Figure 3-1 shows a schematic of a standard rotary drill string used to drill deep boreholes with direct circulation. Such a drill string would be used on large drilling rigs. At the bottom of the drill string is the drill bit. The drill bit is threaded (made-up) to a bit sub. The drill bit has a male thread or threaded pin pointing up. The bit sub is a short thick wall pipe that has a female thread or threaded box on both ends. Above the bit sub are the drill collars. Each of the drill collars and most
of the remainder of the components in the drill string are designed with a threaded pin down and a threaded box up. The bit sub is used to protect the bottom threads of the bottom drill collar from the wear caused by the frequent drill bit changes that are typical for all deep drilling operations. A drill collar is a thick wall pipe that provides the weight or vertical axial force on the drill bit allowing the drill bit to be advanced as it is rotated (see Figure 1-2). Usually there are a number of drill collars in a drill string. The number of drill collars in a drill string depends on how much weight-on-bit (WOB) is required to allow the drill bit to be advanced efficiently (drill string design will be discussed in Section 3.6). The drill collar lengths are in accordance with the range designations of Table 1-1 .
Generally the drill collars in a drill string have the same thread design. Above the drill collars are the drill pipe joints. The drill pipe joint lengths are also in accordance with the range designations of Table 1-1. The threads of the drill collar connections are usually not the same as the threads of the drill pipe joint connections (tool joints). Therefore, a special crossover sub must be used to mate the drill collars to the drill pipe. The crossover sub is a short thick walled pipe with a threaded pin down (with the drill collar threads) and a threaded box up (with the drill 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 .
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 .
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.
The kelly cock sub has a threaded pin connection down and a threaded box connection up. Above the kelly cock sub is a swivel sub. The swivel sub protects the swivel and has a threaded pin connection down and a threaded pin up. Above the swivel sub is the swivel. The bottom of the swivel has a threaded box connection down. The swivel is split into two sections, a rotating section on the bottom and a non-rotating section on the top (see Figure 1-5). The non-rotating section of the swivel is held in the mast by the traveling block and hoisting system. A sealed bearing allows the bottom section of the swivel to rotate while the top section is held by the traveling block. The swivel allows the circulation fluid (drilling mud or compressed air or natural gas) to flow through the swivel to the rotating drill string.
For direct circulation, the circulation fluid flows down the inside of the drill string to the drill bit, flows through the drill bit orifices (or nozzles), entrains the rock cuttings from the drill bit, and flows up the annulus between the outside surface of the drill string and the inside surface of the borehole.
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