Cathodic protection separation for Offshore to Onshore Pipeline

It would seem that the anodes would tend to reach an equilivent voltage, one somewhere between the voltages of the two anode materials. Therefore you would increase the protection afforded by one material at the expense of the other. One section of piping would then effectively be underdesigned CP system, the other overdesigned. I would design both sections using one type of anode and not use an insulation flange until you go above ground and stay up there. Its not long enough to need two different CP systems.

Sparking due to cathodic protection mismatch voltages I would think would not be a problem as it would be somewhere around 0.5 volts, but now that problem is solved too.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

Dear Big Inch,

I don't think it is effective to use aluminium anode for onshore buried section. That's way in the initial design we want to separate those two section using insulating gasket.

In case we remove the insulating gasket, the magnesium anode (onshore section) is more negative compare to the aluminium anode and the quantity is very small compare to offshore section, therefore I assume it will be depleted lot faster than the original design. Am I correct?


Rgds,
Wyn

The subsea anodes will have some 'throw' up the beach since there is some continuity of electrolyte from water to soil. The question is: is it far enough to reach the road crossings? Probably not. So, you will need to supplement the CP. In using two sacrificial anode systems, there shouldn't be a need to insulate between the two. If it was impressed current onshore there probably would be.

Having said that, what is happening beyond the road crossings in terms of entering a plant etc because you will need to isolate there otherwise the magnesium will disappear rapidly? Also, watch out that the pipe supports don't contact the pipe and draw current; or, allow for it in the anode calculations for the magnesium.

Steve Jones
Materials & Corrosion Engineer

Where's the onshore CP survey data?

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

We are using concrete support. However, steel plate will be still there on top of the concrete, which is in contact to the pipeline (separated only by coating).
Soil resistivity is 30 ohm-m

Concrete saps up water from the soil and puts it in contact with the pipe. You'll need neoprene between the bottom of the pipe saddle and the steel plate embedded in the concrete.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)