The rotary system is used to rotate the drillstring, and therefore the drillbit, on the bottom of the borehole. The rotary system includes all the equipment used to achieve bit rotation (Figure 9).

The swivel is positioned at the top of the drillstring. It has 3 functions:
1. Supports the weight of the drill string
2. Permits the string to rotate
3. Allows mud to be pumped while the string is rotating

The hook of the travelling block is latched into the bail of the swivel and the kelly hose is attached to the gooseneck of the swivel.

The kelly is the first section of pipe below the swivel. It is normally about 40′ long, and has an outer hexagonal cross-section. It must have this hexagonal (or sometimes square) shape to transmit rotation from the rotary table to the drillstring. The kelly has a right hand thread connection on its lower [pin] end, and a left hand thread connection on its upper [box] end. A short, inexpensive piece of pipe called a kelly saver sub is used between the kelly and the first joint of drillpipe. The kelly saver sub prevents excessive wear of the threads of the connection on the kelly, due to continuous make-up and breakout of the kelly whilst drilling. Kelly cocks are valves installed at either end of the kelly to isolate high pressures and prevent backflow from the well if an influx occurs at the bottom of the well. The rotary table is located on the drill floor and can be turned in both clockwise and anti-clockwise directions. It is controlled from the drillers console. This rotating table has a square recess and four post holes. A large cylindrical sleeve, called a master bushing, is used to protect the rotary table.

drilling rig rotary

The torque from the rotary table is transmitted to the kelly through the four pins on a device which runs along the length of the kelly, known as the kelly bushing. The kelly bushing has 4 pins, which fit into the post holes of the rotary table. When power is supplied to the rotary table torque is transmitted from the rotating table to the kelly via the kelly bushing.

Slips are used to suspend pipe in the rotary table when making or breaking a connection. Slips are made up of three tapered, hinged segments, which are wrapped around the top of the drillpipe so that it can be suspended from the rotary table when the top connection of the drillpipe is being screwed or unscrewed. The inside of the slips have a serrated surface, which grips the pipe (Figure 9).

To unscrew (or “break”) a connection, two large wrenches (or tongs) are used. A stand (3 lengths of drillpipe) of pipe is raised up into the derrick until the lowermost drillpipe appears above the rotary table. The roughnecks drop the slips into the gap between the drillpipe and master bushing in the rotary table to wedge and support the rest of the drillstring. The breakout tongs are latched onto the pipe above the connection and the make up tongs below the connection (Figure 10). With the make-up tong held in position, the driller operates the breakout tong and breaks out the connection.

tubing making and breakout

To make a connection the make-up tong is put above, and the breakout tong below the connection. This time the breakout tong is fixed, and the driller pulls on the make-up tong until the connection is tight. Although the tongs are used to break or tighten up a connection to the required torque, other means of screwing the connection together, prior to torquing up, are available:

1. For making up the kelly, the lower tool joint is fixed by a tong while the kelly is rotated by a kelly spinner. The kelly spinner is a machine which is operated by compressed air.
2. A drillpipe spinner (power tongs) may be used to make up or backoff a connection (powered by compressed air).
3. For making up some subs or special tools (e.g. MWD subs) a chain tong is often used.

Institute of Petroleum Engineering, Heriot-Watt University

The mud pits are usually a series of large steel tanks, all interconnected and fitted with agitators to maintain the solids, used to maintain the density of the drilling fluid, in suspension. Some pits are used for circulating (e.g. suction pit) and others for mixing and storing fresh mud. Most modern rigs have equipment for storing and mixing bulk additives (e.g. barite) as well as chemicals (both granular and liquid). The mixing pumps are generally high volume, low pressure centrifugal pumps.

At least 2 slush pumps are installed on the rig. At shallow depths they are usually connected in parallel to deliver high flow rates. As the well goes deeper the pumps may act in series to provide high pressure and lower flowrates.

duplex and triplex pump

Positive displacement type pumps are used (reciprocating pistons) to deliver the high volumes and high pressures required to circulate mud through the drillstring and up the annulus. There are two types of positive displacement pumps in common use:
(i)     Duplex (2 cylinders) – double acting
(ii)    Triplex (3 cylinders) – single acting

Triplex pumps are generally used in offshore rigs and duplex pumps on land rigs. Duplex pumps (Figure 7) have two cylinders and are double-acting (i.e. pump on the up-stroke and the down-stroke). Triplex pumps (Figure 8) have three cylinders and are single-acting (i.e. pump on the up-stroke only). Triplex pumps have the advantages of being lighter, give smoother discharge and have lower maintenance costs.

Institute of Petroleum Engineering, Heriot-Watt University

The circulating system is used to circulate drilling fluid down through the drill string and up the annulus, carrying the drilled cuttings from the face of the bit to surface. The main components of the circulating system are shown in Figure 6. The main functions of the drilling fluid will be discussed in a subsequent chapter – Drilling Fluids. However, the two main functions of the drilling fluid are:

1. To clean the hole of cuttings made by the bit
2. To exert a hydrostatic pressure sufficient to prevent formation fluids entering the borehole

Drilling fluid (mud) is usually a mixture of water, clay, weighting material (Barite) and chemicals. The mud is mixed and conditioned in the mud pits and then circulated downhole by large pumps (slush pumps). The mud is pumped through the standpipe, kelly hose, swivel, kelly and down the drillstring. At the bottom of the hole the mud passes through the bit and then up the annulus, carrying cuttings up to surface. On surface the mud is directed from the annulus, through the flowline (or mud return line) and before it re-enters the mudpits the drilled cuttings are removed from the drilling mud by the solids removal equipment. Once the drilled cuttings have been removed from the mud it is re-circulated down the hole. The mud is therefore in a continuous circulating system. The properties of the mud are checked continuously to ensure that the desired properties of the mud are maintained. If the properties of the mud change then chemicals will be added to the mud to bring the properties back to those that are required to fulfil the functions of the fluid. These chemicals will be added whilst circulating through the mud pits or mud with the required properties will be mixed in separate mud pits and slowly mixed in with the circulating mud.

circulating system

When the mud pumps are switched off, the mud will stop flowing through the system and the level of the mud inside the drillstring will equal the level in the annulus. The level in the annulus will be equal to the height of the mud return flowline. If the mud continues to flow from the annulus when the mud pumps are switched off then an influx from the formation is occurring and the well should be closed in with the Blowout preventer stack. If the level of fluid in the well falls below the flowline when the mud pumps are shut down losses are occurring (the mud is flowing into the formations downhole).

The discharge line from the mud pumps is connected to the standpipe – a steel pipe mounted vertically on one leg of the derrick. A flexible rubber hose (kelly hose) connects the top of the standpipe to the swivel via the gooseneck. The swivel will be discussed in the section on rotary system below.

Once the mud has been circulated round the system it will contain suspended drilled cuttings, perhaps some gas and other contaminants. These must be removed before the mud is recycled. The mud passes over a shale shaker, which is basically a vibrating screen. This will remove the larger particles, while allowing the residue (underflow) to pass into settling tanks. The finer material can be removed using other solids removal equipment. If the mud contains gas from the formation it will be passed through a degasser which separates the gas from the liquid mud. Having passed through all the mud processing equipment the mud is returned to the mud tanks for recycling.

There will be at least two pumps on the rig and these will be connected by a mud manifold. When drilling large diameter hole near surface both pumps are connected in parallel to produce high flow rates. When drilling smaller size hole only one pump is usually necessary and the other is used as a back-up. The advantages of using reciprocating positive displacement pumps are that they can be used to:
1. Pump fluids containing high solids content
2. Operate over a wide range of pressures and flow rates
and that they are:
1. Reliable
2. Simple to operate, and easy to maintain

The flowrate and pressure delivered by the pump depends on the size of sleeve (liner) that is placed in the cylinders of the pumps. A liner is basically a replaceable tube which is placed inside the cylinder to decrease the bore.

The hoisting system is a large pulley system which is used to lower and raise equipment into and out of the well. In particular, the hoisting system is used to raise and lower the drillstring and casing into and out of the well. The components parts of the hoisting system are shown in Figure 3. The drawworks consists of a large revolving drum, around which a wire rope (drilling line) is spooled. The drum of the drawworks is connected to an electric motor and gearing system. The driller controls the drawworks with a clutch and gearing system when lifting equipment out of the well and a brake (friction and electric) when running equipment into the well. The drilling line is threaded (reeved) over a set of sheaves in the top of the derrick, known as the crown block and down to another set of sheaves known as the travelling block. A large hook with a snap-shut locking device is suspended from the travelling block. This hook is used to suspend the drillstring. A set of clamps, known as the elevators, used when running, or pulling, the drillstring or casing into or out of the hole, are also connected to the travelling block.
drilling rig hoisting system

Having reeved the drilling line around the crown block and travelling block, one end of the drilling line is secured to an anchor point somewhere below the rig floor.Since this line does not move it is called the deadline. The other end of the drilling line is wound onto the drawworks and is called the fastline. The drilling line is usually reeved around the blocks several times. The tensile strength of the drilling line and the number of times it is reeved through the blocks will depend on the load which must be supported by the hoisting system. It can be seen from Figure 3 that the tensile load (lbs.) on the drilling line, and therefore on the fast line, Ff and dead line Fd in a frictionless system can be determined from the total load supported by the drilling lines, W (lbs.) and the number of lines, N reeved around the crown and travelling block:

 

There are many individual pieces of equipment on a rotary drilling rig (Figure 1). These individual pieces of equipment can however be grouped together into six sub-systems. These systems are: the power system; the hoisting system; the circulating system; the rotary system; the well control system and the well monitoring system. Although the pieces of equipment associated with these systems will vary in design, these systems will be found on all drilling rigs. The equipment discussed below will be found on both land-based and offshore drilling rigs.

rotary drilling rig