Future riser pressure build up

[Jela123]

Hi guys. Need a bit of assistance on explanation for this. I am looking into the internal integrity preservation of future risers for a client.

The future risers were installed about 6-7 years and were pressure tested in-situ using freshwater to 230 barg. Documents at my disposal says that the risers were depressurized after the hydrostatic tests, anchor flanged at the topside and blind flanged subsea.

Recent pressure measurement on the risers showed internal pressure of between 0.8 - 2 barg. This was not expected since the risers contain only freshwater and was depressurized initially. Thinking that it was not properly depressurized 7 years by testing company, we depressurised it to zero barg but checks after a week interval showed the internal pressure to be back to 0.4 - 1.3 barg.

My question is where could the pressure build-up be coming from since both ends of the risers are flanged off?

 

[LittleInch]

Temperature change from when you bled them off to now? 1 bar is about 3-4C so doesn't take much / a bit of sunshine / change in air temp. Even a few metres of tidal difference in temperature might do it.

Are these still water filled?

Or you have some sort of gas producing bugs in there.

You probably need to connect them to a pressure TX / recorder and take a months worth of readings to see how or if the pressure changes over a day or month and match that to some temperature changes or some other change.



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Also: If you get a response it's polite to respond to it.

 

[Jela123]

Thanks for the response @LittleInch.

M understanding from person on board is that there hasn't been much of a temperature variation over last week.

The risers are still filled with freshwater left in there after hydrotest as they couldn't discharge into sea since an equilibrium pressure will eventually come into play between the water in the sea and freshwater in the future riser. So the hydrotest company just depressurized and shut the freshwater in.

We have looked at the aspect of gas producing bugs and intention is to biocide treat once the source of gas build-up is 'possibly' found.

Will have a look at your last suggestion as well. Thanks

 

[LittleInch]

As said even 3 to 4 c is enough for 1 bar.

Whatever it is I don't think you've discovered a new form of energy....

What size? How much is out of water compared to below?

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

 

[Jela123]

Thanks once again @LittleInch.

The size of the 2 risers is 8inches. Not sure how much is below the sea to be honest but total length of risers to sub-cellar is approx 60metres.

The other point that is of confusion is both of them are same size, filled with same freshwater and in the same location and yet,one has new build-up pressure of about 1barg and the other was 2barg. Same results before they were bled down a week ago. Would have thought they should at least be close after venting down.