Kelly drive – Drilling Equipment – Readyzone

Kelly drive


kelly - drive

Fig. 1 – Typical drill-stem assembly.

Figure 1 shows the main component of the drill string including the position of the Kelly drive within the assembly.

A Kelly is used to transmit rotation and weight to the drill bit via drill pipe and drill collars. Weight on bit and rotation are the principal factors in breaking the rock and making hole. The Kelly drive also carries the total weight of the drill string and is, therefore, the most heavily loaded item. Kellys are manufactured either from bars with an as-forced drive section or from bars with fully machined drive sections.

Kellys are manufactured from high grades of Chrome-Molybdenum by one of two processes: (1) the full length quenched and tempered: or (2) the drive section normalised and tempered and the ends quenched and tempered.

Kellys that are treated by process (1) show greater impact strength and are suitable for use on drill ships subjected to stresses from pitch and roll of the ship. Most kellys have a Brinell hardness ranging from 285 to 341.

Two different shapes of Kelly are also available: square and hexagonal. The drive section of a hexagonal Kelly is stronger than the drive section of the square Kelly when the appropriate Kelly is selected for a given casing size.

kelly – drive

Table 1 – Outside diameters of kellys ( inches )

Kellys are manufactured in two lengths: 40 ft (12.2m) and 54 ft (16.5 m). Kellys are also manufactured in various sizes, as shown in Table 1.

Rotation of the Kelly ( and, in turn, the drill string ) is derived from the rotary table by means of a Kelly drive bushing and a master bushing. The master bushing ( Figure 2 ) fits in a recess in the rotary table. It serves two purposes: (1) it provides engagement of the Kelly drive bushing with the rotary table; and (2) it provides a tapered seating for the slip which hold drill pipe in the rotary table.

The Kelly drive bushing ( Figure 3 ) engages with the master bushing by either (a) drive pins fitted on the bottom of the Kelly bushing which fit into holes bored into the master bushing or (b) a square section on the bottom of the Kelly bushing which fits into a square recess in the master bushing. As the table turns, the Kelly drive bushing turns with it to drive the Kelly. At the same time as the Kelly works down, the rollers in the Kelly bushing allow the Kelly free movement and keep it centred in the rotary bore.

 Kelly drive accessories include:

Kelly saver sub — a small sub connected to the bottom of the Kelly to protect its threads from excessive wear as successive joints of drill pipe are made up and broken out during tripping and drilling operations.

Kelly cock — a small sub installed on top of the Kelly, or below the Kelly saver sub. When used above the Kelly, it acts as a back-pressure value, protecting equipment above the Kelly ( e.g swivel and rotary hose ) from the high surging pressures coming from below. When installed below the Kelly, a Kelly cock can be used to shut off drill pipe pressure, as in kick situations. Figure 4 shows typical positions where a Kelly cock is normally installed and a section through a Kelly cock.


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Reference  : H. Rabia, Oil Well Drilling engineering-Principles and Practice

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