There are a number of reasons that drillers have the need to buy another string of drill pipe. Whatever the reason, when selecting new or used pipe, an understanding of what the pipe actually is and does can help.

There are a number of reasons that drillers have the need to buy another string of drill pipe. It may be a new rig, or a used one that didn’t come with pipe, or they lost their drill string in a hole, or that the pipe they have now is just not doing the job. Whatever the reason, when selecting new (or used) pipe, an understanding of what the pipe actually is, and does, helps. Drill pipe does a lot more than just connect the bit to the rig.

Most drill pipe today uses what are called rotary shouldered connections. This means that there is a flat face on the tool joint that transmits the torque, and provides the seal. The diameter and width of this face dictates the make-up torque and the maximum torque that the string will stand.

These joints usually are attached to the tube of the pipe by one of three methods. The strongest, and most expensive, is inertia welding. In this method, the tube of the pipe is rotated against the tool joint (rotating in the opposite direction), and forced together until the metal is fused by heat and pressure. It makes a very strong joint, and is inherently straight. It also has the ability to fuse dissimilar metals, such as steel tool joints to aluminum tube.

Welded tool joints also are very common, and can be very strong, but, while they save money, there can be some problems. They must be applied by a very experienced welder, who has done this before. Straightness is very important, so welding jigs must be used. Welding also may affect the heat treatment of the joint or the tube, and should be taken into account. This is very important when using high-grade steel.

An older, though still fairly common, method of tool joint attachment is screw-on threads. The advantage is the ability to change a tool joint quickly in the event of damage, but there also are some serious disadvantages. The threaded end of the tube will have a reduced cross-section, and thus be weak in both torque and tensile strength. These joints generally are seen only on small, straight-hole rigs.

The next consideration is upset. This means that the tool joint usually will have a greater cross-sectional area than the body of the pipe. Among the reasons for this: The tool joint should be designed so that when new, the joint is stronger than the pipe. This allows for normal wear, and allows redressing when necessary. It also transfers the bending moment to the body of the pipe. The method of increasing the cross-sectional area of the tool joint usually is by either internal upset, external upset, or both. External upset is most common, and generally a better choice, if you have the room in the hole. Internally upset pipe often is very restricted as to flow, and requires more pump horsepower to flow well. A common example of this in N rod. It has a 23⁄8-inch O.D. but only 7⁄8-inch I.D. On deep holes, the pump pressure goes through the roof.

Consideration should be given to the body of the pipe, as to weight, grade and yield strength. Obviously, the pipe must be strong enough to drill as deep as you need to go, and have a significant safety factor built in to handle emergencies. The best way to make pipe stronger is to use more steel. The strength of drill pipe usually is expressed in terms of yield point. Since steel is elastic, and can be stretched like a spring and spring back to its original shape, the yield point is the point at which the steel does not spring back – it stretches or twists, and stays there. This is not the failure point, but the steel is permanently damaged, and will fail sooner or later at this point. Because adding more steel obviously takes room, the pipe will have either a larger O.D. or a smaller I.D., and added weight, this might not be possible. It may be too heavy for the rig, or it might be too big to work in the hole. In this case, better grades of steel are required. The steel industry and the oilfield have developed some remarkably strong grades of pipe, and some remarkable prices to go with them. Knowing the grade of pipe you are buying will tell you what you can do with it and what to expect from it.

Another important factor is stiffness. Larger O.D. pipe is inherently stiffer. This is good for straight-hole drilling, and has the added benefit of producing higher annular velocities. It is not designed for directional drilling, and will resist “going around corners.” For this use, consider externally upset pipe; it is more flexible.

Once the pipe has been selected, some consideration should be given to handling tools. Slips, elevators, lift plugs, tongs and other things have an effect on your pipe. They all must fit the pipe. Slips must have enough gripping area to keep from crushing the pipe, and tongs should only be used on the tool joint area, not the tube – they will crush it. Elevators should be the type designed for the pipe; I have seen some pretty bad accidents happen when drillers used square-shoulder elevators on bottleneck pipe.

One common problem I have seen is drillers complaining about having trouble breaking out the pipe when coming out of the hole. While this can be caused by extreme drilling torque, I have found two other, much more common causes. The commonest cause is improper make-up. A lot of drillers just make up the pipe hand-tight, and let the rotary torque it up. This is almost certain to over-torque the pipe because of the inertia of the drill string slamming the pipe together. Pretty soon, you see them coming out of the hole, heating each joint with a torch to get it to break out. Whatever heat-treated properties the steel had before now are gone. Proper make-up torque is important to the life of the pipe.

Another common problem is proper lubricant. Correct application of pipe dope is critical to proper make up and life of your pipe. I’ve seen drillers run out of dope and just keep on drilling. I guess they figured they were on a dope-free rig.

One last consideration for drill pipe is critical speed. This is a rotary speed that causes a whipping action in the drill pipe, and certainly will damage the pipe and probably the hole. It is easy to spot; torque goes way up, and the vibration can’t be missed at the surface. It is caused by a combination of pipe cross-sectional area, rotary speed and depth, and should be avoided. It usually is noticed in deep holes with slim-hole pipe, but can be observed in many holes if the rotary speed is too high.

When buying drill pipe, the choice between new and used always is a factor. If you are buying used pipe, make sure it is inspected and graded. Ungraded pipe is a poker hand with your money – and not a winner. Double white band is premium pipe; it is essentially new. White band pipe is within the tolerances of good, used pipe, and will last a long time (and save you some money). Yellow band pipe essentially is a fence post looking for a home. It may be round, straight and look something like pipe – it might even screw together – but it usually is junk. 
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