×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Two Stuck Scrapers!!!!

Two Stuck Scrapers!!!!

Two Stuck Scrapers!!!!

(OP)
Hello all

Here is the situation:

Pipeline Diameter 36", lenth of 16 km. Sour gas service 300psig MAOP.

Shutdown in order to replace a 12meter joint with a crack and the reciever valve. Scrapable but haven't been scraped in a while because the reciever valve was changed a couple of years back with a valve which turned out to be not full bore (32").

After shutdown, it was deprussrized to 0psig. a scraper was sent using N2(disks, Bi-directional) status unknown. Another scraper same type was sent after it, missing inside the pipe. N2 is received at the end of the pipeline.

There is a mainline valve and a bypass, close to 2/3 the pipeline length stuck while trying to close it at 60%, after the scrapers were sent. An xray was done on it to see if something is hendring it, but nothing was found. The pipejoint to be replaced is around 2km before the mainline valve.

What are my options now? Please list everything because I came in after the fact and I am not sure what has been done to resolve the situation.

RE: Two Stuck Scrapers!!!!

I'm more than a little puzzled as to how the scraper at 36" was supposed to get through a 32" valve before you replaced it or have I read that wrong?

Anyway, you're not in a good place. First off you need to try and find out where your missing pigs are. Do you have flow when you put Nitrogen on the launcher side? Fit pressure gauges in as many locations on the pipeline as you can and see if you can plot pressure versus length to see where the pressure drop across the stuck pigs is, or at least some idea of where it might be.

If you haven't tried reverse flow it's always worth a shot, but if you have two pigs stuck then they are not likely to move.

If you can open your main line valve, try sending a tracker pig down fitted with a radio or magnetic sender and track it at locations not exceeding 500m. One more stuck pig won't make much of a difference but at least you'll find it.

Then get the cutting torches out.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Two Stuck Scrapers!!!!

(OP)
The idea was to send the scraper, once it reaches the receiver, it is kept there and the defective joint is replaced, then send it back to the launcher so we can replace the receiver valve.


What we are going to try now is pressurize the line (From launcher) with N2 (High flow rate). Pigs might move and also we ll be able to check pressure along the pipe. (Pipeline is underground so will be difficult)

Sending another scraper while the mainline valve is still not fully open is not an option, still working on that but seems like a dead end so far.


RE: Two Stuck Scrapers!!!!

What about sending in one of those little 4x4 trucks with a wireless camera? Might just end up with more crap in the pipe but you never know.
Long shot I am sure.

Regard
Brad Stone

RE: Two Stuck Scrapers!!!!

Please be very very careful and think seriously before you try the high pressure / flow "push". If by some miracle it actually works - you can then get a very high velocity of what is not a small thing (A 36" bidi). I for one would not want to have an uncontrolled pig in my pipeline.

You need to recognise that you're in a bad place right now and all this will do is elongate the time taken to get your line back in service.

You should start now to mobilise some crews and hot tap gear to install 2" taps every km to monitor what is happening. You might need to go for 8" or 12" taps if that doesn't work and send a pig WITH TRACKING DEVICE, venting at each of your taps in turn until it stops.

what tends to happen with stuck pigs is that everyone goes round at high speed trying all sorts of brutal options without any real idea of where the pig is or why it is stuck. If you start planning now and do it properly, you'll have a much better outcome in a shorter time. Doing all this is a complete b:tch, but it's where you are so start doing it properly. You will need to fix your mid point valve though and don't listen to whoever sent two pigs down a line without tracking equipment. One maybe, but not the second...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Two Stuck Scrapers!!!!

I'm not real big on warnings, but I've seen a bunch of people using nitrogen lately without paying much attention to injection temperature. Liquid nitrogen is kind of cold, and there have been several fatalities in the last couple of years caused by pressuring up a line with -40C (or F) nitrogen and having brittle failure at way below system MAWP.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

RE: Two Stuck Scrapers!!!!

And another couple of accidents from introducing pressurized gases from exterior sources having maximum pressures well above pig launcher and pipeline design pressures which were not fitted with pressure relief valves.

I hate Windowz 8!!!!

RE: Two Stuck Scrapers!!!!

fyi http://www.wfaa.com/news/investigates/Pipeline-Fly... [If gas pressure is used to move anything in, or remove anything from lines, I guess it is best to make sure there is some sort of effective "catcher" on end, lest new meaning be ascribed to "launcher". Sounds like this projectile(s) went through or to two homes 500 feet away, and "Christina" was fortunate!]

RE: Two Stuck Scrapers!!!!

I really wish there was some way to make the people on a job follow the clearly defined procedures that Engineers write. I'm sure that the guy that was responsible for launching that projectile has been fired. And he should have been. But next time he is in that situation will he follow the procedure or figure that there is no way he'll get hit by a 1 in a thousand accident twice? I've seen a number of near misses that were only near-misses because the procedure anticipated the possibility of the pig approaching the receiver at terminal velocity and put in appropriate safeguards.

You hear about this kind of crap periodically and I don't think there is any way to stop it.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

RE: Two Stuck Scrapers!!!!

(OP)

is it really that bad with with about 30gal/min and 50 psig? About 1-2 m/s speed and open drain at the end of the pipe.

I am actually against that approach but can't win the argument with the idea of cutting the pipe to get the pigs out.

There is a vaporizer so it is gas nitrogen and don't think it ll pressurize high enough to liquidify (plz correct me if am wrong)

And like I said, we are trying on the mainline valve, once it opens we can send a tracker.

And I too wish ppl follow procedures correctly.

RE: Two Stuck Scrapers!!!!

Probably not, but your post initially said "pressurise" with "high flow rate". However if the seal is good, work out what the force is at 50psi across a 36" pig - 50,000 lbs force...

A vapouriser is exactly that - it just vaporises the liquid - the key point is what is the temperature. You can get, as zdas04 says, very cold gas coming from the nitrogen provider. If your inlet line freezes - it is quite possibly too low...

I agree that a lot of people run around trying not to do, or at least plan for, what is often required - i.e. find it then cut the thing out. One pig you might just get out by huffing and puffing, but two is very difficult. Everyone just tries to do this on the cheap, without realising that what you have here is a train wreck.

Good luck and keep us informed.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Two Stuck Scrapers!!!!

​I believe many of us were taught in our science classes many years ago that natural tendencies are towards minimum energy and maximum entropy (disorder) of systems. I guess the profundity of that natural tendency (or daresay law?) becomes more and more clear or encompassing with experience. While I’m sure one does the best one can, it is really difficult and maybe particularly when a whole lot of potential energy is involved to make procedures or anything else foolproof, as it appears fools get increasingly ingenious. Not sure when all this will change, but maybe (not) “when pigs fly”?

RE: Two Stuck Scrapers!!!!

Anybody in the market have an 8 km boroscope? Duct-tape it to a radio-control car ....

RE: Two Stuck Scrapers!!!!

(OP)
Thank you all for the advice.....

We found the 2nd pig under the main line valve, its a gate valve and the pig was a grotesque inside lol. Almost cut in half.

Work is resumed and we will find the 1st pig no doubt..

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources