Duplex to Carbon Steel Pipe Destec Hub Connections 2

[Mozzie]

Hello All,

We operate an offshore FPSO and are modifying some of our topsides production pipework (high pressure hydrocarbon). Unfortunately the pipework has been designed with a spec break - Duplex to Carbon Steel. The pipe connection at the spec break is in the form of a Destec Hub. P&IDs indicate that in order to isolate the two dissimilar metals and thus prevent galvanic corrosion, that we coat the duplex side of the spec break (over a length of 1 metre). Is this an acceptable measure, or would it be better to use some sort of insulating gasket kit at the pipe connection (if such a thing is available in Destec specs).

Appreciate any feedback.

Cheers

 

[YLWEA]

I'm not familiar with DESTEC, but we have used similar internal coating approaches to preventing galvanic corrosion. Our rule of thumb is coat the wetted surfaces for 3x the pipe diameter.

 

[Mozzie]

Destec is a brand name, I believe Grayloc Hubs are another. These hubs are an alternative to standard flanges especially for high pressure applications where the size of standard flanges is restrictive. Basically these connections consist of two connecting pipe hubs, two "G"-clamps with two studbolt connections either side and a seal ring gasket.

I am not a corrosion engineer but I would have thought we have a corrosivity concern on the external of the pipe (seawater environment as the electrolyte?)moreso than the internal (hydrocarbon oil/gas). Shouldn't we paint the duplex pipe externally?

Cheers

Kim

 

[YLWEA]

If this is immersed in seawater and the OD is wetted then I retract my comment about coating. I was thinking only the ID was wetted. For immersion service and disimilar metal connections my first choice would be an insulating gasket, although I don't have experience with FPSO systems.

 

[Mozzie]

Thanks for your input.
Cheers

 

[MetalMickey]

In subsea pipelines we produce we would always use an insulating gasket wherever possible. The use of a coating is higher risk due to possible damage or breakdown, or complications due to any CP applied. However knowing the Destec design I think the use of a gasket is going to be pretty difficult, those guys will be the best to advise. In a similar situation we ended up using welding cladding.

Hope this assists