Introduction

Rig masts and derricks are tall because they have to accommo­date long lengths of pipe the rig crew raises into it during the drilling process. A mast or derrick can be as high as a 16-story building—about 200 feet (60 metres) tall. Most, however, are closer to 140 feet (43 metres) high. Even so, in flat country, a structure as lofty as a 16-story building is conspicuous.

Upon arriving at the rig, the first step is to check in with the boss. He or she is probably in a mobile home or a portable building on the site that serves as an office and living quarters. The rig boss may have the intriguing title of "toolpusher"; or, rig workers may call him or her the "rig superintendent," or the "rig manager." (Currently, most toolpushers, or rig su­perintendents, are men; but that's changing.) Toolpusher is the traditional term for the rig boss. It probably originated from the fondness rig workers have of calling practically every inanimate thing on a rig a tool. Thus, one who bossed the personnel using the tools also pushed the tools, in a symbolic, if not actual, sense. Nowadays, the drilling industry leans towards the term rig superintendent or rig manager for the person in charge, but you'll still hear rig hands call him or her the toolpusher (or, in Canada, the "toolpush").

Figure 4. Personal protective equipment (PPE) includes hard hats and safety glasses.

Now don your hard hat, which is a very tough plastic cap with a brim to protect your head. Also, put on your steel-capped boots, which keep your toes from being crushed, and your safety glasses to safeguard your eyes. This style of dress is de rigueur for everyone. Whether working on a rig or merely visiting it, everyone must wear personal protective equip­ment, or PPE for short (fig. 4). Rig workers also wear gloves to protect their hands and you may want to wear a pair, too.

A PRIMER OF OILWELL DRILLING

Figure 5. Steel stairs with handrails lead up to the rig floor.

With protective gear on and the rig superintendent's permission, let's go up to the rig floor. The floor is the main work area of the rig and it usually rests on a strong foundation, a substructure, which raises it above ground level. Accordingly, we have to walk up a set of steel stairs (fig. 5). Keep a hand on the handrail as you walk up and don't hurry. It could be a 40-foot (12-metre) climb. Once on the floor, stop for a minute to catch your breath and take a good look around the floor. You may see the crew handling several lengths, or joints, of drill pipe, the steel tubes that put the bit (the hole-boring device) on the bottom of the hole. On the other hand, the rig may be drilling, or "making hole," as they sometimes say. If it's drilling, from time to time you may hear the distinctive and loud squawk or squeal of the drawworks brake as it slacks off the drilling line to allow the bit to drill ahead. The drawworks is a large, powerful hoist that, among other things, regulates the weight the drill string puts on the bit (fig. 6). A loud screech comes every time the friction brake bands ease their grip on the steel hubs of the drawworks drum to apply weight. It's loud, but it's music to the ears of the rig owner because it usually means the bit is drilling ahead without problems.

Regardless of what's occurring on the rig floor, take time to observe, for you're standing in a place that is vital to the oil and gas industry. Certainly, many operations besides drilling are involved in getting crude oil and natural gas out of the ground and into forms we can use, such as gasoline and heating fuel. However, without a drilled well—a hole in the ground—oil companies could not obtain oil and gas, or petroleum, at all.

At this point, you may not know what the equipment is for or what the personnel are doing, but don't be troubled. This bookwill identify most of the people and tools it takes to drill, and will give you a better appreciation of oilwell drilling. Before launching into equipment and processes, however, let's cover a little drilling history.

Figure 6. The drawworks is a powerful hoist.

The story of oilwell drilling in the United States begins in the mid-i8oos, at the dawn of the industrial revolution. It was a time when people were beginning to need something better than candles to work and read by. Responding to the demand for reliable lighting, companies began making oil lamps that were brighter than candles, lasted longer, and were not easily blown out by an errant breeze.

One of the best oils to burn in these lamps was sperm-whale oil. Sperm oil was clear, nearly odorless, light in weight, and burned with little smoke. Virtually everyone preferred whale oil, but by the mid-18oos, it was so scarce that only the wealthy could afford it. The New England whalers had all but hunted their quarry to extinction. Thus, the time was ripe for an inexpensive lamp oil to replace whale oil. At the same time, steam-powered machines that required good-quality lubri­cants were becoming common.

About this time—1854—a New York attorney named George Bissell received a sample of an unusual liquid from a professor at Dartmouth College. Bissell and the professor had met previously and had discovered a mutual interest in finding a whale-oil substitute. The professor wanted Bissell's opinion of the liquid's value as a lamp oil and lubricant. The sample had been collected near a creek that flowed through the woods of Crawford and Venango counties in northwestern Pennsylvania. Besides water, the creek also carried an odor­ous, dark-colored substance that burned and, when applied to machinery, was a good lubricant. The substance was, of course, oil. Because it flowed out of the rocky terrain in and near the creek, people called it "rock oil." Indeed, so much oil flowed into the stream that settlers named it Oil Creek (fig. 7).

^History

Figure 7. Oil Creek as it looks today

A PRIMER OFOILWELL DRILLING

The sample came from land next to the creek just southeast of the town of Titusville, where the oil seeped from the rocks in the form of a spring.

THE DRAKE WELL, 1850s

After examining the oil sample, Bissell was convinced that refined rock oil would burn as cleanly and safely as any of the oils available at the time, including whale oil. He also believed that it would be a good lubricant. Bissell thus began raising money to collect the oil from the Titusville spring and to market it for illumination and lubrication. It was a difficult proposition; after a false start or two and much wheeling and dealing, Bissell, a Connecticut banker named James M. Townsend, and others formed what ultimately became the Seneca Oil Company, in New Haven, Connecticut.

One problem the company faced was how best to produce the oil from the land. The company directors knew that it was not efficient to simply let the oil flow out of the rock and scoop it from the ground. Others who had collected oil in this manner obtained merely a gallon (a few litres) or two a day. Seneca Oil's purpose was to produce large amounts of oil and market it in the populous northeastern U.S. Somebody in the company—no one knows who—came up with the idea of drilling a well to tap the oil. Drilling was not a new concept, for people had been drilling saltwater wells in the Titusville area foryears. Interestingly, many of these saltwater wells also produced oil, which the salt drillers considered a nuisance because it contaminated the salt.

Another issue facing the fledgling oil company was the need to hire someone to oversee the drilling project in Titusville. Eventually, board member Townsend met and hired Edwin L. Drake to represent Seneca's interests at the Oil Creek site. At the time, Drake was an unemployed rail­road conductor, but he had two things going for him. First, because he was out of work, he had plenty of time to devote to the project. Second, Drake had a railroad pass, which allowed him free travel to Pennsylvania. As a final touch, Townsend gave Drake the rank of honorary colonel, which sounded considerably more prestigious than just plain mister. With that, Colonel Drake went to Titusville.

History

By the spring of 1859, Drake employed William A. Smith to be his well driller. Smith, a blacksmith and an experienced brine-well driller, was known to most everyone as Uncle Billy. He showed up at the well site in Titusville with his sons as helpers and his daughter as camp cook. One of the first things Drake and Uncle Billy did was drive a length of hollow steel pipe through the soft surface soil until it reached bed­rock. If they had not used this pipe, this steel casing, the loose topsoil would have caved into any hole they tried to drill. (To this day, drillers still begin oilwells by casing the top of the hole.) Drake and Smith then built the drilling rig (fig. 8), ran the drilling tools inside the casing, and drilled the rock.

By Saturday, August 26, 1859, Drake and Smith had drilled the hole to a depth of about 69 feet (21 metres). Near the end of the day, Smith noted that the bit suddenly dropped 6 inches (15 centimetres). It was near quitting time, so he shut the operation down, figuring he and the boys would continue drilling the following Monday. On Sunday, which in those days was a well driller's holiday, Smith decided to check on the well. He looked into the top of the casing and found the hole full of oil. Overnight, oil from a formation some 69У2 feet (21.2 metres) below the surface had flowed into the well casing and filled it to the top. The well's being full of oil signaled success. No one knows for sure how much oil it produced, but it was probably around 800 to 1,200 gallons (about 3,000 to 4,800 litres) per day, which far outstripped the gallon or two that could be collected off the ground. Regard­less of how much oil the well actually produced, it demon­strated that a drilled well could yield ample amounts of oil.

As far as we know, Drake's was the first well in the United States drilled for the sole purpose of finding and producing oil. News of the accomplishment spread rapidly and, because a ready market existed for refined rock oil, dozens of new rigs sprang up in the area to take advantage of the demand for it. Saltwater drillers formerly reluctant to drill oilwells changed their bias, and the first oil boom in the U.S. was underway. Refined rock oil soon became the primary lamp oil. And, as machines became more common, refined rock oil became a much sought after lubricant. Colonel Drake's well in Titus­ville marked the beginning of the petroleum era in the United States.

Figure 8. A replica of the original Drake well; it was built in 1945 on the original site of the well. The area is now a state park.

A PRIMER OF OILWELL DRILLING

CALIFORNIA, LATE 1800s

Reports of drilling for oil in Pennsylvania soon reached all parts of the U.S., Canada, and abroad. Interest in oilwell drilling was particularly high in California, where the popu­lation was rapidly growing. After prospectors found gold at Sutter's Mill in 1849, immigrants flooded into California. Unlike the northeastern U.S., which had plenty of coal for heating and for firing boilers and other machinery, California had none. Luckily, many oil and gas seeps, similar to those in Pennsylvania, occurred in California. Therefore, as word of Drake's successful drilling venture spread, enterprising Cali-fornians applied the technology to their fields. The first successful well was drilled in 1866. It was 550 feet (168 metres) deep and produced 15 to 20 barrels (about 2 to 3 cubic metres) a day. It was considered a great success and prompted the drilling of many more wells. Oil and gas production provided much of California's energy.

THE LUCAS WELL, 1901

Before long, almost everyone in the U.S. came to depend on oil as a plentiful and inexpensive source of energy. Individuals and companies were drilling wells all over the country. Virtually anywhere entrepreneurs could erect a rig, they were drilling an oilwell. Texas was no exception.

The area around Beaumont, Texas is flat, coastal plain country. When something interrupts the flatness, people tend to notice. Consequently, practically everyone in late nineteenth-century Beaumont knew about Big Hill. Big Hill, whose formal name was Spindletop, was a dome rising about 15 feet (4.5 metres) above the surrounding plain. Enough gas seeped out of the dome that a lighted match easily ignited it.

One person particularly fascinated by Spindletop was Patillo Higgins, a self-taught geologist who lived in the region. He was convinced that oil and gas lay below Spindletop about 1,000 feet (300 metres) deep. Around 1890, Higgins obtained land on top of the dome and, with several financial partners, drilled two unsuccessful wells. The problem was that at about 350 feet (100 metres), the bit encountered a thick sand formation that the drillers called "running quicksand."

History

The sand was so loose it caved into the drilled hole to make further drilling impossible. Drillers ran casing, just as Drake had, attempting to combat the cave-in. The formation was so bad, however, that it crushed the casing. Discouraged, but still certain that oil lay below Spindletop, Higgins put out the word that he would lease the property to anyone willing to drill a 1,000-foot (300-metre) test well.

Ultimately, an Austrian mining engineer answered Higgins's call. Named Anthony Lucas, the engineer visited Spindletop and agreed with Higgins that the hill was a salt dome surrounded by geologic formations that trapped oil and gas. After another frustrating and costly failure, Lucas finally spudded (began drilling) a new well at Spindletop on October 27,1900. He hired the Hamil brothers of Corsicana, Texas to drill the well. Aware that the running quicksand would cause trouble, the Hamils paid close attention to the mix of their drilling fluid. Drilling fluid is a liquid or a gas concoction that, when employed on the type of rig the Hamils used, goes down the hole, picks up the rock cuttings made by the bit, and carries the cuttings up to the surface for disposal. The type of rigs Drake and the early California drillers used did not require drilling fluid, which, as you will learn soon, all but doomed such rigs to extinction.

At Spindletop, the Hamils used water as a drilling fluid. They hand dug a pit in the ground next to the rig, fille d it with water, and pumped the water into the well as they drilled it. The Hamils knew from their earlier drilling experiences, however, that clear water alone wouldn't do the job: they needed to muddy it up. They were aware that the tiny solid particles of clay in the muddy water would stick to the sides of the hole. The particles formed a thin, but strong sheath— a wall cake—on the sides of the hole, much like plaster on the walls of room. The wall cake stabilized the sand and kept it from caving in (fig. 9). Legend has it that the Hamils ran cattle through the earthen pit to stir up the clay and muddy the water. Whatever they did to make mud, it worked and they successfully drilled through the troublesome sand.

Figure p. Wall cake stabilizes the hole.

IO

A PRIMER OF OILWELL DRILLING

Figure 10. The Lucas well is estimated to have flowed about 2 million gallons (8,000 cubic metres) of oil per day.

So it was that by January 1901 the new well reached about 1,000 feet (300 metres). On January 10, the drilling crew began lowering a new bit to the bottom of the hole. Suddenly, drilling mud spewed out of the well. A geyser of oil soon followed it. It gushed 200 feet (60 metres) above the 60-foot-high (18-metre-high) derrick (fig. 10). As Lucas watched the gusher from a safe distance, he estimated that it flowed at least 2 million gallons (nearly 8,000 cubic metres) of oil per day. In oilfield terms, that's about 50,000 barrels of oil per day. (One barrel of oil equals 42 U.S. gallons.) That's a lot of oil.

Thus, Spindletop's first claim to fame was that it flowed absolutely unheard of amounts of oil. Before Spindletop, a big producer flowed 2,000 barrels (320 cubic metres) per day. The Lucas well produced 2 5 times that amount. Spindletop's second claim to fame was that it showed the effectiveness of a type of rig, which, before Spindletop, drillers had not used much.

The Hamil's equipment was a rotary drilling rig; most drillers used cable-tool rigs. Unlike cable-tool rigs, rotary rigs require drilling fluid to operate, and particles in the drilling fluid prevent formations from caving. The Lucas well showed that rotary rigs could drill wells that cable-tool rigs could not. Consequently, oilwell drillers began using rotary rigs more than cable-tool rigs. Today, almost all wells are drilled with rotary rigs. Because rotary rigs are so dominant, and because cable-tool rigs drilled a lot of wells before being supplanted by rotaries, let's find out more about them.

Cable-Tool and Rotary Drilling

Not counting picks and shovels, two drilling techniques have been available since people first began making holes in the ground: cable-tool drilling and rotary drilling. Both methods originated a long time ago. Over 2,000 years ago, for instance, the Chinese drilled wells with primitive yet efficient cable-tool rigs. (They were still using similar rigs as late as the 1940s.) To quarry rocks for the pyramids, the ancient Egyptians drilled holes using hand-powered rotating bits. They drilled several holes in a line and stuck dry wooden pegs in the holes. They then saturated the pegs with water. The swelling wood split the stone along the line made by the holes.

CABLE-TOOL DRILLING

Colonel Drake and Uncle Billy used a steam-powered cable-tool rig to drill the Oil Creek site. Early drillers in California and other parts of the world also used cable-tool rigs. To understand the principle of cable-tool drilling, picture a child's seesaw. Put a child on each end of it and let them rock it up and down. This rocking motion demonstrates the principle of cable-tool drilling. To explore it further, take the kids off the seesaw and go to one end of it. Tie a cable to the end and let the cable dangle straight down to the ground. Next, attach a heavy chisel with a sharp point to the dangling end of the cable. Adjust the cable's length so that when you hold the end of the seesaw all the way up, the chisel point hangs a short distance above the ground. Finally, let go of the seesaw. Releasing the seesaw lets the heavy chisel hit hard enough to punch a hole in the ground. Pickup the seesaw and repeat the process. Repeated rocking of the seesaw makes the chisel drill a hole. The process is quite effective. A heavy, sharp-pointed chisel can force its way through a great deal of rock with every blow.

11

12

A PRIMER OF OILWELL DRILLING

Figure ii. A cable-tool rig

A cable-tool rig (fig. 11) worked much like a seesaw. Of course, cable-tool rigs had more parts and, instead of a seesaw, a cable tool had a powered walking beam mounted in a derrick. At Drake's rig, a 6-horsepower (4.5-watt) steamboat engine powered the walking beam (fig. 12). The walking beam was a wooden bar that rocked up and down on a central pivot, much like a seesaw. The derrick provided a space to raise the cable and pull the long drilling tools out of the hole. As the beam rocked up it raised the cable and attached chisel, or bit. Then, when the walking beam rocked down, heavy weights, sinker bars, above the bit provided weight to ram it into the ground. The bit punched its way into the rock. Repeated lifting and dropping made the bit drill. Special equipment played out the cable as the hole deepened.

Figure 12. A 6-horsepower (4.5-watt) steamboat engine thought to be similar to Drake V original, powers the Drake well replica.

Cable-Tool and Rotary Drilling

Cable-tool drilling worked very well in the hard-rock formations such as those in eastern U.S., the Midwest, and California. Indeed, a few cable-tool rigs are probably drilling wells somewhere in the world even now, although their use peaked in the 1920s and faded thereafter. Figure 13 pictures a 1920's cable-tool rig that drilled wells in Ohio and Pennsyl­vania until the 1950s.

In spite of cable-tool drilling's widespread use in the early days, the system had a couple of drawbacks. One was that cable-tool drillers had to periodically stop drilling and pull the bit from the hole. They then had to run a special basket, a bailer, into the hole to retrieve and remove the pieces of rock, or cuttings, the bit made. After bailing the cuttings, they then ran the bit back to bottom to resume drilling. If the crew failed to bail out the cuttings, the cuttings obstructed the bit's progress. Bailing cuttings was not a big hindrance, however, because the cable-tool system allowed the crew to do it quickly. Since the cable was wound onto a winch, or windlass, called the "bullwheel" (see fig. 11), the crew simply reeled cable on and off the bullwheel to raise and lower the bit and bailer. Reeling cable was a fast operation.

A far bigger problem than bailing, and the one that led to cable-tool drilling's demise, was that the cable-tool technique didn't work in soft formations like clay or loose sand. Clay and sand closed around the bit and wedged it in the hole. This limitation led to the increased use of rotary rigs because more wells were being drilled in places like Spindletop where cable-tool bits got stuck. The wall cake created by circulating drilling fluid prevented formations from collapsing.

Figure 13. A 1p20 's California standard cable-tool rig; it is located on the grounds of Drake Well State Park in northwestern Pennsylvania.

A PRIMER OF OILWELL DRILLING

■л ;-.• j:-a :•■'- '■:■/.':-.'■■ ■:'л :-.- ''.'А :•.- .■■:'■/. ■■.'-

Figure 14. A bit for a rotary drilling rig

Figure 7j- 7^^e drill stem puts the bit on bottom to drill.

Figure 16. Two rotary helpers lift a joint of drill pipe out of the mousehole prior to adding it to the active drill string.

ROTARY DRILLING

Rotary drilling is quite different from cable-tool drilling. For one thing, a rotary rig uses a bit that isn't anything like a cable-tool's chisel bit. Instead of a chisel, a rotary bit has rows of teeth or other types of cutting devices that penetrate the formation and then scrape or gouge out pieces of it as the rig system rotates the bit (fig. 14). Further, a rotary rig doesn't use cable to suspend the bit in the hole. Rotary crew members attach the bit to the end of a long string of hollow pipe. By screwing together several joints of pipe, they put the bit on the bottom of the hole (fig. 15). As the hole deepens, they add joints of pipe (fig. 16).

Rotating Systems

With the bit on bottom, the rig can rotate it in one of three ways. Many rigs use a machine called a "rotary table," a sort of heavy-duty turntable (fig. 17). Others rotate the bit with a top drive, a device with a powerful built-in electric motor that turns die pipe and bit (fig. 18). And, in special cases, a slim downhole motor,

Figure 17. Components in the rotary table rotate the drill string and bit.

Figure 18. A powerful motor in the top drive rotates the drill string and bit.

16

A PRIMER OF OILWELL DRILLING

Figure ic/. A downhole motor placed near the bit rotates the hit.

usually powered by drilling fluid but in some cases by elec­tricity, rotates the bit (fig. 19). A long metal housing with a diameter a little less than the hole's holds the motor. The bit screws onto the end of it.

Generally, the latest rotary rigs use a top drive to rotate the pipe and bit. However, rigs using rotary tables have been around a long time and many drilling companies still own and use them. Moreover, rotary tables are simple, rugged, and easy to maintain. Rotary rig owners often use downhole motors where they have to rotate the bit without rotating the entire string of pipe. Such situations occur when the rig is drilling a slant, or directional hole, a hole that is intentionally diverted from vertical to better exploit a reservoir. (A later chapter in this book covers directional drilling in more detail.)

Regardless of the system used to rotate the bit, the driller, the person operating the rig, allows some of the weight of the pipe to press down on the bit. The weight causes the bit's cutters to bite into the formation rock. Then, as the bit rotates, the cutters roll over the rock and scrape or gouge it out.

Fluid Circulation

By itself, rotating a bit on pipe does not get the job done. The cuttings the bit makes must be moved out of the way. Other­wise, they collect under the bit cutters and impede drilling. Recall that the crew on a cable-tool rig had to stop drilling and bail the cuttings. A rotary rig crew does not have to bail cuttings, because the rig circulates fluid while the bit drills and the fluid carries the cuttings up to the surface. As mentioned earlier, crew members attach a rotary bit to hollow pipe, instead of to braided cable. The pipe is thus a conduit: a powerful pump on the surface (fig. 20) moves fluid down the pipe to the bit and back to the surface (fig. 21). This fluid picks up the cuttings as the bit makes them and carries them to the surface where they are disposed of. The pump then moves the clean mud back down the hole.

Figure 20. Two pumps are available on this rig to move drilling fluid down the pipe; normally only one at a time is used. If more volume is needed, however, the other pump can also be put into service. (Some very large rigs have three or four pumps.)

Figure 2i. A pump circulates drilling mud down the drill pipe, out the bit, and up the hole.

i8

A PRIMER OFOILWELL DRILLING

Figure 22. Drilling mud

The fluid is usually a special liquid called "drilling mud" (fig. 22). Don't be misled by the name, however. Although the earliest drilling muds were not much more than a plain, watery mud (recall that the Hamil brothers supposedly filled a pit with water and ran cattle through it to make it muddy), drilling mud can be a complex blend of materials. What's more, sometimes it isn't a liquid, which is why a better name for drilling mud is "drilling fluid." A fluid can be a liquid, a gas, or a combination of the two.

As you now know, one advantage of a rotary rig is that workers do not have to worry about soft formations caving in on the bit and sticking it. Just as the Hamils prepared the mud to stabilize the hole at Spindletop, today's drillers also pre­pare, or condition, the drilling mud to control formations. Besides keeping boreholes from caving in, circulating mud performs several other important functions. For example, it moves the cuttings away from the bit and cools and lubricates it. It also keeps formation fluids from entering the hole and blowing out to the surface. Indeed, circulating drilling fluid has so many advantages that cable-tool drilling is virtually obsolete. Although companies may use a cable-tool rig in a few special cases, more often they use rotary rigs. Several kinds of rotary rig are available for drilling on land and offshore. Let's look at the major types.

Many kinds of rotary drilling rigs are available, particu­larly offshore where the marine environment plays an important role in rig design. Two broad categories of rig are those that work on land (fig. 23) and those that work offshore (fig. 24). Some experts like to create a third category: rigs that work in inland waters (fig. 25). Inland rigs usually drill in lakes, marshes, and estuaries, places that are neither land nor offshore, places where, as one wit put it, "it's too wet to plow and too muddy to drink." For our purposes, though, dividing rotary rigs into land and offshore types is acceptable, because inland rigs also drill in water, even if it is shallow.

Rotary Rig Types

F igure 24. An offshore rig

Figure 23. A land rig

Figure 25. An inland barge rig

2О

A PRIMER OF OILWELL DRILLING

Table 1

Land rigs classified by drilling depth

Rig Size	Maximum Drilling Depth, Feet (Metres)
Light duty	3,000-5,000 (1,000-1,500)
Medium duty	4,000-10,000 (1,200-3,000)
Heavy duty	12,000-16,000 (3,500-5,000)
Very heavy duty	18,000-25,000+ (5,500-7,500+)

LAND RIGS

Land rigs look much alike, although details vary. A major difference is their size, and size determines how deep the rig can drill. Well depths range from a few hundred or thousand feet (metres) to tens of thousands of feet (metres). The depth of the formation that contains, or is believed to contain, oil and gas controls well depth. Classified by size, land rigs are light duty, medium duty, heavy duty, and very heavy duty. Table 1 arranges them according to this scheme and shows the depths to which they can drill.

Keep in mind, though, that a rig can drill holes shallower than its maximum rated depth. For example, a medium-duty rig could drill a 2,500-foot (750-metre) hole, although a light-duty rig could also drill it. On the other hand, a rig cannot drill too much beyond its rated maximum depth, because it cannot handle the heavier weight of the drilling equipment required for deeper holes.

Another feature land rigs share is portability. A rig can drill a hole at one site, be disassembled if required, moved to another site (fig. 2 6), and be reassembled to drill another hole. Indeed, land rigs are so mobile that one definition terms them "portable hole factories." The definition sounds odd, but it is accurate.

Figure 26. То move a rig, crew members disassemble it and move it piece by piece.

Rotary Rig Types

21

MOBILE OFFSHORE RIGS

A widely used offshore drilling rig is a mobile ojfshore drilling unit, or MOD U, for short (pronounced "mow-du"). Another is a platform. Although drilling occurs from platforms, com­panies mainly employ them on the producing side of the oil and gas business. This book concentrates on drilling, so it does not cover platforms. However, more information about platforms is available in the PETEX publication, A Primer of Ojfshore Operations.

MODUs are portable; they drill a well at one offshore site and then move to drill another. MODUs are eitherfloaters or bottom-supported. When drilling, floaters work on top of, or slightly below, the water's surface. Floaters include semisub-mersibles and drill ships. They are capable of drilling in waters thousands of feet (metres) deep. MODUs that contact the ocean bottom and are supported by it are bottom-supported. Bottom-supported units include submersibles and jackups. Submersibles are further divided into posted barges, bottle types, inland barges, and arctic. Generally, bottom-supported rigs drill in waters shallower than floaters. Table 2 lists MODUs.

Table 2

Types ofMODU

M obile Offshore Drilling Units (MODUs)

Bottom-Supported	Floating
Units	Units
Submersibles	Semisubmersibles
Posted barges	Drill ships
Bottle types
Arctic types
Inland barges
Jackups

Bottom-Supported Units

Submersibles and jackups contact the seafloor when drilling. The lower part of a submersible's structure rests on the sea-floor. In the case of jackups, only the legs contact the seafloor.

Submersibles

A submersible MODU floats on the water's surface when moved from one drilling site to another. When it reaches the site, crew members flood compartments that submerge the lower part of the rig to the seafloor. With the base of the rig in contact with the ocean bottom, wind, waves, and currents have little effect on it.

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A PRIMER OF O1LWELL DRILLING

Figure 27. The first MODU was a posted-barge submersible designed to drill in shallow waters.

Posted-Barge Submersibles

The first MODU was a submersible. It drilled its initial well in 1949 off the Gulf Coast of Louisiana in 18 feet (5.5 metres) of water. It was a posted-barge submersible—a barge hull and steel posts (columns) supported a deck and drilling equipment (fig. 2 7). It proved that mobile rigs could drill offshore. Posted barges are now virtually obsolete, however, because newer and better designs have replaced them.

Bottle-Type Submersibles

About 1954, drilling moved into water depths beyond the posted barge's capabilities, which was about 30 feet (9 metres).

Rotary Rig Types

23

So, naval architects designed bottle-type submersibles. A bottle-type rig has four tall steel cylinders (bottles) at each corner of the structure. The main deck lies across several steel supports and the bottles. The rig and other equipment are placed on the main deck. When flooded, the bottles cause the rig to submerge to the seafloor (fig. 28).

In their heyday in the early 1960s, the biggest bottle-type submersibles drilled in 150-foot (45-metre) water depths. To­day, jackups have largely replaced them; jackups are less expen­sive to build than bottle-types and can drill in deeper water. Rather than completely scrap their bottle types, however, rig owners modified some of them to drill as semisubmersibles, which are still in use. (Semisubmersibles are covered shortly.)

Arctic Submersibles

A special type of submersible rig is an arctic submersible. In the arctic, where petroleum deposits lie under shallow oceans such as the Beaufort Sea, oil companies knew that jackups and conventional barge rigs would not be suitable. During the arctic winter, massive chunks of ice form and then move with currents on the water's surface. Called "floes," these moving ice blocks exert tremendous force on any object they contact. The force is great enough to destroy the legs of a jackup or the hull of a conventional ship or a barge.

Arctic submersibles therefore have a reinforced hull, a caisson. One type of caisson has a reinforced concrete base on which the drilling rig is installed (fig. 29). When the sea is

Figure 28. When flooded, the bottles cause a bottle-type submersible to submerge to the seafloor.

Figure 29. A concrete island drilling system (CIDS) features a reinforced concrete caisson.

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A PRIMER OF OILWELL DRILLING

ice-free in the brief arctic summer, boats tow the submersible to the drilling site. There, workers submerge the caisson to the sea bottom and start drilling. Shortly, when ice floes form and begin to move, the arctic submersible's strong caisson hull deflects the floes, enabling operations to continue.

Inland Barge Rigs

Figure 30. Drilling equipment is placed on the deck of a barge to drill in the shallow waters of bays and estuaries.

A fourth submersible is an inland barge rig. It has a barge hull—a flat-bottomed, flat-sided, rectangular steel box. The rig builder places a drilling rig and other equipment on the barge deck (fig. 30). Inland barge rigs normally drill in marshes, bays, swamps, or other shallow inland waters. By definition, barges are not self-propelled; they have no built-in power to move them from one site to another. Therefore, boats tow them to the drilling location. When being moved, the barge floats on the water's surface; then, when positioned at the drilling site, the barge is flooded so that it rests on the bottom ooze. Since they often drill in swampy shallow waters, drilling people often call inland barges "swamp barges."

Rotary Rig Types

Jackups

A j ackup rig is a widely used mobile offshore drilling unit. It floats on a barge hull when towed to the drilling location (fig. 31). Most modern jackups have three legs with a triangular-shaped barge hull; others have four or more legs with rectangular hulls. A jackup's legs can be cylindrical columns, somewhat like pillars (fig. 32), or they can be open-truss structures, which resemble a mast or a derrick (fig. 33).

Figure 31. Four boats tow this j ackup to its drilling location.

Figure 52. A j ackup rig with four columnar legs

Figure 33. Thisjackup has open-truss legs.

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Figure 34. The hull ofthis jackup is raised to clear the highest anticipated waves.

Whether it has columnar or open-truss legs, when a jackup's barge hull is positioned on the drilling site, the crew jacks down the legs until they contact the seafloor. They then raise, or jack up, the hull above the height of the highest anticipated waves (fig. 34). The drilling equipment is on top of the hull. The largest jackups can drill in water depths up to about 400 feet (about 120 metres), and are capable of drilling holes up to 30,000 feet (10,000 metres), or close to 5У2 miles, deep.

Rotary Rig Types

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Floating Units

Floating offshore drilling rigs include semisubmersibles and drill ships. Semisubmersibles, because of their design, are more stable than drill ships. On the other hand, drill ships can carry more drilling equipment and supplies, which often make them the choice in remote waters.

Semisubmersibles

Most semisubmersible rigs have two or more pontoons on which the rig floats. A pontoon is a long, relatively narrow, and hollow steel float with a rectangular or round cross section (fig. 35). When a semisubmersible is moved, the pontoons contain mostly air so that the rig floats on the water's surface. In most cases, towboats then tie onto the rig and move it to the drill site. However, some semisubmersible rigs are self-propelled—they have built-in power units that drive the rig from one site to another.

Figure 35. A semisubmersible rig floats on pontoons.

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A PRIMER OF OILWELL DRILLING

Figure 36. The pontoons of this semisubmersible float a few feet (metres) below the water's surface.

Figure 3 7. The main deck of a semisub­mersible is big.

Semisubmersibles get their name from the fact that in the drilling mode the rig is not submerged to the point where its pontoons contact the sea bottom. Instead, rig personnel care­fully flood the pontoons to make them submerge only a few feet (metres) below the water's surface (fig. 36). Thus, the rig is "semisubmerged." (If the pontoons contacted the sea bottom, the rig would be "submerged.") With its pontoons submerged below the waterline, waves do not affect the rig as much as they do when it floats on die surface. A semisubmersible rig therefore offers a more stable drilling platform than a drill ship that drills while floating on the water's surface.

Large cylindrical or square columns extend upward from the pontoons. The main deckrests on top of the columns. The main deck of a semi is big (fig. 37). Semis (short for semi­submersibles) often use anchors to keep them on the drilling station. Workers release several large anchors from the deck of the rig. An anchor-handling boat crew sets the anchors on the seafloor.

Besides being good rough-water rigs, semis are also capable of drilling in water thousands of feet (metres) deep. While many semis work in water depths ranging from 1,000 to 3,500 feet (300 to 1,000 metres), the latest are capable of drilling in water depths of 8,000 feet (2,500 metres). Semis can drill holes up to 30,000 feet (10,000 metres) deep. Indeed, semisub­mersibles are among the largest floating structures ever made.

Rotary Rig Types

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The biggest ones soar to over 100 feet (30 metres) tall and their main decks can be almost as big as a football field—that's 3,000 square yards (2,500 square metres).

Drill Ships

A drill ship is also a floater (fig. 3 8). Drill ships are very mobile because they are self-propelled and have a streamlined hull, much like a regular ocean-going ship. A company may there­fore choose a drill ship to make hole in remote waters, far from land. A drill ship is a good choice for drilling remote locations. For one thing, it can move at reasonable speeds under its own power. Secondly, its ship-shaped hull can carry a large amount of the equipment and material required for drilling. Frequent resupplying from a shore base is therefore not necessary.

While many drill ships operate in water depths ranging from 1,000 to 3,000 feet (300 to 1,000 metres), the latest can drill in water depths approaching 10,000 feet (3,000 metres), or nearly 2 miles (3.2 kilometres). They can drill holes over 30,000 feet (10,000 metres) deep. These big drill ships are more than 800 feet (250 metres) long, which is almost as long as three football fields laid end to end. They measure some 100 feet (30 metres) wide, or a little wider than a football field.

Figure 38. A drill ship

ЗО A PRIMER OF OILWELL DRILLING

Their hulls tower more than 60 feet (18 metres) high, about that of a six-story building.

Anchors keep some drill ships on station while drilling, but those drilling in deep water require dynamic positioning. Dynamically positioned drill ships use computer-controlled thrusters and sophisticated electronic sensors. Thrusters are power units with propellers that the builder mounts fore and aft on the drill ship's hull below the waterline. Once the dynamic positioning operator tells a computer exactly where it should keep the rig positioned, the computer, using infor­mation transmitted by the sensors, automatically controls the thrusters. The thrusters offset wind, wave, and current forces that would move the rig away from the desired position.

Whether on land or offshore, and whether large, me­dium, or small, all rigs require personnel to operate them. The people who drill wells usually work for a company whose business involves drilling, either directly or indirectly. So, let's look next at companies involved in drilling and the personnel who do the work.

People who work for companies involved in drilling work all over the world. They drill wells on land and ice, in swamps, and on waters as small as lakes or as large as the Pacific Ocean. Drilling is demanding; it goes on 24 hours a day, 7 days a week, in all types of weather. Moreover, drilling is complex; so complex that no single company is diverse enough to perform all the required work. Consequently, many companies and individuals are involved. Companies include operating companies, drilling contractors, and ser­vice and supply companies.

People and Companies

OPERATING COMPANIES

An operating company, or an operator, is usually an oil company, a company whose primary business is working with oil and gas, or petroleum. An operating company may be an indepen­dent or a major. An independent company may be one or two individuals or it may have hundreds of employees. Major companies, such as ExxonMobil, BP Amoco, or Shell, may have thousands of employees. Besides size, another difference between an independent and a major is that, in general, an independent only produces and sells crude oil and natural gas. A major, on the other hand, produces crude oil and natural gas, transports them from the field to a refinery or a plant, refines or processes the oil and gas, and sells the products to consumers.

Whether independent or major, an operator must acquire the right to drill for and produce petroleum at a particular site. An operating company does not usually own the land or the minerals (oil and gas are minerals) lying under the land. It therefore has to buy or lease the rights to drill for and produce oil and gas from the landowner and the mineral holder. Individuals, partnerships, corporations, or a federal, state, or local government can own land and mineral rights. The operator not only pays the landowner a fee for leasing, it also pays the mineral holder a royalty, which is a share of the money made from the sale of oil or gas.

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32 A PRIMER OF OILWELL DRILLING

DRILLING CONTRACTORS

Drilling is a unique undertaking that requires experienced personnel and special equipment. Most operating companies therefore find it more cost effective to hire expertise and equip­ment from drilling companies than to keep the personnel and equipment under their own roof. So, almost everywhere in the world, drilling contractors do the drilling.

A drilling contractor is an individual or a company that owns from one to dozens of drilling rigs. The contractor hires out a rig and the personnel needed to run it to any operator who wishes to pay to have a well drilled. Some contractors are land contractors—they operate only land rigs. Others are offshore contractors—they operate only offshore rigs. A few contractors operate rigs that drill both on land and offshore. The contractor may have different sizes of rigs that can drill to various depths. A drilling contracting company may be small or large; it may own rigs that drill mainly in a local area or it may have rigs working all over the world.

Regardless of its size, a drilling company's job is to drill holes. It must drill holes to the depth and specifications set by the operating company, who is also the well owner. An operating company usually invites several contractors to bid on a job. Often, the operator awards the contract to the lowest bidder, but not always. Sometimes a good work record may override a low bid.

DRILLINC CONTRACTS

The operator usually sends a proposal to several drilling contractors. The proposal describes the drilling project and requests a bid. The contractor then fills out the proposal, signs it, and sends it back to the operator. If the operating company accepts the bid, it becomes a contract between the operator and the drilling company. This signed agreement clearly states the services and supplies the contractor and the operator are to provide for a particular project.

The International Association of Drilling Contractors (IADC) supplies popular contract forms (fig. 39). IADC is an organization whose membership is made up of drilling con­tractors, oil companies, and service and supply companies with an interest in drilling. Headquartered in Houston, Texas, and with offices throughout the world, IADC provides many services to its members, not only in the U.S., but also in other parts of the globe. Its mission is "to promote a commit­ment to safety, to preservation of the environment, and to advances in drilling technology."

F igure 39. An IADC drilling bid form; when signed by all parties, it becomes a contract.