Pipeline C.P. Design

[Hariharan]

I am looking for the basis for determining the coating breakdown factor for a submarine pipeline coated with about 500 microns of Fusion bonded epoxy and
having a concrete weight coating of 1-3 inches. This type of system, I understand,
is common in GOM. The coating breakdown factor is required for design of the Cathodic Protection System.

DNV RP B401 is not explicit for this combination. Any other standard, or interpretation
and GOM Practice or references would be useful.

Thanks

Hariharan

 

[roscoe]

I remember that we used DNV RP B401, and although there was no breakdown factor explicitly called out for FBE, we used the most similar coating (I seem to recall DNV lists a rubberized coating system, I don´t have a copy of DNV in front of me). Since FBE is better than that coating system, the assumption is conservative.

The project was also for submarine pipelines in GOM.

Your FBE thickness sounds about right. I recall we used 26 mils FBE coating thickness with a non-slip layer to bond the FBE and concrete and prevent damage during pipelaying activities.

You might try contacting your Anode and/or FBE fabricator/installer.

Regards

 

[Hariharan]

Roscoe,

Thanks. When did you execute this project? DNV RP B401 (1993) has
a definite breakdown factor for FBE coating without any overwrap
and for FBE with adhesive and polypropylene type overlays. There is
none for FBE + concrete coating. There is however, a category called
'any equivalent protective coating + mechanical protection or weight
coating" which is somewhat vague.

An additional point is - both concrete cab absorb water. Can this
result in additional current drainage, and call for a higher
"equivalent" coating breakdown factor ?

TIA

Hariharan

 

[narenr]

The coating breakdown factor for CP design is dependent on the coating specification and the protection provided, generally the weight coat concrete.

Apart from the corrosion coating having inherent protection capability, coating is prone for damage. Coating specification defines the quality requirement such as for holiday detection and acceptance. This paramater greatly affect the coating breakdown factors.

For offshore pipelines concrete coating provides good protection against abuse during handling and installation. However in case of thin coatings such as FBE, 3PP, PU or PE, the coating is prone for concrete slippage and damage in spite of providing antislip bands.

The generally used coating breakdown factors for thick corrosion coating + Concrete are as per DnVRPB401 clause 6.5.3. which for example of 30 years design life is 5% avg and 11%final.

Although pipe coating is very well established procedures, DnV seems to be confusing Owners by including clauses such as 6.5.4. I have recently come across Exxon-Mobil requiring 23%avg and 41%final for FBE with or without concrete coating [ridiculous].

Therefore it is safer to use coating breakdown factors as per clause 6.4.3 for thin coatings.

The water absoption of concrete has not effect on breakdown factors, however current drainage may occur. Additional CP may be required for the drainage due the reinforcement surface area, Refer NORSOK M-CR-503 and DnVRPB401 clause 7.5.3.



Narendranath R
narenr@narendranath.itgo.com

http://www.narendranath.itgo.com

Pipeline engineering is made easy with state of the art computer software, visit

http://www.narendranath.itgo.com.

 

[Hariharan]

Naren.

Thanks a lot for the info. I am trying to gather info against an
Exxonmobil type of situation! Could you refer me to some
Oil company specs which may be available on the net, which
specify Cl. 6.5.3

I did raise the point of current drainage due to reinforcement
with my colleagues. Their response is that the reinforcement
is not electrically connected to the steel pipeline, and hence
there would be no current drainage to the reinforcement.

Let me check about M-503 and DNV and revert.

M. Hariharan

 

[Hariharan]

Here normally 1.5mm bars @ 50 mm spacing are used as a mesh
for reinforcement. This is bare steel and works out to nearly
20 % of the steel surface area! Not considered by anyone.

M. Hariharan

 

[narenr]

Hari,

I have not come across any spec avialable in the public domain.

http://www.polcor.gr/poloff.htm

gives very basic CP design details. Try searching for US military specs most of which are avialable in the net. The ARAMCO specs and SHELL DEPs are also quite detailed, but not avialable on the net.

Narendranath R
narenr@narendranath.itgo.com

http://www.narendranath.itgo.com

Pipeline engineering is made easy with state of the art computer software, visit

http://www.narendranath.itgo.com.