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.
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
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
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
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.
The World According to Wayne: Drill Pipe Selection
January 3, 2011