Page 75 - Air and Gas Drilling Manual

Chapter 3: Downhole Equipment 3-3
pipe threads). The number of drill pipe joints is determined by the depth of the
borehole to be drilled. Only the drill collars can be placed in compression (to place
weight on the bit). The drill pipe joints are always kept in tension [1].
All of the threaded connections in drill strings are API threaded shoulder
connections. There are a variety of these connections and they will be discussed in
detail in Sections 3.3 and 3.4. Figure 3-2 shows a typical API threaded shoulder
connection for a drill pipe. As can be seen, the connection has matching flat
shoulders on the pin and on the box. When a pin and box are made up, the flat
surfaces of the shoulders mate against each other and seal to form a strong structure
that is also leak proof. The shouldered connection protects the thinner walled body
of the drill pipe and the threads inside the connection from damage when the drill
string (and the connection) are flexed when bent in a deviated borehole [2].

Figure 3-2: Cutaway of a made-up API shouldered connection.
At the top of the drill pipe section is the kelly cock (or saver) sub. The kelly
cock sub is another crossover sub. But this sub is used to protect the bottom
threads of the kelly. Even if the threads at the bottom the kelly are the same as the
drill pipe threads, this special crossover sub is usually used. As drilling progresses
additional elements of drill pipe are added to the top of the drill string. The kelly is
a special type of drill pipe with a square or hexagon outer surface. The rotary table
grips the outside of this pipe and provides the torque to the drill string to make it
rotate. Thus, as additional drill pipe are added to the drill string as the bit advances
in the borehole, drill pipe must be disconnected and a new pipe joint added. The
bottom threaded box of the kelly save takes the wear of these repeated connections of
drill pipe. All of the threaded components below the top threaded connection of the
kelly are right hand threads. The rotary table rotates to the right (clockwise from the
top view of the table). This rotation tightens the right hand threads below the table.
At the top of the kelly is a left-hand thread connection (threaded box). As
drilling progresses, the rotary table, in addition to providing torque to rotate the drill
string, also allows the kelly to slide through the table allowing the borehole to be
deepened. Since the torque is applied along the square or hexagon outer surfaces of
the kelly, the left hand thread at the top of the kelly is tightened by the inertial drag
of the non-rotating components above the kelly. All of the components above the
kelly are left hand thread connections. Above the kelly is a kelly cock sub
(optional). The kelly cock is a special valve that allows the sealing off of the inside
of the drill string in a blowout event during oil or natural gas drilling operations.