Cathodic protection for Inner pipe in PIP

[ronfrend]

We have an application where DoT requires cathodic protection on the inner pipe on a 12x16 "pipe in pipe" subsea pipeline. Does anyone have any experience of this? I've talked to several suppliers and our own pipeline engineers with no luck. The only suggestion the contractor has come up with is galvanizing the inner pipe but I have bad experience of galvanizing causing blockage of impulse lines due to oxidation. The annulus is to be under vacuum (only a couple of inches water) and nitrogen filled so any leakage would allow water in.
Any thoughts?

Ron Frend

http://www.predicon.net

 

[LittleInch]

I would assume the PiP is required for some purpose.

If the annulus fills with water, that purpose has now been breached and hence the pipe has essentially failed and will need to be withdrawn from service / repaired / replaced, otherwise why have the Pipe in pipe in the first place?

As such the requirement for CP of the internal pipe is ludicrous and serves no purpose. Point out these salient facts to the DOT. Of curse if your pipie can actually survive and continue in operation after such an event then you're in trouble.

In any event CP only works when there is a current circuit. I there is no water there is no circuit and no corrosion.

You could always just clamp on a few anodes and pretend that the inner pipe is in open sea, but I guess it is bare pipe and the anode size would be huge.

Unfortunately Pipe in Pipe ( a drawing would be good to see exactly what you mean) is not covered in any design code or document and therefore needs some adjustment of standard design thinking. Clearly this is currently beyond whoever at the DOT is coming up with that, but you need to engage them and point out the error of their ways.

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

 

[ronfrend]

Thanks LittleInch, that's exactly my thoughts too but I thought I would see if anyone else had this problem. I also checked the ABS "Guide for Building and Classing Subsea Pipeline Systems" and in Chapter 3, Section9 paragraph 5.13 it states
"Cathodic protection design is to be performed according to relevant codes, e.g., NACE RP0176. Cathodic protection together with an appropriate protective coating system is to be considered for protection of the external steel surfaces from the effects of corrosion.
The sleeve pipe is to be protected from corrosion using chemical inhibitors within the carrier annulus fluid. Pipelines within the sleeve pipe are to be maintained in a dry environment, and a cathodic protection system will therefore not be required."
Looks a (very) long chat with DoT.


Ron Frend

http://www.predicon.net