DNV-RP-B401- CATHODIC PROTECTION SYSTEM

Question

DNV-RP-B401- CATHODIC PROTECTION SYSTEM

Sadik899 (Mechanical)

(OP)

9 Jan 15 11:38

Hi everyone,
I am trying to interpret the DNV-RP-B401 Cathodic Protection System Code. I am hoping someone could elaborate the following part, 'When the design life of the CP system exceeds the actual calculated life
of the coating system according to (6.4.2), fcm may be calculated as.....'

My understanding is that if tf is more than t, then we apply eq 2. But then how do we calculate t if we don't know f in eq 3.

Below is the statement:

[i]6.4.4 Once a and b are defined, mean and final coating breakdown factors, fcm and fcf, respectively, to be used
for CP design purposes are to be calculated by introducing the CP design life, tf (yrs):
fcf = a + b · tf (eq 1)
For certain protection objects, with large uncoated surfaces, the initial coating breakdown factor, fci = a, may
be applied to calculate the initial current demand to include coated surfaces.
6.4.5 If the calculated value according to 6.4.4 exceeds 1,
fcf = 1 shall be applied in the design. When the design life of the CP system exceeds the actual calculated life
of the coating system according to (6.4.2), fcm may be calculated as:
fcm= 1-((1-a)^2/2btf) (eq 2)
6.4.2 The coating breakdown factor is a function of coating properties, operational parameters and time. As a
simple engineering approach, fc can be expressed as:
fc = a + b · t (eq 3)
where t (years) is the coating age and a and b are constants that are dependent on coating properties and the
environment.
Guidance note:
The effect of marine growth is highest in the upper 30 meters where wave forces may further contribute to coating
degradation. Another factor is periodic cleaning of marine growth in this zone.[/i}

All efforts to tackle this is much appreciated. My first thread. Do let me know if you need any clarifications.

Kind regards,
Sadik Hussain

BigInch (Petroleum) · Answer 1 · 2015-01-13 22:43:01

It appears that you need to know a and t to do this problem. You probably should also know what b is. I'd say that if you don't know a, b and t, you should not be trying to use equation 3.

sundaram15887 (Mechanical) · Answer 2 · 2015-02-24 05:45:23

I think the Cathodic Protection Systems were designed previously without considering the influence of coating on the corrosion behavior. So this lead to increased demand on the number of anodes and the design current etc. But over the last few years Coatings have been found useful in contributing positively to the CP cause which led to a considerable reduction in the usage of anodes. So the point is basically that you need two sets of design life , one for CP sytem and the other for the coating you apply to the structure. Ideally you would like both durations to be similar since it is beneficial to replace them together.

BigInch (Petroleum) · Answer 3 · 2015-02-24 09:13:30

You should size the anodes based on an assumed amount of coating damage and a anode lifetime. What has been done looks like two calculations, picking the biggest number.

What's the point in having anodes lasting longer than the coating?
That's what you want to do? Fine.

sundaram15887 (Mechanical) · Answer 4 · 2015-02-24 13:25:46

@ Biginch,
Yes it would be preferable if you size the anodes based on the coating life. But sometimes Owners tend to specify different durations say 5 years for the anodes and 3 for the coating. So you consider the influence of the coating for the first 3 years and for the remainder, calculate without it. Dry-docking once every 5 years (rather than 3) is more beneficial to the owner since the ship/vessel may have some more issues (routine maybe) that need to be addressed.

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DNV-RP-B401- CATHODIC PROTECTION SYSTEM