Buttress threading on 7" casing 1

[nottoobright]

I am threading buttress on 7" seamless SS304 casings. As I get to the end of the thread there are areas where the cutting tool doesn't come into contact with the casing (due to tolerance in straightness / roundness)
What does API 5B allow? I"ve got the specs but I haven't been able to find reference to this.
K-55 API casings with BTC couplings that I've purchased in the past all seem to have this same problem of the threads 'fading away'.

 

[desertfox]

Hi nottobright

Before the thread fades have you already achieved the required thread length for that component?
Is it possible you can skim the case before you thread it
thereby eliminating the out of straightness problem.

regards

desertfox

 

[nottoobright]

Hi Desertfox

Appreciate the prompt response. No, we can't skim them any more, not enough OD.
I'm not sure if we've enough thread length to do the job, if you ask the boys in the factory the answer is "YES!" I'm more concerned about what API 5B or 5CT has to say about this.

 

[desertfox]

Hi nottobright

Haven't got access to API 5B or 5CT.
But if you have made it to drawing and you have sufficient
thread length and meet the tolerances over that length then I can't understand what your concern is, although I might
be missing something if I am please explain.

regards

desertfox

 

[nottoobright]

Hi again Desertfox

I quoted this job "as per API" which means that my threads have to comply with the standard for depth, pitch, taper etc. Normally I don't thread onto casings, I roll and weld my own blanks and then thread these. No problem with the fading threads as I control the OD. These are then welded to the casings.
This job however doesn't allow welding, hence I'm threading on the casing. Aesthetically speaking, I think it looks terrible. Will it do the job? Probably.
However, the customer is in a country where the government strictly regulates the product and it must conform to API specs. This is why I'm wondering if anyone at eng-tips can help.

 

[desertfox]

Hi again

I did a search on those specs it seems you have to subscribe
to get the info although I did find this passage:-

Specification API 5B, also from the API, specifies the corresponding thread and a method for inspecting it.

Specification API 5B specifically mentions, for each pipe dimension, the value of the thread nominal pitch diameter in a cross-sectional plane located at the end of the perfect male threads on the pipe body side, male threads exceeding this plane having an incomplete height and fading out.

In the remainder of the present document, this plane is termed the "reference plane of the pitch diameter", abbreviated to "reference plane". The term "first thread" relates to the side of the threaded portion directed towards the free end of the corresponding male or female element; the term "last thread" refers to the side of the threaded portion oppositely directed to the free end of the corresponding element.

The last perfect male thread is thus located in the reference plane while the last male thread corresponds to the end of the threaded portion on the pipe body side.

Threads produced in accordance with API 5B must be inspected by hand tightening of gauges such as ring gauges comprising an internal thread in the case of inspecting male threads or plug gauges comprising an external thread in the case of inspecting female threads.

The relative axial position when the gauge has been screwed on is checked with respect to the inspected thread and specification API 5B defines a value and a tolerance for this relative axial position.

The inspection method specified by API 5B has advantages, namely simple and rapid global inspection of the thread, but it also has a number of economical and technical disadvantages.

Firstly, for each thread diameter to be inspected, the hard gauge inspection method requires sets of gauges for different levels, namely master gauges and secondary or working gauges, the working gauges having to be discarded when the wear on them exceeds a critical value.

This means that an extremely large number of high precision gauges have to be made and they have to be managed depending on their state of wear; thus costs are high.

That thread inspection method provides a global result which depends on a number of parameters including the pitch diameter and also the taper and ovality, which parameters interact and thus do not facilitate fine interpretation of the inspection results.

Thus when inspecting a male thread, if the taper of the male thread to be inspected is less than that of the gauge, the first roots of the male threads are in contact with the gauge threads while the last roots of the male threads have a radial clearance with respect to the corresponding threads of the gauge. In contrast, if the taper of the male thread to be inspected is higher than that of the gauge, the roots of the last male threads are in contact with the threads of the gauge but not the roots of the first threads.

In both cases, the pitch diameter of the male thread in the corresponding plane on the gauge in the reference plane is less than the nominal pitch diameter but, in addition in the second case, the pitch diameter at the first male threads is not accurately known.

Similarly, in the case of inspecting a female thread, when the taper of the female thread to be inspected is less than or greater than that of the gauge, the pitch diameter of the female thread in the corresponding plane on the gauge to the reference plane is higher than the nominal pitch diameter but, if is its lower, the pitch diameter at the first male threads is not accurately known.

The manufacturers of particular threaded connections such as the connections described in European patent EP-A-0 488 912, which are known to have service performances which are superior to those of API connections, have been forced to use inspection methods similar to those specified for API connections because of international recognition and imposition of API specifications.

The cost of implementing those methods is considerable; the manufacturer must have available complete sets of gauges for itself and for sub-contractors.

Thread inspection methods have thus been developed which do not use hard gauges but which carry out a direct determination of the pitch diameter in the reference plane or at another set position.

Don't know whether its of any use to you or not, but if your quoting to that spec won't your company have copies?

regards

desertfox

 

[Cockroach]

Yeah, typical of K-55 casing, obviously an oilfield application for a liner connection.

Try to take a cut to the minimum thread allowance on minor diameter, then use a Coupling as a gauge and buck it up to torque allowance. This will determine if the thread is functional without the possiblity of thread jump.

Typically the end is scrap. You may wish to do what DesertFox has suggested and take a light skim next time. If length of your end is not an issue, you can cut off the thread and make-up length with a pup.

Otherwise, the piece may be scrap. And I have a yard full of this stuff.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada