Air trapped-piepline repairs

[Gilles12]

Good morning,
I hope you're doing well.
I am working on a project to replace a 24 m section of a damaged 12'' pipeline. this pipeline is buried and transports diesel. The point to be repaired is located 14 m from a shut-off valve. the other valve is located 30 km from the first. remember that the pipeline is 250 km long and that the repaired point is 154 km from the head station.
. the profile is quite uneven (see diagram). I evaluated the ka volume recovered (around 200 m3). My problem is getting it back into service. due to drainage, there will necessarily be air trapped at different high points of the profile which can seriously disrupt operations (pressure pulsation and possible excessive suppression in the event of a shutdown). here are my questions:
1- my emptying procedure consists of making a tap at the damaged point then sucking the product into tanks. Do you have any suggestions or ideas for a better procedure because I want to minimize the drain volume and therefore the air intake.

2- I thought about using scrapers at a good position to reduce the flow of liquid to be drained. however I am thinking of the risk of ignition (air + fuel vapor+ friction) due to the scraper if it ends up in the section where the air is trapped. Are my fears justified, there are other easy ways to implement to resolve this pb.

What do you think?

pipeline characteristics

-ID: 11.75"
Length: 250km
leak point: 154 km
nearest bblock vavle 151km and 181km
profile see file attached of section between block valve. Repairs point in yelow.

 

[akpipelineengr]

You need to pig the line or use one of several different isolation options (e.g., freeze plug, stopple tool, etc.). Your fears about ignition when introducing oxygen to the pipeline are well-founded as you have all three components of the fire triangle (oxygen, fuel, and heat sources from friction/static, cutting, welding, etc.). This risk is typically mitigated by some combination of operational controls, inert gas purging, cleaning, and appropriate use of isolation tools.

This is a complex and potentially dangerous operation that needs to be well-planned before you put people in the ditch. You need a competent pipeline isolation and intervention company.

 

[1503-44]

Not to throw shade on the danger, but it's actually quite difficult to explode diesel. Some guerrillas in Colombia threw dynamite at my diesel tank. Nothing. Diesel rain. Good thing they couldn't read, or maybe they would have bombed the gasoline tank.

I can read, but not docx. It's far better to post pdf.

Getting a pig in the right place can be tricky. A stopple is probably easiest. No need to drain very much. Draining a lot increases spill risk. That's probably the biggest danger. Costs a lot to clean up spilled diesel.

Can you disconnect at the valve, is it flanged or welded?



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Also before I chip in, what is the status of the bit you want to replace?

Is there a hole/ leak?

Is the piece weakened?

Why are you replacing it?

Also you say the bit is 14 metres from a valve, but then say about 3kn. Which is correct?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Gilles12]

Good evening everyone,

Thank you for your answers.

@ littleinch actually there was an act of vandalism that was discovered. it is at Pk 154, 3 km after the shut-off valve. yes the part is weakened. there was first a temporary repair by bandaging with a resistant HP material. the final solution chosen is replacement of the Percé tube.

@44 all shut-off valves are welded.
@akpipelineengr noted and thank you.

Find attached the profil

 

[LittleInch]

There are many ways to do this and the correct one will depend on what is available at each end and how long you can reasonably take this line out of service, but you're looking at 7 to 10 days whatever you do.

Your drain down idea is, IMHO, a poor one.

1) line valves rarely seal 100% and cannot be relied up to seal unless you've actually tested them. Close the valve, depressurise the D/s side, pressure up the u/s side and then monitor the pressure for 24 hours to see if the valve is passing. 99% of the time they pass.
2) Drain down takes for ever to have the last dribble of product stop, especially diesel. You then need to insert a pig or seal into either end once you've cut the line to stop weeping product from appearing. On your profile you will be pulling a vacuum from the high point unless you also hot tap there and insert a vent. This could allow you to vent air out again when refilling. Pulling a vacuum is not a great idea and you could end up with glugs of liquid coming down the line.
3) Risk of spillage and contamination is high
4) Your pipe will spring when cut so you need to excavate about 60m of line to give you some flexibility when replacing the damaged section. Is that possible?

Your options as I see it are:
A) Simply clear the entire line of product. Will take a lot of Nitrogen and about 3 days at 1m/sec, but then there should be very little product when you cut and then you simply re-fill the line as though you were commissioning it in the first place. Definitely the lowest risk approach.

B) insert a water plug between two pigs of approx length 1000m. Carefully monitor volumes pumped and stick trackers on both pigs. Once the first one goes past stop pumping and shut down. Close the isolation valves and then stick two freeze plugs on about 100m apart. Drain the water down. Do the repair using your pre tested pipe. Do the NDT. Take the freeze plugs off and start pumping. You don't care about the air in the water (but if you do then insert two plugs for fill and vent between the ice plugs. When the diesel / water interface hits the end terminal divert to a large open tank and let the water and air flow in until you get diesel again, then divert to the diesel tank.

You could use two pigs on each end with maybe 150m of water between them. Jus be aware that pigs pass about 0.5% of the product they run. So e.g. if your line was 1000m3 voplume you need 1050 m3 to push it to the end. So you will end up with watery diesel in front of the first pig.

C) You could look at some fancy plug type pigs (STATS do them), but these are expensive and not sure if they come in 12" size. Also 150km is a long way to push them


Whatever you do you need to de a good set of cleaning runs to remove all the dirt and gunk you could get as welding will create a lot of smoke otherwise.

These operations are not easy and it is a little bit worrying you don't have more experienced personnel on hand to run through these options or pas son experience. Also bring in your contractors and listen to them. They aren't always right, but will tell you when you're wrong.

Please let us know what you decide to do and when you do it post some photos!


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Gilles12]

Good evening,

Thanks for all your ideas.

Totally agree with you@littleinch

WE thinking about the use of several pig. But i heard that as empiric laws 2 pigs must be separated about 100km of pipeline.

I will keep you in touch

 

[LittleInch]

"But i heard that as empiric laws 2 pigs must be separated about 100km of pipeline."

Utter rubbish.

The issue is simply the time taken to drain down a trap and remove the first pig, or have a longer trap or stop pumping.

To be fair over 250km, if you don't have about 1to 2 km, the second pig could catch the other one up if the first of started slipping more and that can cause problems but 100km! No way

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Pretty sure stopples will be the easiest.

Why would you not do that.

???

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

To be fair, the OP is coming up as the Ivory Coast. Pretty sure the cost of bringing in a stopple crew is too high.

Biggest issue to me is that you end up leaving extra flanges buried and higher connections that can get damaged or suffer corrosion.

Freeze plugs are much simpler but this is quite a long pipeline to get a water plug in the right place.

Doing a full line clearance or even replacing with water is far less risk. But do all your other repairs at the same time, but not for 24m of pipe.....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Gilles12]

Hi,

@littleinch thank you.

 

[1503-44]

Minimum pressure, diesel, one stopple, its a DIY ... especially If you are in Africa.

The distances involved and the general lack of sufficient local capability makes it a necessity to be self-sufficient in basic maintenance skills. Doing work in Africa has to be seen to a great extent as a BYOB party.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Gilles12]

Hello everyone,

Thanks !!!

Finally, to carry out the repair work, we isolated the leak point by closing the shut-off valves on either side of it. then, we carried out drilling under load to decompress and then drain the section concerned. The product was recovered by tank trucks. the drain lasted 36 hours. Then the residual volume was blocked using concrete. The work is in progress and we are fully considering filling the line. If anyone has a clue I admit I don't really see how to manage the air that will be trapped unless I use a foam scraper to move this air blockage. Thank you for your ideas

 

[Gilles12]

hello,

As I announced in my last post we were thinking about the filling of the pipeline section emptied during works.
We started filling the line getting one side valve closed and the other side opened. We fed the pipeline's opened side with fuel at a low flow rate when purging air before the closed side. I think we got a maximum of trapped air out but a non-negligible rest in the pipeline. we then opened slowly the other side with the aim of getting the remaining air during transport. we also put a foam pig in line to ensure that air will be pushed by the pig.
as the pig arrived at the end station, we noticed an abnormal increase in operating pressure coupled with a drastic diminution of flow rate. After stopping the pumps and opening the pig receiver we noticed that a mixture of the foam pig and the mixture of bentonite and polystyrene used to isolate the working zone got stuck and solid in the bypass line of the receiver. I do not understand why:

- the bentonite got solid in the bypass line ( flow condition 40°C, 60 bars, gasoil)
- the foam pig got sucked into the line ( the receiver ID = 16", the pipeline ID = 12" bypass line = 8")

If anybody has an explanation, I will be happy to hear it !

Thanks

 

[LittleInch]

I read this and then gave up when I read "Then the residual volume was blocked using concrete." Eh?? Care to define what actually happened as "concrete" doesn't tend to move.

What do you mean exactly?

Got a diagram or sketch or something? I can't understand your sequence and whilst "one side" and "the other side" makes sense to you, it doesn't to me....
Where did you vent the air? At the high point or somewhere else? Not a lot of this makes sense to anyone who wasn't there.
Where was the foam pig inserted?

Sweeping out dirt and gunk, especially with a foam pig can easily lead to blocking of the outlet. The foam pig could easily have ridden up over the dirt and then been forced into the bypass nozzle and then moved when you opened the pig trap.

A build up of dirt in the trap could do the same thing. Something happened like that many moons ago on a line I know where for lots of reasons the line had a lot of sand in it. Pigging the line using a "standard" pig, the pig stopped about 3km from the trap. They opened up the trap and had to dig out with a pickaxe the dirt which had compressed itself into the back end of the trap and blocked the bypass line. Suspect the same thing happened to you.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.