Pipeline CP Coating Breakdown Factors (Again)

[DarrenHeal]

Working on a pipeline coated with 2.5 mm 3-layer polypropylene (nothing outstanding about it).

Using RP-F103 Table A.1 and a 3-layer FBE/PP coating I get:

a=0.1x10E-2
b=0.003*10E-2

giving

fcm = a +0.5*b*T = 0.1x10E-2 + (0.5*0.003x10E-2*25)
fcm = 0.001375
and

fcf = a + b*T
fcf = 0.1x10E-2 + 0.003x10E-2*25
fcf = 0.00175

If I do the calculations to ISO 15589-2, however, I get

fcm = 0.0075
fcf = 0.01

So the coating breakdown factors under ISO 15589 are an order of magnitude higher than those under RP-F103 (and a lot closer to what I would expect).

Is there simply an error in Tables A.1 and / or A.2 of RP-F103 or am I missing something?

 

[SJones]

There is no error. You have a perception of what the coating breakdown factor should be based on some belief; DNV have their perception based on their belief. And then there is ISO 15589-2. Why not throw NORSOK M-503 in for good measure and have x1.5 and x3 factors on the total current demand?!!!!!

It's all down to the black art of CP and how lucky do you feel with each standard

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[DarrenHeal]

As for perception, yes, I agree we are all driven by that. On the other hand, very few pipelines have failed due to over-protection and anodes cost buttons compared to the rest of the flowline and its installation.

DNV would have you believe a couple of old aluminium takeaway food trays somewhere in the Barents Sea and a couple of beer cans in the Serpentine would protect a 2 km flowline in the North Sea!

Compromise I'm taking is to use the worst case of DNV and ISO coating breakdown factors and plug those into the calculations. A bit mix'n'match, I admit, but I rather tend to believe six (twelve for spacing limits) 10 kg anodes are needed for a 1.8 km 8-inch pipe than one 5 kg anode!

Then, of course, I have to assume half the anodes will fall off or become disconnected in service or be pulled off when the flowline is trenched. Don't believe the latter will happen? In the mid 90s a certain offshore construction contractor trenched a pipeline and due to under-specification of the anode attachment bolts ended up pulling all the anodes to one end of the line!

M-503 doesn't really come into it - that's for subsea structures, painted, with attendant coating breakdown factors based on both water absorption by, and mechanical damage to, the paint.

 

[SJones]

Like I said, it's all about perception. You don't trust DNV even though it does have strict requirements for coating performance and anode manufacture; your prerogative and, indeed, a few kg of aluminium extra is no big deal. Regarding NORSOK M-503, you may wish to review clause 8 of the 3rd edition, May 2007.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[DarrenHeal]

Right, calculations checked, answers are:

RP-F103:

For 3-layer polypropylene anti-corrison coating (CDS 3) and 300 mm long IMPU field joint coatings (3D system) 25 year design life I get:

fcm = 1.375E-3 for the line pipe
fcm = 0.01375 for the FJC

For a 25 mA/m2 maintenance current density this gives an anode net mass requirement of 15.2 kg.

According to ISO 15589-2 the corresponding mean coating breakdown factor for both line pipe and FJCs is 0.0075 (~5.5 x higher than RP-F013. With a maintenance current density of 20 mA/m2 (80% of RP-F103 value and pipeline is buried so 20 mA/m2 used per ISO 15589-2 recommendation) the net anode mass required is 43.4 kg.

Moreover, due to the very low CBFs under RP-F103 I get an attenuation separation of 1.8+ km using RP-F103 values in the RP-F103 method, compared with ~124 m using the ISO values in the RP-F103 (pick and mix problems acknowledged here).

The ISO values for mass still seem quite low, but the attenuation separation at about 10 pipe joints seems much more in line with what I would expect.

Comments anyone?

 

[DarrenHeal]

One last question: How on Earth (or should that be "under water"?) does one use the attenuation formulae in ISO 15589-2? Plot potentials against distance and see where they cross the magic line or is there a better way?

 

[SJones]

Which all seems to point to sticking to the default anode separation of 300 metres! You might want to go to the ~124 spacing for a kilometre or so if the pipeline runs from a structure with CP and you don't trust the structure CP design!

Looking at the ISO 15590-2 attenuation, it would appear to be simply confirming that you can achieve protection at the point L in much the same way as 5.6 of F103.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04