03-Rotary Drilling Rig

Shell Intensive Training Programme Well Engineering Rotary Drilling Rigs

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7.0 Rotary Drilling Rig 7.1 Introduction A rotary drilling rig is a portable factory that is built to make hole. The requirement of portability (being able to be moved from site to site) places a limitation on rig design, as to both weight and size of each component. The total weight of the rig is a factor for overland moves, but the weight and bulk of each unit of assembled equipment is even more important. Each unit assembly is limited in weight because of truck and highway limitations on gross weight. Rotary drilling rigs must be disassembled for any component or subassembly so that weight limits does not exceed a move. The most efficient rotary rig design is not necessarily the most effective. Other factors have to be considered. Rotary rig design should- 1) allow for rapid erection and take-down, and provide for packaging in as few pieces as

practical; 2) Not require special cranes for assembly (rig-up) or disassembly (tear-down); 3) Enable drill pipe to be run into the hole or pulled out with minimum time wasted; 4) Providing the maximum amount of available power for the circulating fluid to the bit (goo

hydraulics) when drilling. Many factors determine a rig’s portability. Wheel-mounted rigs can be used for drilling to depths of 10,000 feet or more and for completion/workover service on 15,000-foot wells. These rigs have self-erecting, telescoping masts; and the mast, drawworks, and engines are built on a trailer or self-propelled unit. Equipment such as mud pumps must be handled as packages; therefore, efficient planning and design are necessary. The relationships of the major systems of a land rig are shown in figure 7.1. These systems accomplish the three main functions of rotary drilling: hoisting, rotating, and fluid circulation. (figure 7.1) The hoisting system is used to raise and lower the drill stem, and also to support and lower pipe that is used for casing and tubing. A mast or derrick supports the hook by means of the traveling block, wire rope, crown block, and drawworks. The drawworks is powered by two or three engines (called prime movers) to raise or lower the drill stem so that the bit can drill. The drill stem is the whole assembly from the swivel to the bit, including the kelly, drill pipe, drill collars, and bit sub. A principal feature of the rotating system is the rotary table, or rotary. The rotary table is powered by the prime movers to rotate the kelly, which is raised or lowered through the kelly drive bushing. The rotation of the kelly causes the drill stem and bit to turn and thus “make hole” as the bit grinds away the rock formation. The kelly is supported by the hoisting system. Drilling fluid is pumped down the drill pipe to the bit and then up the annulus, the area in the hole that is outside the drill pipe. The circulating system sends drilling fluid from a mud pit through the mud pump, standpipe, rotary hose, swivel, kelly, drill pipe, drill collars, bit, annulus, and back to the pit. The hydraulic power of the drilling fluid passing through the bit cleans the bottom of the hole and produces more effective drilling. Under special circumstances, a mud motor or turbodrill is used to turn the bit. In this case, hydraulic power of the drilling fluid (instead of rotation of the drill stem) turns the bit.


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If rigs did not require mobility and quick rig-up and tear-down capability, they could be designed to require less power for hoisting, pumping and other jobs. Hydraulic rigs have been built, but they are heavy, slow, and troublesome to operate. The best means of hoisting drill pipe is the block-and-tackle arrangement that is generally employed. 7.2 Derrick, Mast, And Substructure Standard drilling rig derricks are tall steel structures with four supporting legs standing on a square base. The derrick and substructure plays an important role in drilling operations. the derrick provides the vertical height necessary for the hoisting system to raise and lower the pipe. The derrick is assembled piece by piece at the drilling site. A drilling mast, which is partially assembled when it is manufactured, usually has a smaller floor area. It is raised from a horizontal to a vertical position in one lift, as a beam supported at one end can be lifted. The standard derrick has become rare today except for extremely deep wells and offshore drilling. The mast (fig. 7.2) has almost completely replaced the conventional derrick for drilling on land because- 1) it can be quickly dismantled and erected on

another location by the regular rig crew; and 2) it can be moved in large units without

complete disassembly. Masts 135 to 145 feet in height are the most common size. The mast or the derrick rests on a substructure, which also supports the rig floor and the rotary. The rig floor provides an area for handling the drill stem and related equipment. Blowout preventers and wellhead fittings are located under the substructure. The substructure supports the weight of the casing as it is being run in the hole. Drill pipe is suspended from the rotary table, which is supported by the beams of the substructure. Heavy-duty masts and substructures may have capacities of 1,200,00 pounds. The normal capacity is in excess of 500,000 pounds. The derrick and the substructure must have enough strength to support all loads, including the


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hook load, pipe set in the derrick, and wind loads. The API has developed size classifications for the derrick shown in Fig 7.3. the specifications are summarized in Table - 7.1. In addition, table 7-1 contains data used in determining wind loading.


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The derrick and substructure must be able to support the imposed by pipe weight on the block a portion of the drillstring standing in the derrick. Due to the manner in which the hook load is distributed over the derrick, the effective load may exceed the actual. When heavy casing strings are run, it may be necessary to lay down some drillpipe initially so the derrick loading capacity is not exceeded. The derrick load resulting from a hook load can be evaluated with Fig 7-4. The force on the derrick ( FD ) includes the hook load, L; the tension in the fast line, TF; and the tension in the line, TD;

FD = L + TF + TD Eq. 2.1

The tension in the fast line in a nonideal friction, is shown below: TF = __L__ NEB where EB = efficiency factor of block system L = hook load, lb N = number of lines strung over the block system TF = fast-line tension, lb

Since the dead line does not move, the tension is shown in eq. 2.2

TD = L/N Eq. 2-2can be rewritten as follows;

FD = L + _L__ + L Eq. 2. 3 NEB N = L(1 + EB + EBN) EBN


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The total force on the derrick (FD) is not evenly distributed over each of the fourlegs (Fig 7-5). the fast-line tension is distributed evenly between legs C & D , since the drawworks is commonly positioned between the legs. The dead-line tension is near a leg. The force on each leg can be summarized as follows:

Load Source

Total Load

Load on Each Derrick Leg

A B C D Hook Load

L L 4

L 4

L 4

L 4

fast line L__ EB N

L__ 2EB N

L__ 2EB N

Dead Line

L N

L N

___ ___ ___

Total L ( NN+ 4

4) L/4 L E N

E NB

B

( )+ 24

L E NE NB

B

( )+ 24

The load on leg A is greater than any other leg if EB > 0.5. Therefore, the maximum derrick load can be defined as four times the strength of the weakest leg:

FDE = 4LN

N+

44

= LN

N+

4; where FDE = effective derrick load


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The derrick will be exposed to loads created by wind acting horizontally on pipe set back in the derrick. the Wind load (LW) is calculated from :

LW = 0.004V2 Where Lw = wind load, lb/ft2 V = wind velocity, mph

7.3 Power and Power Transmission Drilling rigs, and their support vessels in the case of barge and floating vessels, have high power requirements. Some of the equipment requiring power includes the drawworks, mud pumps, rotary system, and life-support system. The power loading may be continuos or intermittent.


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The power system on a drilling rig usually consists of a prime mover as the source of raw power and some means to transmit the raw power to the end-use equipment. the prime movers used in the current drilling industry are diesel engines. Steam is no longer a source of rig power, since natural gas (which was used to fire the boilers) has increased dramatically in cost. Internal-combustion engines and electricity now power most drilling rigs. Large rigs and most wheel-mounted assemblies are generally powered by diesel engines. Diesel-electric motors for the different rig machines. Most prime movers are diesel engines, although engines that use natural gas or liquefied petroleum gas (LPG) in the form of propane or butane drive some rigs. Drilling rig engines range from 250 to 2,000 horsepower (hp) each; total rig power may be 500 to 5,000 hp. In figure 7.6, a conventional arrangement of the engines, compound, and drawworks for a mechanical-drive-drilling rig is shown. In a diesel-electric system for rotary drilling (fig. 7.7), note the relative loads.

On a mechanical-drive rig, a means of transmitting the power from the engines to the drawworks, pumps, and rotary must be provided. This transmission is usually accomplished through an assembly known as the compound, which consists of clutches, couplings, shafts, chains and sprockets. The most widely used system on new rigs or large marine rigs is the AC-SCR system. The mechanical horsepower requirement for the prime movers must be determined from an evaluation of the loads and the overall system efficiency:

mechanical horsepower = ____load____ efficiency

where the efficiency is less than 1.0. Although the above eqn is straightforward, it is difficult to implement due to problems in establishing the load and efficiency factor.

The efficiency factor (E) describes the power losses from the prime movers to the end-


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use equipment. It can be calculated from :

E = energy output - energy input energy output

where output is from the prime mover and input is the amount remaining for actual usage after some losses. The system losses result from friction, gears, and belt line losses. The efficiency factors range from 0-1. Some drilling personnel assume that efficiency factors for oil-well drilling machinery are 0.98 per shaft and chains. If more than one engine is used, an average value is calculated. The mechanical horse power requirements must be modified for harsh temperature environments or altitudes. According to API Standard 7B-11c, approximate conversions for altitude temperature of naturally aspirated engines may be made as follows:

• deduct 3% of the standard brake horsepower for each 1000 ft rise in altitude above sea level.

• deduct 1% of the standard brake horsepower for each 100 rise in temperature above 850 F or add 1% fall below 850 F

The engine manufacturer should be consulted for specific variances. 7.4 Hoisting Systems and Drawworks The hoisting system is a vital component of the rig equipment. It provides a means for vertical movement of pipe in the well, i.e., to lower and raise the drillstring and casing. The principal items in the hoisting system are as follows:

• drawworks • crown and travelling blocks • wireline • ancillary equipment such as elevators, hooks and bails.

The hoisting system, in conjunction with the circulating equipment, consumes a portion of the rig’s power. A drawworks on a rig is known in other industries as a hoist. The main purpose of the drawworks is to lift and lower pipe in and out of the hole. The hoisting drum either reels in wire rope to pull the pipe from the hole or lets out wire rope to lower the traveling block and attached drill stem, casing, or tubing. The drawworks includes a transmission, which uses chains sprockets, and gears to allow speed changes of the hoisting drum. Often, the drawworks has a drive sprocket to power the rotary table. This arrangement is common, even on diesel-electric rigs. The drawworks brake system makes it possible for the driller to control a load a several hundred tons of drill pipe or casing. Most rigs are equipped with two brake systems for the drawworks hoisting drum: one that is mechanical and one that is hydraulic or electric. The mechanical system consists of compounded levers to tighten brake bands to bring the drum to full stop. The hydraulic or electric brake can control the speed of descent of a loaded traveling block, although it is not capable of stopping the drum completely. Another of component of the drawworks is the catheads. The makeup, or spinning, cathead is located on the driller’s side of the drawworks and is used to tighten the drill pipe joints. The other cathead, located opposite the driller’s position, is the breakout cathead. It is used to loosen the drill pipe when it is pulled from the hole. Air hoists are provided on many rigs for handling light loads.


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7.5 Blocks and Drilling Line The traveling block (fig. 7.8), crown block, and drilling line within the derrick raise and lower loads of pipe out of and into the hole. During drilling operations, these loads usually consist of drill pipe and drill collars. The blocks and drilling line must also support casing while it is being run in the hole. This casing is often heavier than the drill stem. Drilling line is reeved around sheaves (pulleys) in the crown block at the top of the derrick or mast and in the traveling block .(Fig. 7.9) The blocks and drilling line assembly must have great strength to support the heavy loads. The number of sheaves is determined by the weight to be supported. Five is the most common, but deeper wells often require six or seven. Friction is minimized in the blocks by heavy-duty bearings. Large-diameter sheaves are provided to lessen wear on the drilling line, which is usually a multistrand steel cable, 1 ¼ to 1 ½ inches in diameter.


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API Specification 8A recognizes the maximum load rating in tons (2000 lb units) as follows:

5 40 250 10 65 350 15 100 500 25 150 650 750

The maximum load rating may be based on either the tensile strength or the yield strength of the material at the manufacturer’s option. The rig must be evaluated with respect to the block-and-tackle system to ensure that it meets the designed safety requirements. Specifications for safety factors are as follows:

Calculated Load Rating, tons Tensile Strength Design Safety Factor 0-150 4.00

151 - 500 4.00 - ( R - 150) Eq. 5.1 350

501 and over 3.00 where R is the calculated rating in 2000 lb tons for Eq. 5.1. The maximum load rating of the pipe must be calculated before applying the safety factor and evaluating the equipment rating. The travelling block is free to move and has hook, bails, and elevators attached to the bottom for latching to the pipe. Both blocks have 4-12 sheaves. The no. of lines strung vary with load, with fewer on shallow wells and maximum for heavy casing loads. The block system is not a frictionless system, i.e., its efficiency factor is less than 1.0. It is often assumed that the efficiency factor is computed from:

EB = (0.98)n where n is the number of sheave pairs. the following table indicated EB for various pulley systems.

No. of Lines EB 6 0.886 8 0.850 10 0.817 12 0.785

Wire Rope. Drilling rigs have many applications for wire ropes. The more common uses for wire ropes are as drilling lines and guideline tensioners. The drilling line connects to the drawworks and the dead-line anchor. It is pulled through the crown and travelling block sheaves so that the travelling block can be raised or lowered as necessary. Wire rope is made from cold drawn carbon steel of various grades, depending on the strength required. The API classifies the various grades as follows:


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• extra improved plow steel (EIPS) • improved plow steel (IPS) • plow steel (PS) • mild plow steel (MPS)

Generally, the first two higher-strength grades, EIPS and IPS, are used currently for

drilling lines due to the rugged service encountered. The primary element of wire rope is the individual wires. Wires are carefully selected,

sized, and layered into strands. After stranding, the strands are layered together around a core to form wire rope. The core may be a fiber rope (either natural grown fibers or man-made fibers), a plastic core, a spring steel core, a multiple-wire strand, or an independent wire rope (IWRC). The independent wire rope is the most widely used because it resists crushing and distortion.

The wire rope is usually described by type of core, the number of strands wrapped around the core, and the number of individual wires per strand (Fig 7.10). For example, a 6 x 19 with an independent IWRC is a typical type of rope used as drilling line. It contains one independent wire rope core, six strands, with nineteen separate wires per strand.

Wire rope is usually furnished preformed but can be furnished non-preformed upon special request. A preformed rope has the strands shaped to the helical form they assume in the finished rope before the strands have been fabricated in to the rope. The strands of the preformed will not spring from the normal position when the sizing bands are removed.

The lay of the rope describes the direction of the strand wrap around the core and the direction of the wire rope around within the strands. (Fig 7.11). The strands may be right or left lay. The individual wires can be regular or Lang lay. The length of the lay is usually 7.25-8 times the nominal diameter.


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The nominal strength of the wire rope depends on the material used in construction, the

number of strands and wires, and the size of the rope. The API has published tables for breaking strengths of various wire ropes Fig 7.12. As an example, the nominal strength of 1 3/8”, 6 x 37 drawn galvanized IWRC rope is 192,000 lb.

The API has established minimum design factors for wire ropes operating under oil-field conditions. These design factors are specified in API Recommended Practice 9B.

Type of Service Minimum Design Factor Cable tool line 3 Sand line 3 Hoisting service other than rotary drilling 3 Mast hoisting and lowering 2.5 Rotary drilling line when setting casing 2 Pulling on stuck pipe and similar infrequent operations

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When working near the minimum design factor, consideration should be given to the efficiencies of wire rope bent around sheaves, fittings, or drums.

The primary function of the wire rope in conjunction with other components of the hoisting system is to provide a mechanical advantage (M) for raising or lowering the drillstring or casing. If the tension line in the fast line attached to the drawworks is defined as TF , then the mechanical advantage is as follows:

M = L TF where L = hook load, lb TF = fast-line tension, lb M = mechanical advantage

The fast-line tension can be computed, if an ideal system is considered:


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TF = L_ N

where N = number of lines strung over the block system.

Since block efficiency (EB) must be considered in a non-ideal case, the fast-line tension is as follows:

TF = L NEB


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Example A 1 3/8”, 6 x 37 galvanized IWRC rope is to be used when running a 425,000 lb casing string. The company intends to rig-up a 10 line system. Determine if the rope meets the design factor criteria of 2.0. Assume an efficiency of 0.98 per sheave. Use cols. 8 - 10 of Fig 7.12. Solution: 1. The efficiency factor (EB) for a 10 line system is:

EB = (0.98)n EB = (0.98)10 EB = 0.817

2. The fast-line tension (TF ) is compute from :

TF = L NEB = 425, 000 lb (10) (0.817) = 52,019 lb

3. The load factor is:

192,000 lb = 3.69 52,019 lb

4. therefore, the rope meets the design factor 2.0 The horsepower required to lift a load, L, at some velocity is given by:

HP = _LV__ 33,000 where V = Velocity, ft/min 33,000 = ft-lb/min/hp

This equation is very useful in determining the amount of input horsepower requirements from the prime moves. Block system and draw works efficiency must be considered:

HPE = HPB (33,000)(EB)(ED) where HPB = block horsepower HPE = engine horsepower


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EB = block efficiency ED = drawworks efficiency

Wire rope requires lubrication to extend its useful life. The strands rub against one

another as the rope flexes over sheaves in the traveling and crown blocks. Because wire rope eventually becomes too worn for use, it is an expensive, renewable item in the drilling process. A planned program of moving the wire rope allows longer service life.

The usual practice is to evaluate the number of ton-miles of work performed by the wire rope. A ton-mile is defined as the amount of work needed to move a 1-ton load over a distance of 1 mile. After a rope has reached a specific ton-mile limit, it is removed from service. the limits vary for different operations and may range from about 500 for 1.0” rope to about 1,800 for 1 3/8” rope. Drilling line is cut ( a portion is retrieved) before any critically strenuous job. The major factors affecting ton-mile wear on the wire rope are round trips, setting casing, and drilling. The following equation computes ton-miles during a round trip:

TR = D L D W D M CS M( )

, ,( / )

, ,+ + +

10 560 0001 2

2 640 000

where TR = ton-miles during a trip D = hole depth, ft LS = length of drill pipe stand, ft WM = effective weight per foot of drill pipe, lb/ft M = total weight of travelling block-elevator assembly C = effective weight of drill-collar assembly minus the effective weight of the same length of drillpipe, lb/ft

Similar equations are provided in API RP 9B for coring, drilling, and setting casing. M is the weight of the travelling block assembly. It includes the traveling block, hook, links and elevators. if the actual weight of the traveling block assembly is unknown, the following approximate values may be used:

Travelling Block Capacity, tons Assembly Weight, lb 100 6,000 150 9,000 250 12,000 350 19,000 500 28,000 650 35,000 750 48,000

In addition to fatigue wear from accumulated ton-miles of service, the wire rope will

wear more at lap and pickup points. the pickup points are on the top side of the crown block when the weight of the drill string is lifted from the supports in the rotary table during tripping


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operations. the lap points on the draw works drum occur when the line begins to new wrap. Slip and cut programs are designed to

avoid excessive wear at the lap and pickup points. Slipping involves loosening the dead-line anchor and placing a few more feet of line into service from the storage reel. Cutting requires that the line on the draw works reels be loosened and a section cut and removed. Slipping changes the pickup points, and cutting changes the lap points. A line is usually slipped several times before it is cut. 7.6 Rotary, Kelly, And Swivel The rotary is the piece of equipment that gives the rotary drilling rig its name; it is the machine that turns the drill stem and the bit in order to make hole. A rotary table is fitted with a drive bushing (fig.7.13). The three-, four-, six-, or eight-sided kelly fits through the bushing and is thus turned by the rotary. The rotary is a basic yet extremely rugged machine that is distinguished by its ability to withstand hard service. The drive bushing may fit in a square opening in the rotary tale, or four pins that fit in the openings of the table may drive it. The drive bushing permits vertical movement of the kelly as the hole is deepened, at the same time rotating the drill stem. The rotary serves two main functions:

1) to rotate the drills stem; and 2) to hold friction-grip devices called

slips to support the drill stem or casing.

A sprocket and chain may mechanically drive the rotary from the drawworks. However, many drilling rigs provide power to an electric motor that drives the rotary directly. In some cases, an independent engine is used to drive the rotary. The kelly is the tope member of the drill stem. It is about 40 feet long and may be either triangular-, square-, hexagon-, or octagon-shaped to fit its drive bushing. The kelly can move freely up and down through


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the drive bushing while the rotary is turning it. (fig. 7.14). The swivel (fig. 7.15) hangs from a hook under the traveling bloc, and serves several vital functions.

1) It supports the weight of the drill stem.

2) It allows rotation of the drill stem.

3) It provides a passageway for drilling fluid to enter the drill stem.

The rotary hose is connected to a gooseneck-fitting on the swivel; drilling fluid is pumped into the gooseneck, through the swivel, and down the kelly. This fluid may be under pressure exceeding 3,000 psi. 7.7 Circulating System An essential feature of the rotary drilling process is the circulating system, commonly called the mud system. In order for rotary drilling to proceed, the drilled cuttings must be lifted out of the hole. Fluid must be pumped down through the annulus (the space outside the drill string). The principal purposes of circulating fluid are-

1) cleaning the bottom of the hole by washing the cuttings back up to the surface;

2) cooling the bit; 3) supporting the walls of the

well; and 4) preventing entry of formation

fluid into the borehole. The circulation fluid is usually a

liquid, but it may be air or gas. Water is the usual base, though occasionally oil is used. A pump forces the drilling fluid up through a standpipe hose into the swivel, down through the drill stem, and back to the surface again (where it returns to the mud pits). The mud pits or tanks are usually fitted with solids-control equipment, which removes cuttings and other solid material in mud brought up from the hole before it is recirculated into the well by the mud pump. When air is used as drilling fluid, compressors replace the mud pump and there is no need for storage pits and settling tanks. Compressed air is forced down the drill stem to the bit and up the annulus by air pressure. Most mud pumps currently used in the drilling industry are duplex or triplex positive


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displacement pumps. 7.8 Blowout Preventers Drilling fluid in the hole helps prevent formation fluid from entering the borehole. If formation fluid does enter the well, it may rise to the surface and cause some of the drilling fluid to flow out of the hole. If the drilling crew cannot control the flow, it is called a kick. If the drilling crew can control the flow, it is called a kick. If the flow is continuous and cannot be controlled, a blowout has occurred. A blowout preventer (BOP), in conjunction with other equipment and techniques, is used to shut off and control a kick before it becomes a blowout. Several BOPs are usually installed on top of a well, with an annular preventer above and two or more ram preventers below. An annular preventer has a resilient sealing element. When activated by fluid pressure, the sealing element closes on the kelly, drill pipe, or drill collars. Ram preventers have two steel ram segments that are pushed together from both sides to seal around drill pipe. Both annular and ram preventers are operated by hydraulic fluid pressure. Blind ram preventers can be used to close an open hole (hole with no drill pipe in it). Blowout preventers are opened and closed by hydraulic power. The fluid is stored under pressure in an accumulator. High-pressure lines carry the hydraulic fluid from the accumulator to the BOP stack. When the driller turns the proper valves, the fluid operates the BOPs. Because the preventers must be able to close quickly, the hydraulic fluid is put under 1,500 to 3,000 psi of pressure by nitrogen gas in the accumulator unit. 7.9 Auxiliaries Electric Generators: Modern rotary rigs provide power for auxiliaries with AC generators that are usually diesel-powered. Most of these generators have capacities of 50 to 100 kilowatts, although larger units are sometimes installed. The generators have enough capacity to carry the main power load of the rig (excluding hoisting, pumping, and rotating functions). A second engine and generator unit are held in ready reserve. AC electricity is used for rig lighting, shale shaker motors, mud pit stirrers, centrifugal pumps, rig instruments, engine-cooling fans, air conditioning for bunkhouses and other purposes. Air Compressors A small compressor is usually mounted on the engine compound for supplying air to the pneumatic controls and clutches. The compressor has a volume tank to allow reserve storage of compressed air. Large rigs usually have another electrically powered compressor to furnish high-pressure air for other purposes, such as starting the main engines and operating air-powered hoists, air slips, BOP equipment, water wells and air-operated tools. Water Pumps Water supply is an important item for drilling rig operations. Water is usually obtained from a well, stream, lake, or pipeline from a remote source. A stored supply of several hundred barrels is maintained at the rig. This may be in a pit or tank(s) of sufficient capacity to maintain operations for a short time if the primary supply is interrupted. Low-pressure water pumps are usually provided for washdown and for cooling the brakes of the drawworks. High-capacity pumps are generally used for mud and cement mixing and mud transfer. Other Equipment Drilling rigs also include such facilities as fuel storage tanks, a house for changing work clothes, a doghouse ( a small structure on the rig floor that serves as an office for the driller), a


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place to store parts for the pumps and other equipment, and other facilities. Most large rigs are provided with an office trailer where the supervisors can maintain communications with the head office. 7.10 Rig Design Consideration Factors When assembling equipment and machinery to make up a rotary drilling rig, the following factors that affect performance and mobility must be considered: 1) range of well depths to be drilled; 2) sizes of drill pipe and drill collars to be used for expected hole diameter; 3) casing sizes and loads expected to be handled; 4) size of table opening, rotary speed and power needed; 5) circulating fluid volume, pressure, pump size and power required; 6) mud system requirements, (i.e., mud pits, piping, pit accessories, dry mud storage, etc.); 7) conventional derrick or mast and type of substructure that will be used 9their rated

capacities and heights); 8) blowout preventers and control equipment necessary; 9) auxiliary services required (i.e., electricity, compressed air, water supply); and 10) miscellaneous items such as instruments, housing and pipe racks needed for efficient

operation. Hole depth, drill pipe sizes, and expected sizes and weights of casing will determine the derrick load capacity. The size of casing will establish the rotary table opening. Rotary revolutions per minute (rpm) and drilling weights to be used will dictate rotary power and speed options. The volume and pressure of mud circulation will establish the size of the pump and its pressure rating. Deep, large-diameter holes will require more mud in the active mud system than shallow, slim holes. Conventional derricks require rig builders for assembly; and often a crane is required on the new location before the rig is transferred. Most drilling masts are cantilever types that can be assembled quickly by the rig crew and raised to vertical position by using the drawworks. If high-pressure BOPs are required, wellhead clearance beneath the rig floor may exceed 20 feet. Manually operated blowout preventers may be used on shallow, low-pressure wells, but larger preventers with pressure ratings up to 5,000 psi are operated hydraulically. Rig lighting, shale shakers, and mud mixers require electricity, usually 110- or 220-volt AC power. Miscellaneous electrical power totals as much as 100 kilowatts for heavy-duty rigs. Many rigs require compressed air for rig controls, brakes, air hoists, and other purposes. Washdown, mixing procedures, and brake cooling require readily available fresh water under pressure. Miscellaneous items of equipment that must be considered include instruments such as the weight indicator pressure gauges, mud monitors, and rate-of-penetration indicators, usually placed in the doghouse on the rig floor. A crew change facility and quarters for supervisors and others are usually self-contained units. High Substructures High substructures pose two major problems: 1) getting the drawworks on to the rig floor; and 2) aligning extra-long chain drives from the engines to the drawworks. Step-down substructures for the engines can circumvent these problems. Derrick or mast substructures with 20 to 30 feet of clearance for the wellhead and blowout preventers require cranes or precarious truck ramps for placing the drawworks on the


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rig floor. Using rig layouts with alternative designs may reduce substructure height. If the drawworks is on the derrick floor on a mechanical rig, the engines must be close to the same height to avoid long chain drives. Electric-drive rigs cut down on substructure size by eliminating engines on the derrick floor. They can be removed to ground level, since no alignment is necessary between the engines and the rig motors. When the drawworks is mounted at, or near, ground level, the substructure need be no larger than that needed for the derrick floor. Mast erection can also be simplified by hinging it near ground level. A rotary rig with either a derrick or a mast can be assembled with hoist, transmission, and engines on the ground. A cat-works with a drive arrangement for the rotary is placed on the rig floor. The drawworks brake lever is operated from the floor, using an extension linkage. Mechanical rigs that use this design, with a propeller shaft from the ground-mounted transmission to drive the cat-works and rotary, are available. Ground-mounted hoist assemblies not only eliminate unnecessary substructures but also lighten the main substructure and speed up erection and dismantling. Offshore Platforms Since offshore drilling rigs are so expensive to install, they are often designed for the use of multiple-well derricks. A multiple-well derrick consists of a standard derrick that has been widened to provide space to drill two, four, six, or even more than twenty wells without the necessity of moving the derrick. All the holes are drilled from the same platform. Only the crown block, the racking platform, and the rotary table must be moved for the different wells located within the one derrick (fig. 7.16). For drilling in deep water from a floating platform, rigs must be designed to withstand the side-to-side whipping action produced by the roll of the sea. Some method must be provided to prevent the traveling block from striking the side of the derrick during drilling under rough sea conditions. One means is a traveling block guide, which may be either a wire-line strung from the crown to the derrick floor or a rigid track-and-roller arrangement. Offshore derricks are also subject to loosening of bolts from sea action during drilling or trauma while mobile units are moved from place to place. Specially welded derricks have been designed to withstand such problems. Rig Drive Systems The major power trains in a modern rotary drilling rig are- 1) drawworks and mechanical transmission from the engines or electric motors; 2) wire rope system, including crown block, travelling block, wire rope for hoisting; 3) mud pump drives; 4) rotary drives; and 5) power transmission devices.


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Today the efficiencies of the first four components are greatly influenced by the type of power transmission devices chosen. Rigs are referred to according to their method of power transmission, as follows:-a) mechanical drilling rig, b)DC/DC drilling rig; or c) SCR electrical drilling rig.

03-Rotary Drilling Rig

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