Rotary system is responsible for rotating drill string. Drilling rig may have either TDS (Top drive system) or Kelly-swivel to transmit rotation to drill string. Rotary system components are TDS or kelly and drill string. The parts of drill string are drill pipe, heavy weight drill pipe, stabilizer,drill collar, x/o, bit sub and bit. 

TDS Vs Kelly Swivel

Top drive rotate drill string from top  where as kelly system rotation is provided at rig floor with help of rotary table.Top drive capable to drill with one stand of drill pipe and kelly can drill with one drill pipe. Therefore kelly system is time consuming as compared to TDS. Most of new rig coming with Top drive but still kelly system exist in drilling industry.

Drill string components

Drill Pipe 

Drill pipe are hollow steel pipe that make major part of drill string . It have pipe  body,threaded pin and box tool joint to make up drill pipes together. A drill pipe length is normally 30 ft. 

Heavy weight drill pipe

As the name indicates heavy weight drill pipe are heavier than same size of drill pipe. Heavy weight drill pipes are  part of BHA and are located between drill collar and drill pipe to work as transition zone.  

Drill collar 

Drill collars are most heaviest part of drill string and are also part of  bottom hole assembly. Drill collar have thick steel wall and less inside diameter  as compared to heavy weight drill pipe and drill pipe. Drill collar provide weight to bit  for drilling action. 

Stabilizer 

stabilizer provide weight and mainly control hole deviation. There are  two main types of stabilizers. 

Near Bit Stabilizer 

It is used just above bit, it have float valve inside it and have  box x box connection on both ends. 

String stabilizer

It is used between two drill collar and is located 30 or 60 ft  away from bit. it have pin x box connection. 

Cross over 

are drill string components used to connect two tubular of different threads. Cross over  can be of pin x box, pin x pin or box x  box connection. 

Bit

Drill bit is bottom most part of drill string, it drill formation or rock when apply rotation and weight on bit with circulation. 

Bits are classified as 

Dr