I got this idea after speaking with a driller at a trade show. He had stopped by my booth and asked me about our drill rods. I was explaining some of our features and how they could help him get more wear. He cut me off saying that he wasn’t interested in that because he guaranteed that he would do something to ruin that drill rod long before it ever wore out.

That got me thinking. Are we dooming our drill pipe because of issues that we are just not aware of? These notes will help.

What Drill Pipe Thread is Best for You?

When you buy a rig, how much thought do you give your drill pipe and, in particular, to your connections? Drill pipe outside diameter (OD) can dictate the best hole size range to meet the needs of the type of work you do. What about your connections?

Connections will dictate the internal diameter (ID), most likely the torsional strength of your drill pipe, how well it withstands bending stresses, how it fits in with your tools and the availability of drill string components.

If you’re an air driller, the ID may not be so important. But if you need fluid volume, then you want to look at the connection ID.

For example, looking at 4½-inch diameter flush-joint drill pipe, there are typically three options: 3½ Reg (1¾-inch ID), 27⁄8 IF (21⁄8⅛-inch ID) or 3½ IF ( 211⁄16-inch ID). How much volume do your jobs require and will these IDs permit it? (I note that the official standard ID for the 3½ Reg is 1½ inches but 1¾-inch ID is commonly used.)

Going from 1¾-inch ID to 21⁄8-inch ID doesn’t seem like much of a difference, but the 2⅛-inch ID would allow 50 percent more volume than the 1¾-inch. The 211⁄16-inch ID would allow 60 percent more volume than the 21⁄8-inch ID.

Another factor for thread selection is strength. For water well drillers, API and similar connections have oodles of strength for most typical applications — as long as you are drilling vertically and maintain recommended OD/ID combinations. Now, if you are drilling large-diameter holes, drilling in difficult conditions or have concerns, then check into the tensile and torque strengths.

For flush-joint drill pipe, balancing connections is tricky because most API threads were not designed for flush-joint sizes. They were designed for elevator-shouldered drill pipe. For example, the 27⁄8⅞ IF would commonly be found on a 27⁄8-inch external upset tube with a 41⁄8-inch OD by 21⁄8-inch ID tool joint. Here the connection is in balance.

Are You Balanced?

When in balance and properly torqued, the pin and box act in unison to absorb the tensile, rotational and bending stresses. Get them out of balance and one member will take more than his share. The 27⁄8 IF and 3½ Reg on 4½-inch OD would have a stronger box than originally intended, while the 3½ IF would have a weaker one.

Let’s go back to the 4½-inch flush joint. Say we need the big ID and opt for the 3½ IF. This thread has a standard OD of 4¾ inches. We can put it on 4½-inch OD (recommended minimum).

First, know the options available and their respective recommended OD/ID combinations. Try to keep them in balance. Next, look at the ID requirements as they fit your work load.

This brings up another consideration: OD wear. That 3½ IF might work great on 4½-inch OD, but what if we experience a lot of wear? We would think a large ID requirement might mean mud which does not produce a lot of wear, but sometimes the large IDs are required to run sampling tools.

Lots to think about. First, know the options available and their respective recommended OD/ID combinations. Try to keep them in balance. Next, look at the ID requirements as they fit your work load. Are we OK if we get some OD wear?

In oilfield applications where drill pipe design is more critical, engineers try to keep connection strength within a ratio of body (tube) strength. If we go from an E75 grade of tube to a much stronger S-135 grade, engineers will beef up the tool joint dimensions to better coincide with tube strength. They do this by decreasing the ID and increasing the OD.

To gain pin strength, decrease the ID. To gain box strength, increase the OD.

What if you need more torque strength but do not have the flexibility to change the OD or ID? You can use a stronger alloy steel but that can be expensive and be less weldable. With the advent of the HDD industry doing so much more, a boatload of connections have been designed to provide the additional torque strengths and help connections withstand the extra bending stresses.

A quick way to add torque strength is through a double shoulder. Here an additional shoulder is created with the end of the pin and a landing at the back of the box. During use, the usual shoulder is engaged (the one formed by the face of the box and end of the pin tool joint at the base of the pin) and, as the torque starts to climb, that second face becomes the engaged. This can add 30 percent or more to a standard connection’s torque strength.

Double shoulders are usually not needed in standard drilling applications, as these strengths were built into the API set of connections. The double shoulder could also be utilized to maintain connection strength when going with an oversized ID. Running that 27⁄8⅞ IF and want a 2¼-inch ID?

There has been lots of discussion regarding the best connections for drilling through a dogleg or for HDD applications. The typical strength calculations are done through the wall thickness of the connection at a certain location and material strength. Connections with certain OD/ID combinations were assigned a “bending strength ratio” or BSR. BSRs are supposed to be in the 2.25:1 to 2.75:1 range for good balance. This can vary depending on the application.

Studies have found that certain connections that actually had a traditionally less desirable BSR did better that those with better BSRs in dogleg situations. So if you are drilling “round corners” or “through the bend,” then some research into this aspect would be recommended. Here thread geometries in how the bending stress is distributed becomes more important than just the material strength and thickness alone.

On a final note, try to pick connections that are well known and available in the industry. Going with Billie Bob’s Modified Mayhew might work great, but good luck locating tools with that connection in a pinch. If it is a proprietary connection, ask who is licensed to cut it. Have some tools left over from a rig you are trading in or do you run other rigs? Does this connection allow you to utilize these tools or borrow tools rig to rig?

When in doubt, ask for advice. Talk to your mud guy on ID requirements to suit your mud pump and project range. Talk to a drill pipe guy concerning whether your connections are well balanced and what the options are if they aren’t. Now, where can I find a drill pipe guy? Ummmmmm … let me think.

Call me. Talk is free.