Pipeline Pig Stuck

[adtw777]

I work in an older refinery 1950's. We have old u/g piping. Inspection methods have been limited to LRUT. We have just started a pigging program to get a better handle of what our current risks are. The lines have never been pigged before from our records. We have researched the lines and given the ILI contractor all the data we have. We agreed to run foam cleaning pig, then gauge pig, then def, then a multi-data set tool with go/no-go's between each run depending on information gathered after each run. When the contractor got to site he was leary of running the foam pig given we only run at 150psi. After much discussion we ran the gauge pig first. After successfully running the gauge pig, we had a go by the contractor and stuck the deformation tool. We were prepared for the situation but ended up cutting out the section of line to remove the pig. An investigation is underway. The tool passed through back to back 45's and then 10 feet later ended up stuck in a 1.5 D 90 degree elbow. It had passed through 3 similar 90's without trouble. After the cut out it was observed that the drive cups were inverted but the tool couldn't be pulled from the elbow using significant force. it was jammed. The elbow had to be cut apart and it looks like the "finger" portion was jammed up against the elbow. The contractor is baffled and we don't have a lot of experience with pigging. Is sticking a pig a common occurrence ?? Why would it pass through several elbows then get stuck in one? We are leary of continuing with this contractor until we find out what went wrong and why.

 

[racookpe1978]

The first 3x elbows were "just big enough" for the inspection pig to pass, the fourth?

Just a smidgen too little. Or the butt weld had a glob of weld metal inside.

You have an idea. But run a "too little" AND "too short" pig FIRST through every pipe. When it passes through, then run a "just a little longer" pig at the same "too small dia". Then, after both have passed through, then run a "minimum dia" full-length pig to measure the wall thicknesses. It might get through most of the pipes.

That is, you are now trying to run a "gage pig" through an unknown system that has been running 60 years. Is every pipe and every elbow and every weld joint "perfect" and "in spec"?

No. And you don't know which ones are just a little too small. yet.

 

[LittleInch]

Some data and pictures would be very useful to comment further. Things like line size and wall thickness, pig dimension etc Pigging 1.5D elbows is difficult and prone to getting things stuck.

What did the guage plate look like when it came out?

I assume by a deformation pig you mean a caliper pig? I'm a little surprised you seem to running a uni-directional pig ("cups inverted") which doesn't allow some reversal.

Although it is not cheap, a crawler camera unit may be what you need to make sure that there are no unexpected changes in diameter, valves not aligned, excessive weld metal etc. Also a towed unit able to be pulled back and forth may be better than trying to flow - what is the fluid by the way - gas or liquid or 2 phase?

My motto: Learn something new every day

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

 

[brimmer]

Check the wall thickness on the 90's (or any records). It may that the fourth elbow was thicker, had greater ID restriction and that was just enough. The ILI vendor can put smaller cups on the tool for the next run. Field 90's are difficult to get through, if you are sticking the geometry tool, you would surely stick the MFL tool. At those pressures, not much of an obrstruction could hold up the tool. Another thing to check is the weld bead on the ID, if the weld(s) on the 90 protrude into the ID more on this elbow, the tool could get stuck against that as well, but if cups were inverted, sounds like an ID restriction issue. Try smaller cups next time, hopefully they can still get a seal.

 

[adtw777]

ILI vendor has provided a failure report. It is suspected that the drive cups inverted going through the back to back 45's creating an "offcenter" situation prior to reaching the 90. Combination of offcenter and loss of propulsion caused the hangup. They are recommending minimizing bypass on cups and additional drive cups. Welds were perfect with very little protrusion. Also the wall thickness was same as rest of run .375". We've asked the vendor for failure frequency. Is it a common occurrence??

 

[786392]

Dear adtw777, Hello/ Good Day,

Though very much belated reply but may be helpful;
at times there exist minor dents in the piping, even in some fittings(e.g. ell's etc.)

accordingly cups inversion could have been due to this and final outcome appearing as few inverted cups& a finger stuck at the specific spot.

Also the cups inversion and axially off-centering may have been caused from such dents.

We long time back (early 80's) suffered similar episode; as an intelligent pig got stuck mid way of my ex-employer's 18.6 Kilometer 6 inches diameter U-G pipeline& we resorted to pipe cut for gadget removal.

Also a thorough survey of 'design' vs 'as built' drawing was conducted;few identified short radius bends(as against the intelligent pig's requirement) were replaced before further inspection/pigging.

I understand this will prove helpful.

Best Regards
Qalander(Chem)

 

[lhwingking]

I am new to piping and pipeline engineering but I have a doubt here. A piggable pipeline is one whose bend radius is atleast 5D right? You say your bend radius is 1.5D. I have not practically worked on pigging but I have theoretically studied that a piggable pipeline's bend radius is atleast 5D.

 

[IFRs]

Here is another thought, although you seem to have it worked out. Elbows change shape when stressed, if your line is heated and the flexibility was expected to be at the elbows, you may experience pig hang-ups.

 

[LittleInch]

adtw,

what sort of sizes are you looking at here? The smaller you get, the worse it tends to get. Pigging 1.5 D bends / elbows is a bit of a lottery and each system there will be some learning about what is and isn't possible. Back to back bends is a known issue which causes problems so those lines may need to be looked at differently or removed prior to inspection of the rest of the line. All depends on how long your lines are and how critical it is to inspect rather than just replace them.

I would suggest you look critically at all the runs and start to grade them in terms of risk where multiple elbows in a short distance or simply the total number of elbows in a run starts to exceed a certain number. Your issue is elbows so there is a cumulative risk of a stuck pig as the number increases.

My motto: Learn something new every day

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

 

[brimmer]

Minimizing bypass on cups should help (though I don't know how much bypass they had in the first run). Some tools have more collapsability than others, don't know what tool you are dealing with. Until the geometry tool gets through, I would not run the MFL tool. Yes, failied ILI runs are a common occurance, we acheive approx 80% first run success rate. When we have 1.5D bends, or field 90's we stick to one vendor in particular as we have had good success with their tool making it through lines like yours, because getting a tool stuck in lines like this is a common occurance/concern. I would be inclined to have another try at it with modified cups.