Thoughts on a gas condensate line being decommissioned

[bouk]

Hi there,

I'm no specialist in decommissioning of pipelines, so would like some thoughts from those with some experience in this. I've got a gas condensate line that's been shut in for several years now and looking to be decommissioned. First task is to actually work out what could actually be in the line having been shut in for so long!

My initial thoughts are water, oil, residual chemicals, iron scale, etc. But I have a few questions:

1. Since this line is no longer at pressure - when operational it was at 70 barg. Currently at 3barg - is it at all possible there would be hydrates present if the line is no longer at pressure?

2. Are any chemicals in particular (for example, corrosion inhibitor, methanol) known to degrade and cause sludge that could also be in the pipeline?

High level comments/thoughts would be appreciated for now.

 

[bouk]

Sorry, let me clarify, it is primarily a gas pipeline with a small amount of condensate in it.

 

[Pmatherne]

When the pipeline was shut down previously what did they do to it. Was it flushed with another fluid or just depressurized and has just been sitting there


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[bouk]

It was just depressurized and has just been sitting there

 

[BigInch]

Regulations are rather sparse on this topic, unless in the North Sea. You should probably at least clean pig it and purge it of explosive gases, at least fill it with air. What you do with it after that... in the absence of further regulations and company policy, pretty much whatever you feel like.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[Pmatherne]

Since it was just depressurized you can see what the pipeline use to carry and start form there. It is hard to take a guess right now. If they pigged it before dropping the pressure there might not be much condensate. If they did not then there might be quite a lot left in the system

If you are looking for some guidelines BSEE has some regulations on decommissioning of pipelines. You might be able to use those as a go-by to develop a strategy for your particular job

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[LittleInch]

Looking at your questions and issues

The fact it is still at 3 bar is good as it shows it's not completely broken. You get hydrates as the pressure increases and temperature decreases, not at 3 bar.

I doubt you have any serious sludge or debris in your line as it wans't flowing.

Your issue is that in order to de-commission the line, first you need to re-commission it(!) This means looking at any records you have of its material, inspections or use to see where issues could arise (low spots, damaged coating etc). What is the CP like?

If you know it holds 3 bar, is this enough to send a foam pig down it and try and clear some of the liquid out of the way or do you need more pressure? Or can you just flow water into it and flush out the contents into an atmospheric tank at this pressure even if slowly? Once you get mostly water you can then pressure test it to the lowest figure you can get away with so that you can clear the pipe of water with pigs, initially foam pigs then perhaps something more like a bi-di to clear it all out of the way using nitrogen normally.

Then depending on the size, you normally need to cut it into sections and seal each section before abandonment or fill it with grout / foam concrete to stop it collapsing.

How you do this and what permissions and approvals you need vary widely by country and jurisdiction and what promises were made (if any) when the line or system was built. Equally how long you remain liable for it after abandonment also varies a lot from nothing to 10 years plus.

Normally it is a good idea to get an environmental study completed to demonstrate that your chosen methd is the BPEO (best practical Environmental Option) to cover you if it starts leaking after you've abandoned it.

My motto: Learn something new every day

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

 

[MortenA]

What we plan to do with our subsea multiphase pipelines (in the North Sea) is to flush them and åig them until the flushing fluid is cleaner than the discharge requirements (max 30 ppm) and then leave them (in situ decommisioning is the fancy term). Our pipelines are buried and the risk of a major rupture (e.g. from an anchor or a trawl) is therefore small except near the WHP and host. Here additional pretection or removal of some of the pipeline may be required. A major rupture could lead to a localised release with an higher concentration of oil. But it is expected that when the pipeline slowly deterioate any leftover will be expesed so slowly that normal biological activity will manage.

 

[dcasto]

Unless the gas in it was dehydrated, there will be water to. Unless there was O2 in the original gas, the water won't oxidize the pipe. Unless you has a nonlubed compressor, there will be lube oils that will do some storage things over time.

Before you start injecting air to run cleaning pairs, make sure you put a buffer of nitrogen before the air. There is a nice post by zzz as on selecting air compressors to clean pipelines. I'd assume you have enough liquid in the pipeline to fill up the highest single elevation change

 

[bouk]

Thanks all for the input - it's looking like we're going to inject them with nitrogen and then either flush with seawater or pig using a gel based pig. There are still quite a few unknowns - in particular because pigging of the line hasn't been done previously so the contents are a guess at the moment, but appreciate your thoughts.