location in the Drill String.
Crossover Subs are the most frequently used tool in the Drill String. They allow different thread types and sizes to be connected to each other. They make up between two sizes of Drill Pipe, two sizes of Drill Collars, between Drill Pipe and Drill Collars, between Drill Collars and specialty tools, between the last Drill Collar and the Drill Bit, etc.
Depending on the two tools being connected by a Crossover, they can have a straight OD, a Bottlenecked OD, be Box x Pin, Box x Box, Pin x Pin, etc. The standard overall length for a Straight Crossover is 36”, and 48” for a Bottleneck, unless otherwise specified.
Wear Subs, also referred to as Throw Aways, are short tools, with just enough Shoulder to Shoulder length to tong on. They are made up to a tool in the string that would be difficult and or expensive to re-work. By making up a Throw Away or Wear Sub to the more expensive tool, it takes the wear. These tools aren’t re-worked. They are ‘thrown away’ and replaced by a new Wear Sub.
Float Subs are primarily used between two joints of Drill Pipe. They have the same thread size & type the Drill Pipe with a Box up and a Pin down. There is a Float Bore inside the Box connection to house a Float Valve. Some drilling programs require a Float Sub at certain intervals along the string to control the upward flow of drilling fluids. The Float Sub has a straight OD and the standard overall length is 36”.
Bit Subs have a Box Connection on both ends. The last tool in the Drill String almost always has a Pin connection down, and the Drill Bit always has a Pin connection up. The Bit Sub is configured to be made up between those two tools. Most drilling programs require the Bit Sub to be ‘bored for a Float’. The lower Box Connection is the one that has this feature so a Float Valve can be installed. We put this feature in all Bit Subs for oilfield application, so the tool will accommodate a Float Valve if required. Straight OD Bit Subs have a standard overall length of 36”. Bit Subs can also be Bottlenecked. If so, the standard overall length is 48”.