Wednesday, June 20, 2012
Posted by Ankit Chugh on 9:57 AM
Drilling rigs like these bore or drill holes into the earth. Usually they drill to find oil or gas. They work both on land and offshore. Some are big and some are relatively small. Big rigs drill very deep holes, 20000 feet (ft), 3000 meters (m) or more; small rigs may only drill to a few thousand ft or meters. People in the oil land describe groups of rigs into 6 basic types: Land, Jack up, Platform, Submersible, Semi-Submersible and Drill Ship.
A land rig drills on dry land. There is the most common rig. Light duty rigs drill holes from 3000-5000ft, or 1000-1500m; Medium duty rigs drill to depth ranging from about 4000-10000ft or 12000-3000m; Heavy duty rigs drill holes from about 12000-16000ft deep or 3500-5000m; Ultra-heavy duty rigs drill holes from about 18000-25000ft or more (5500-7500m or more).
Crew members can move land rigs on trucks, tractors, trailers, barges, helicopters, heavy rolling gear, skids and in rare cases, on specialized air-pressurized equipment.
Small light duty rigs are pretty simple to move. Ultra-heavy land rigs can be difficult to move.
A Jack up rig, drills offshore wells. It has legs that support deck and hole. When positioned over the drilling site, the bottom of the legs rest on the sea floor. Jack up rigs can drill in water depth ranging from a few feet or meters up to more than 400ft, over 120 meters. Boats to a Jack Up rig to a location with its legs up. Once the rig up crew gets the legs firmly positioned on the bottom of the ocean, they can adjust the level of the deck and the hole-height.
A platform rig is a non-mobile offshore structure, that is once built, it never moves from the drill site. Companies drill several wells from the platform. Platform rigs can be Tender-Assisted Rig. The tender floats next to the rigid platform, which is firmly pent to the sea floor. Many platform rigs do not have a tender; they’re so large that they’re self-contained. Big platform rigs include the Steel-Jacket Platform, the Caisson Type, and the Concrete Gravity Type.
In deep water, rig builders have to make platforms that yield to water and wind movements. Two Compliant Platform Rigs are the Guyed-Tower and the Tension-Leg.
A submersible rig rests on the sea floor when it is drilling. Workers flood compartments that cause the rig to submerge and rest on the bottom. When ready to move, workers remove the water from the compartments, this makes the rig float. Boats can then tow the rig to the next site. Rig builders design submersibles to drill in shallow water and in water up to about 175 ft deep, a little lower 50 meters.
Submersible drilling rigs include the Posted Barge Submersible, Bottle-Type Submersible, and the Arctic Submersible.
A Semi-Submersible rig is a floating offshore drilling rig. It has Pontoons and Columns. When flooded with water, the Pontoons cause the unit to partially submerge to a predetermined depth. The working equipment is assembled on deck. On the drill site, workers can either anchor the rig to the sea floor or use a system of thrusters and positioners to keep the rig over the hole. Here, they have it anchored. Crew members mount the wellhead and blow-out preventers on the ocean floor. Special hollow pipe called riser pipe connects the top of the blowout preventer to the rig.
In some cases, the crew uses thrusters to keep the rig over the hole, called Dynamic Positioning. The thrusters, which are connected to an onboard computer, keep the rig in position. Some Dynamically Positioned Semi-Submersibles can drill in water depths of more than 7500ft, or over 2200 meters. When keeping a rig over the hole, drilling crews use the term “On-Station”. Here is a semi-submersible rig loaded on a special carrier. The carrier thus allows moving the rig far distance over the ocean. For shorter moves, the rig owner tows the rig to the drill site, or, some semi-submersibles are self-propelled.
A drill ship is a self-propelled floating offshore drilling unit. It usually uses a sub sea blowout control system similar to the one on the semi-submersible.
Many pieces of equipment make up a rotary drilling rig. Part of it is on surface and part of it is underground, or subsurface. All the equipment has one main purpose: to put a bit at the bottom of the hole, or it can drill or make hole. To put the bit on the bottom, rig crew members screw it into a special pipe. The pipe is called the “Drill String”. Crew members lower the drill string and attach a bit into the hole. For the bit to drill, surface rig equipment has to rotate it, unless it is rotated by a mud motor. Equipment also has to put weight on it to force the bit’s teeth, or cutters into the formation. As the bit rotates, a circulating fluid has to take the drill cuttings away from the bit, otherwise, the hole will clog up. The fluid which circulates is called drilling mud.
To impart rotary motion to the drill string so that the bit can turn, either a top drive or a Kelly & rotary table system is used. Power is transferred from the surface down hole, via the drill string.
Some rigs rotate the drill string with a top drive unit. Top drives are expensive but very efficient. Crew members can add drill pipe and joints to the drill string very quickly and safely and they can drill the well more efficiently with less chance of sticking the drill string in the hole as compared with the Kelly & rotary table. A powerful motor turns the drive shaft which is connected to the top drive. Crew members make up or attach the drill string to the drive shaft. The drive shaft turns the drill string and bit. Notice that the drill string go through an opening in the rotary table, the table does not, however, rotate.
A link system suspends the top drive unit from the rig’s travelling block. Drill mud enters the unit through the gooseneck to the rotary hose, a flexible line that conducts drilling mud from the pump. A motor and a gear box power the main drive shaft; the crew makes up the drill string to the drive shaft. The built-in inside blowout preventers, IBOP or safety valve keeps fluids from back flowing up the drill string when the driller closes it. The crew uses the Torque Wrench assembly to make up and break out (connect & disconnect) the drill string. The elevator links suspend the elevator; the rig crew latches the elevator around the drill string to allow the top drive unit to lift it up or down.
A Kelly, a Kelly Drive Bushing, a Master Bushing and a Rotary Table rotate the drill string and bit on some rigs. The Kelly is a heavy tubular device; it usually has either 4 or 6 sides, that is it either has a square or hexagonal cross section. Square kellys are less expensive than hexagonal ones, but the hex kellys are stronger. So rigs drilling deep holes often use them. Whether four or six-sided, crew members attach or make up the Kelly to the top joint of pipe in the drill string.
The Kelly, four-sided or square as an example, moves through a square opening in the Kelly drive bushing. The Kelly drive bushing meets with the master bushing, which the rotary table turns. This rotates the entire drill string and attached bit. The Kelly moves down as the hole deepens.
There’re many components which make up the drill string as shown in this graphic.
Drill pipe is strong but relatively light weight pipe. Crew members attach it to a top drive or Kelly. Drill pipe forms the upper part of the drill string. Usually the drill pipe rotates, which also rotates the bit. Each section of pipe is called a joint. Crew members screw together or make up several joints and put them into the hole as the bit drills. Drill pipe as well as other tubulars can be specified according to these characteristics: Diameter, Grades or Strength, Weight of steel, Length. The diameter, weight and strength used depends on the size of the hole, the depth of the well and the well properties. Here is a typical oil field tally book, many of these have sections in them which show standard drill pipe specifications.
Drill pipe comes in three ranges of length: range one is 18-22 ft or 5.5-6.7 m; range two is 27-30 ft, or 8.2-9.1 m; and range three is 38-45 ft, or 11.6-13.7m. The most common length is range two, 27-30 ft, or 8.2-9.1 m. Since a hole may be thousands of ft deep, crew members may connect together hundreds of joints of pipe.
Drill pipe diameter can be as small as 23/8 inches or 60.3 mm. This size weighs 4.85 pounds per foot or 7.22 kg per meter. It can be as large as 65/8 inches, or 168.3 mm. This pipe weighs about 27.70 pounds per foot or 41.21 kg per meter. However, 5 in (127mm) drill pipe is one of the more common sizes. It weighs 191/2 pounds per foot or 29.01 kg per meter.
Normal drill pipe grades are E75, X95, G105 and S135. S135 is the strongest.
The rig crew makes up or connects drill pipe using threaded sections at each end of the drill pipe. These threaded sections are tool joints. The female tool joint is the box end at the drill pipe; the male tool joint is the pin end. Tool joints come in several sizes and types.
Tool joints threads are rugged because the crew makes them up and breaks them out over and over during the drilling process. But they have to take care not to damage them. Proper care and handling of drill pipe and other oil field tubulars can prevent corrosion later on the life of the well.
Crew members make up a heavy walled drill pipe in the drill string below the drill pipe. HWDP (often called heavy weight drill pipe) is made up between the drill pipe and drill collars. HWDP is used to provide a transition between the limber drill pipe and the drill collars, which are quite stiff. They use a HWDP reduces the stress that stiff drill collars put on the drill string, as a result, HWDP reduces fatigue on the regular drill pipe. It also helps keep the drill pipe in tension and may sometimes provide weight on the bit, just as drill collars do, especially in directional drilling. Heavy walled DP (or Heavy weight DP) has thicker walls and longer tool joints than standard DP. The longer tool joints reduce wear on the pipe’s body. They keep the body away from the side of the hole. The wear pad also prevents wear; it keeps the middle of the pipe’s body away from the side of the hole.
Spiral HWDP is another type of HWDP. Spiral HWDP has a spiral groove in the pipe’s body. Regular HWDP has no groove, but spiral HWDP has no wear pad. When spiral HWDP contacts the side of the hole, only a small part of the pipe body actually touches it. In fact, only the part of the pipe body between the spiral grooves touches it. The groove doesn’t touch the wall of the hole, thus reducing the surface contact area. Reducing the surface contact area helps prevent the pipe from sticking.
How much weight depends on the type of formation and the size and type of bit, where it can be several thousand of pounds. DCs are normally 30-31 ft (9.4-9.5m) long and have a threaded female connection at on end and a treaded male connection at the other end. It is an interesting observation that in the drilling business tubular equipment diameters and hole-diameters are almost always measured in inches but lengths are usu. measured in meters or ft.
Some DCs are slick. They have a smooth wall; some have spiral groove machined into the wall. The rig uses slick collars under normal circumstances. The rig uses spiral collars when drilling in formations where the collars may stick to the wall of the hole.
Large diameter collars are fairly close to the diameter of the well bore. Under certain circumstances, they can contact the wall of the well bore and get stuck. The spiral in the DC helps prevent the DC from sticking to the wall by reducing its contact area.
Crossover subs go on the drill string between the DP & DCs and other points. A crossover sub has a special box and pin threads. Manufacturers design them to join parts of the drill string that have different thread designs. For example, a drill pipe’s pin may not screw directly into a drill collar’s box so crew members make up a crossover sub in the last joint of the DP where joins the first DC’s joint. The crossover sub’s box threads match the DP’s pin threads and the crossover sub’s pin threads match the DC’s box threads. These matching treads allow crew members to join the drill pipe string to the drill collar string. Drilling rigs typically have a large variety of crossover subs.
Crew members often make up reamers and stabilizers in the drill collar string. Usually they place one or more at various points on the drill collar string near the bottom.
Reamers and stabilizers hold the DC off the wall of the hole to prevent wear on the collars,but even more important, reamers and stabilizers help guide the bit in the direction that should drill.
Reamers have cutters on rollers that actually cut the rock they contact. Stabilizers have blades that touch the wall of the hole but do not cut it.