A long iron tube with threads on the end—how complicated can a piece of drill pipe be? The pipe itself isn’t, but selecting the right kind of drill pipe for your rig and the working conditions can be.

First and foremost, select a pipe designed for your horizontal directional drilling (HDD) unit. Simply selecting a tool joint that will function in the grippers is not enough. The tool joint must be designed to match the rig’s output torque or it will be improperly tightened. Also, the outside diameter of the pipe greatly influences the steering characteristics of the pipe. Too-large or too-small pipe bodies will not be properly matched to the thrust and may create steering problems.

Here are some other important considerations:

Is the pipe lined?
Lining was originally developed to deal with the corrosion caused by the drilling fluids available 15 years ago. Today’s fluids are less corrosive, but lining is still beneficial to the driller. It reduces the size of the cavity that fluid must fill every time a pipe is added to the drill string (lining reduces drilling fluid volume by as much as 65%). This reduces the amount of time required to pressure up a drill string and start drilling again. It also reduces the amount of mud spilled on the ground when pipes are removed from the drill string during pullback operations.

Will the welds hold?
Manufacturing practices play a big role in how a pipe performs. Forging can be used to produce a pipe with no weld seams, or to reinforce a friction weld. However, there is a limited amount of material that can be gathered to form the tool joint each time a pipe is heated. To gather large amounts of material on the end of a drill pipe usually requires several heating and forging cycles, which makes it difficult to produce a robust tool joint. Welding tool joints to a pipe body can produce a good pipe if the weld joint is properly designed. However, weld zones need to be structurally reinforced by stiffening the pipe in the weld zone. Increasing pipe diameter at the weld zone is the most effective method of reducing the working stress on the weld, and helps avoid failures in the weld or the heat-affected zone next to the weld. Pipe designs with unexpanded weld zones almost always fail prematurely in the weld zone, regardless of welding technique.

Is the pipe clean?
The heat associated with forging can cause “scale” to build inside the pipe. Scale is a hard, black layer of brittle iron oxide that is difficult to remove, but tends to pop loose when the pipe is bent. If a pipe is not aggressively cleaned after forging, the scale can come loose and plug the cutting tools, reducing mud flow into the bore. This can cause tracking beacons to overheat and possibly fail.

Do the thread patterns match?
When buying replacement pipe, always replace the entire string with pipe from the same manufacturer. The HDD industry has no established industry standard for threads, so not all threads are made equally. Parts from different manufacturers may appear to assemble, but this doesn’t mean the joint was made up correctly. Thread tolerances also vary widely among manufacturers. And worn threads, of course, do not match any other threads. Mixing new and worn threads increases contact pressure and accelerates thread wear. Also, bend radii of the pipe may be different, which will cause accelerated and uneven wear on all pipes in the string.

Published in the Fall 2007 issue of The Underground.