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Grounding: Soil Resistivity for Water

Grounding: Soil Resistivity for Water

Grounding: Soil Resistivity for Water

(OP)
I am performing ground grid study for an offshore platform. There is no soil, only water. Does anybody know what soil resistivity should I use for ground grid calculations?

RE: Grounding: Soil Resistivity for Water

The conductivity of seawater is 5000 millimhos per meter according to an old (1961) USGS ground conductivity map.  This makes resistivity 0.2 ohm-m.

RE: Grounding: Soil Resistivity for Water

(OP)
Actually the ground grid will be enclosed in the steel structure of the offshore platform. There will be no ground-rods, only ground-conductors. There will definitely be water surrounding the steel structure, but the ground current will travel in the ground conductors and the steel structure, and will come back to the generator. It will not dissipate in water. Maybe I should use the conductivity of steel (instead of seawater)?

RE: Grounding: Soil Resistivity for Water

What ground grid calculations do you need to make?  With all local sources and all current flowing through metallic paths to the source, there will be negligible GPR, touch-voltage, and step-voltage.

RE: Grounding: Soil Resistivity for Water

(OP)
Basically I need to calculate how much ground current is flowing back to the generator from each switchgear, MCC, and transformer. Do I still need to enter the steel conductivity? Is the short circuit current and the ground current the same thing in this case?

RE: Grounding: Soil Resistivity for Water

(OP)
Basically I need to calculate how much ground current is flowing back to the generator from each switchgear, MCC, and transformer. Do I still need to enter the steel conductivity? Is the short circuit current and the ground current the same thing in this case?  

RE: Grounding: Soil Resistivity for Water

So your real problem is not grounding calculations but fault current calculations.  You need to know the resistance, reactance, and mutual reactances of all of the fault current paths to accurately determine the path of the fault current.  Not an easy task.

RE: Grounding: Soil Resistivity for Water

(OP)
Yes that is correct, there will be no ground conductors embedded in the soil, instead there will be insulated ground conductors that will take the current back to the generator. Should I connect all these conductors to the neutral of the generator?

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