Drill Pipe Thread Pitting 1

[MMR Advisor]

I work for a land-based drilling company. Recently, we've had a lot of drill pipe rejected as unservicable during routine inspections due to pitting on the threads. Our drilling mud chemistry is usually oil (diesel) based, with barrite and caustic added, to a ph of about 9-10. We haven't observed any H2S (not that we know of anyway). The joints are lubed every time we make a connection. I just found out about this and have not been able to send anything to a lab yet. Any ideas on what could be causing this? Simple mechanical wear? Galvanic corrosion? H2 cracking? I can email a photo of the threads if needed.

 

[TVP]

A photo would be helpful, but you should use one of the free image posting sites instead of emailing it. I doubt galvanic corrosion, but mechanical wear or H₂S cracking could be possibilities. What is the exact alloy and processing (heat treating, etc.) currently used?

 

[rorschach]

Are these upset threads? Was the pipe fully normalized after upsetting or were just the upset ends normalized? I've seen selective corrosion in the thread area in wet CO2 wells with partially normalized upset pipe. In the industry it even has a name. It is called ringworm corrosion. That is what you get when you buy cheap pipe. It can happen suprisingly fast too.

 

[redpicker]

If this is drill pipe, then the threaded tool joint will be quenched and tempered 4137.

Pitting corrosion of the threads is highly uncommon because of the thread dope (grease) used on every connection and the controlled pH of the drilling fluid. More uncommon when an oil-based mud is used. H2S cracking would be evident as cracking, not pitting, and would not occur if the pH was maintained 9-10.

Pictures would be helpful. Could you be seeing galling? The oil-based drilling fluid can interfere with the thread dope (particularly Zinc-based, check with your dope supplier for recommendations for your mud-system) resulting in metal-to-metal contact of the threads on make-up resulting in galling of threads and shoulders. Under-torque on make-up can also contribute. You should have seen "dry" connections and unusually high break-out torques when tripping out of the hole if this was the case.

It's difficult to tell without pictures, though. It could be simply pitting corrsion due to improper storage.

rp

 

[MMR Advisor]

I've uploaded a photo at the following link:

http://www.photo.net/photodb/member-photos?user_id=2222172

Note the abrasive wear on the threads in addition to the pitting.

The initial concern was galling, but most examples I've seen before have a much more jagged fracture surface. Let me know if you think there is galling (def.-A condition whereby excessive friction between high spots results in localized welding with subsequent splitting and a further roughening of rubbing surfaces of one or both of two mating parts).

Also, could under-torqing during make up contribute?

We use Jet-lube Koprkote pipe dope (

http://www.jetlube.com/NewFiles/eng.html).

I don't see anything to support ringworm corrosion.

After some research, we found out that this lot of pipe was stacked for a while 2004, which leads me to believe there was some corrsion due to moisture. It has been in continuous use for the last year or so.

 

[redpicker]

From the photo, it does seem to be corrosion pitting, perhaps from the storage you mentioned. Whether or not it requires the connections to be recut is not evident from you photograph, however. You need to evaluate the pressure flanks (the thread flanks shown in the photo are the non-pressure flanks). If the pressure flanks are pitted the same, I think I'd recut. The fact the inspection company rejected them makes it difficult to continue using them in this condition.

I believe the copper-based dopes (such as the kopr-kote) are preferred over the zinc for use with the oil-based mud systems. While there is a theoritical possibility that a galvanic couple could be created between the copper and the steel (with the steel being sacrificial), I have never seen it.

rp