0 Comments | Dec 21, 2013

Choosing your connections. 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 diameter can dictate your best hole size range to meet your needs of the type of work you do.  What about your connections?

Connections will dictate the internal diameter, 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 4 1/2” diameter flush joint drill pipe, there are typically 3 options.  3 1/2 Reg (1 3/4” ID), 2 7/8 IF (2 1/8” ID) or 3 1/2 IF ( 2 11/16” ID).  How much volume do your jobs require & will these ID’s permit it?

Going from 1 3/4” ID to 2 1/8” D doesn’t seem like much of a difference but the 2 1/8” ID would allow 50% more volume than the 1 3/4”.  The 2 11/16 ID would allow 60% more volume than the 2 1/8” ID.

Another option for thread selection is strength.  For waterwell drillers, API & similar connections have oodles of strength for most applications as long as you are drilling vertically and maintain recommended OD/ID combinations.

For flush joint drill pipe, this is tricky because most API threads were not designed for flush joint sizes.  They were designed for elevator shouldered drill pipe.  For example, the 2 7/8 IF would commonly be found on a 2 7/8” external upset tube with a  4 1/8” OD x 2 1/8” ID tool joint.  Here the connection is in balance.

When in balance, the Pin & Box act in unison to absorb the tensile, rotational, and bending stresses. Get them out of balance and one member will take more than is share. The 2 7/8 IF  and 3 1/2 Reg on 4 1/2” OD would have a stronger box than originally intended while the 3 1/2 IF a weaker one.

Lets go back to the 4 1/2” flush joint and say we need the big ID and opt for the 31/2 IF.  This thread has a standard OD of 4 3/4”.  We can put it on 4 1/2” OD (recommended minimum).

This brings up another consideration, OD wear. That 3 1/2 IF might work great on 4 1/2” 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 ID’s are required to run sampling tools.

Lots to think about.   First, know the options available & 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 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 my decreasing the ID and increasing the OD.

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

There has been lots of discussion regarding the best connections drilling through a dogleg or HDD applications.  The typical strength calculations are done through the wall thickness at a certain location & material strength.    Connections with certain OD/ID combinations were assigned a “Bending Strength Ratio” or BSR.

Studies have found that  certain connections that actually had a lower BSR did better that those with higher 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 that just the material strength & thickness alone.

On a final note, try to pick connections that are well known & 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 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 & project range.   Talk to  drill pipe guy concerning if your connections are well balanced & what the options are.  Now, where can I find a drill pipe guy.  Ummmmmm Let me think.

Call me, talk is free.

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