Worn threaded connections

[ldmech]

Hello All,

Good to finally get a post in here...I work in the oil and gas industry and recently some equipment was lost in a wellbore (let me know if there is another forum I should be posting in!). The reason given by the service company for the equipment being lost in hole was that the tool had worn threads which had come undone and worked itself off while down hole. The thread is the same at in the picture attached and is a 1-3/16" -12 UN - 28 thread or gearhart owen connection. These threads are made up each time the tool is run in the well, maybe having a lifetime of 3+ years. I am not sure I believe the reason given...

Does anyone have a useful link to some information regarding thread wear over time and how the connection is compromised? How much would a thread need to wear before the friction between the threads locking would be significantly compromised? I had thought that vibration would be the main issue in threaded connections coming undone. Any help or info would be greatly appreciated!

Cheers,

 

[MikeHalloran]

Of course that thread is smaller than any other in the drill string, so if they rotate the string to the left five turns or so, the tool should drop right off. They never rotate the string in reverse, right?

The other possibility that comes to mind is that 12tpi on a 1-3/16 major diameter might not be fine enough to be a self-locking taper, which _might_ make it susceptible to loosening from vibration. I'm too lazy to work out the helix angle right now... A drop of Loctite could make all the difference there.





Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi ctmech

I believe the helix angle is small enough for the thread to be a locking thread, however if the tool is subject to vibration this could reduce the thread friction to zero and cause the thread to work loose,
I've seen a similar thing happen on rolling mill lead screws were the threads were considered self locking.

http://www.loctitesolutions.com/files/AN09/Threadlocking_Guide.pdf

 

[Cockroach]

Oilfield, you should also specify a torque make-up on the thread. This would be the most likely event to a lost in hole scenario, the boys didn't buck the joint properly to the specification.

They do on occasion rotate the string to the left, in cementing operations or to back out of a tight spot. But this is extremely risky, and the rig never applies more torque than 25% of the weakest thread in the string.

Even more reason to specify a torque for this thread. Really doubt it is thread wear, toy would of seen an issue during make-up which would of needed tobeaddressed. Check the service logs, see if this thread has been cross threaded in the past and repaired. It could of been miss cut, or it could of been a make-up issue. Either way, there is more than just saying it is worn.

Good luck with your investigation.

Regards,
Cockroach

 

[ldmech]

Sorry I didn't add in that this was run on wireline, not drill string. There will be rotation of the tool in the well due to unwinding of the wireline from a winch drum, but I have never heard of this being a big problem (yet...).

We have used loctite before, but have not needed to consistently use loctite as we have not thus far had a problem. It was another company which had the problem, but I want to determine if we will have a problem in the future.

The other company has completed an RCA report and the reason given for the equipment loss is worn threads compromising the connection. Is there any way to quantify wear on the threads and at what point they are compromised in their locking?

Cheers

 

[Cockroach]

I think noting we are talking wire rope and not drill sring is a huge omission. I cannot emphasize the need to be diligent and thorough when addressing this forum. It's kind of like going to your doctor to report an illness to which you seek remedy, then adding afterwards, "Oh by the way, did I mention....."

Try not to waste our time.

Regards,
Cockroach

 

[redpicker]

I'm sorry, but I don't buy the worn threads theory. It sounds like a vague, hand-waiving excuse whose purpose is to divert blame away from operations, allowing them to continue. Something to put in the file.

My off-the-cuff theory is they didn't use the proper thread compound or did not make it up properly. It should be designed so that it would never see high enough reverse torque to break it out, so if it was made up correctly, it wouldn't come apart down-hole.

If you really want to get to the bottom, contact the manufacturer. They should, as a minimum, have field guidelines for inspection of the equipment, including threads. I'd imagine they would be very interested, since having their equipment come apart down-hole is not good PR.

rp

 

[ldmech]

Hey Cockroach, sorry for the omission and if I haven't been clear. My main question I have been trying to get across was is there any way calculation in which you can quantify the reduction of the threads locking capacity if you have worn threads, a reduction in the major diameter. I see now there is a far better way to ask the question...

Redpicker, exactly my thoughts. I wanted to make sure before I called the other company out that I had all my information correct. I have heard back from a manufacturer of our threads and am discussing the problem with them. We sat in a meeting to "share information" right after the incident and my first instinct after hearing the description of events, type of thread they use and the outcome from this analysis I think it is far more likely that a connection was not made up correctly.

They are comparing the wear on their threads to a engineering drawing with a thread tolerance supplied, I had always thought that this tolerance described fit conditions and did not have any major bearing on the wear the threads experience (if significant wear).

Cheers for all the help, greatly appreciated.

 

[Cockroach]

I will lay out the calculation next week for you, in detail. Definitely not thead wear. I expect this to be improperly strung together in the field, so an operational or procedural error.

Regards,
Cockroach