Is a double block a safe barrier to environment for subsea structure?

[napoleon1799]

Hi,

We are designing a pressure cap to be used on manifold header for future connection.
Upstream of the header we have a ROV operated valve which will be closed at all time. We want to have a ROV operable needle valve on the pressure cap so we can equalize the pressure prior removing the pressure cap in case the future connection is to be used.

With the above design, we will have basically 2 valves between the oil and the environment. Can a double block be considered a safe barrier to environment?

Some people have mentioned to me that a valve and a blind flange is acceptable but not two valves as they could leak...

I have been trying to look through API 6 A and 17D for guidelines but can't find any clear answer...

Of course one option could be to equalise pressure from the FPSO but in our case this will mean to shut down the full field production which is not the best option...

Any ideas/ advices on what is considered safe barrier to environment and standard references??

 

[BigInch]

Hard to say without knowing the exact conditions of the location and how you will depressure and install the new extension.

I think that 1 DBB is sufficient isolation for normal operations, which apparently you will put between the header main body and the header end cap. BTW, you do mean a welded cap fitting at the end of the header, right? If so, how will you remove that cap in the future, cut and weld?

I do not think that 1 DBB is sufficient for cutting & welding work. Add a blind for that.

A needle valve for releasing pressure to a safe location, if gas, or to an oily-water drain, if liquid, is OK.

I only question the use of a remotely operated valve to release the pressure. Should there be personnal in the area, they might be exposed without some kind of lock-out procedure. I would prefer a manual valve, with a CSO if needed.






**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

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[BigInch]

Sorry, seems like its a subsea component, so there shouldn't be any personnel in the area during pressure release, correct?




**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/