Risk of shock due to the cable fault in the sea bed 1

[krisys]

In an offshore oil filed, we need to take the barge and jack it up near the Well Head Structure (WHT) for working. Sea bed of this area is crowded with 6.6kV live cables. The intention is to safely jack the barge up for the maintenance work in the WHT. The cables shall not get damaged. All the precautions will be taken to avoid the cable damage.

Now the question is that by any chance if the cables gets damaged, what is the risk involved?
Tripping of the supply is not a problem, it is permitted.
All the cables are protected with cable differential protection (primary protection to operate in 40 to 60 msec) and the back up protection to trip in 120 msec.

Now my question is whether there is any risk for the people who are residing in the barge? or any other risk of property damage?

The barge can be earthed by connecting the same to the body of Well Head Structure (WHT) which is a permanent structure located very near and is anchored to the sea bed.

My view is that the sea water is conducting and hence the fault current will flow with in the sea water, hence there is no risk of spark over in the barge.
The barge will be connected to the WHT earth, so the barge will be at earth potential. Cable damage, in case if it happens fine for me, but the safety to persons and the other installations is important for me.

 

[krisys]

Imagine a case of a ship sailing in the ocean. Suddenly the ship's hull (i.e. body) comes into contact with a 6.6kV live conductor. The 6.6kV supply is communing from a 6.6kV generator having resistively earthed neutral. The generator is installed one steel structured platform in the vicinity.

Now the question is what would happen to the people inside the ship? Any risk of shock or fire in the ship? The 6.6kV supply system has standard earth fault and over current protection systems to actuate the circuit breaker and clear the fault.

 

[waross]

Google equipotential zones and Faraday cage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

To get an idea of the extent of asian carp in some rivers, people put probes in into the water around their boat and shock the fish into jumping up out of the water.

https://youtu.be/1BhoCNOjbzo?t=119

"The DNR’s Todd Somers is the foreman of one of these homemade boats. He points out a 240-volt generator near the back of the boat. It can deliver up to 16 amps through the metal poles at the front of the boat; sending electric shocks through the cables into the river. That’ll stun any fish nearby."

http://michiganradio.org/topic/asian-carp-great-lakes#stream/0

Up to 16 amps? That seems kind of scary. I don't think I would stamp that setup.

 

[cuky2000]

The risk is minimum since all the metallic structure of the barge will rise the voltage at the virtually at same potential until the fault is clear by the protective device. Therefore touching any part of the structure at the same potential will be safe, even if the voltage rise very high.
Areas to watch for safe concerns:

1) The barge and the WHT should be at the same potential. Redundant grounding path is recommended.
2) The barge is usual made of steel (ferromagnetic material) that will have non-uniform impedance that change with the frequency. If the barge is too large a non-magnetic dummy grid (let say ~20'x20' Cu) will help to provide a closest equipotential surface on the metallic barge.

 

[krisys]

Thanks cuky2000:

Useful clues, and appreciated.

The following has enlightened me:
The ferromagnetic material has non uniform impedance.

HamburgerHelper:
Your video was interesting that the fish started jumping. But our case is little different. It is electrical protected system. The fault is expected to be cleared even with back up protection within in about 120 milli second.