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Transcription of Drill String Components
There are many components that make up the drill string as shown in this graphic. Drill pipe is a strong but relatively lightweight 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 in to the hole as the bit drills.
Drill pipe as well as other tubulars can be specified according to these characteristics;
- Diameter
- Grades or strength
- Weights 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 to 22ft, or 5.5 to 6.7m. Range two is 27-30ft or 8.2 to 9.1 meters. And range three is 38 to 45ft or 11.6-13.7m. The most common length is range two, 27 to 30ft or 8.2 to 9.1m.
Since a hole may be thousands of feet deep, crew members may screw together hundreds of joints of pipe. Drill pipe diameter can be as small as 2 and 3/8in or 60.3mm. This size weight 4.85 pounds per foot or 7.22kg per meter. It can be as large as 6.58 in or 168.3mm this pipe weighs about 27.7 pounds per foot or 41.21 kg per meter. However 5 in 127mm drill pipe is one of the more common sizes. It weighs 19.5 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 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 of the drill pipe. The male tool joint is the pin end. Tool joints come in seven sizes and types. Tool joint threads are rugged because the crew makes them up and breaks them out over the drilling process.
But they have to take care not to damage them. Proper care and handling of drill pipe and other royal field tubular can prevent corrosion in the life of the well. Crew members make up heavy wall drill pipe in the drill string below the drill pipe. Heavy walled drill pipe, also called heavy weight drill pipe, is made up between the drill pipe and the drill collars. Heavy wall drill pipe is used to provide a transition between the limber drill pipe and the drill collars which are quite stiff. The use of heavy walled drill pipe reduces the stress that stiff drill collars put on the drill string.
As a result, heavy walled drill pipe reduces the fatigue on the regular drill pipe. It also helps keep the drill pipe in tension. And may sometimes provide weight on the bit just like drill collars do just like in directional drilling. Heavy walled drill pipe or heavy weight drill pipe has thicker walls and longer tool joints than standard drill pipe. The longer tool joints reduce wear on the pipes body, they keep the body away from the side of the hole. The wear pad also prevents wear, it keeps the middle of the pipes body away from the side of the hole.
Spiral heavy walled drill pipe is another type of heavy walled drill pipe. Spiral heavy walled drill pipe has a spiral grove in the body. Regular heavy wall drill pipe has no grove but spiral heavy walled pipe has no drill pad. When spiral heavy walled pipe 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 grove touches it. The groove does not touch the wall of the hole thus reducing the surface contact area. Reducing the surface contact area helps prevent the pipe from sticking.
Crew members make up drill collars at the bottom of the drill string. Drill collars have thick walls and are very heavy. They put weight on the bit to make the bit’s cutters bite into the rock and drill. Drill collars range in diameter from three to twelve inches or 76.2 to 304.8 mm. they range in weight from 650 to 11500 pounds or 300 to 5100 kilograms. This particular 6 in drill collar weighs about 2700 pound, 1225kilograms. Since the crew usually installs several drill collars, you can see that a bit requires a lot of weight to drill properly. How much weight depends on the type of formation and the size and type of bit and it can be several thousands of pounds.
Drill collars are normally 30 or 31 ft (9.14 or 9.45 meters long) and have a threaded female connection at one end and threaded male connection at the other end. Its an interesting observation that in the drilling business, tubular equipment diameter and hole equipment diameters are almost always measured in inches but lengths are usually either measured in meters or feet. Some drill collars are slick, the have a smooth wall. Some have a spiral grove machined into their wall. The rig uses slick collars under normal circumstances. The drill uses spiral collars when drilling in formations where 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 drill collar helps prevent the collar from sticking to the wall by reducing its surface contact area.
Cross-over stubs go into the drill string between the drill pipe and drill collars and at other points. The crossover sub has a special box and threads and manufacturer design them to join parts of the drill string that have different thread designs. For example, a drill pipe’s pin is not screwed directly into a drill collar’s box, so crew members make up a cross over sub in the last joint of the drill pipe where it joins the first drill collar joint. The cross over sub’s box threads, match the drill pipe’s pin threads. And the cross over sub’s pin threads match the drill collar’s box threads. These matching threads 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 in various points of the drill collar string near the bottom. Reamer and stabilizers hold the drill collars 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 it should drill.
Reamers have cutters on rollers that actually cut the rock they contact. Stabilizers have blades that touch the walls of the hole but do not cut it. Notice the lower portion of the drill string. It includes the bit, drill collars, stabilizers or reamers and heavy walled drill pipe. Crew members call this part of the drill pipe, the bottom hole assembly or BHA for short. They can make up many different BHA’s, which one depends on the type of formation whether the rig’s drilling a straight or a directional hole and so on. The pipe rack is not part of the drill string but plays an important supporting role. The rig crew cannot put drill pipe and collars on the ground or a deck, dirt and debris would ruin them, so they store them on the pipe rack. They also clean and inspect the drill string in other tubular or pipe on the rack.