Rupturing of 6.6kV Power Junction Box installed at an offshore oil production Well Head Tower

[krisys]

The system:
This is at one of the oil producing offshore Well Head Towers (WHT). The electrical power to this WHT is supplied through 6.6kV submarine cable. The cable is terminated in an Ex 'e' junction box installed above the boat landing level. The system is energized about an year ago.
This is an unmanned tower. The junction box has hinged door, bolted while fully closed.

Incident:
Recently, during the visit to the WHT the junction box door was found ruptured and left hanging on the junction box (JB). However there is no charred marks inside the JB. The connections are found intact.

The 6.6kV connection is not covered with insulation boot. Now there is an argument that there could be some ionization inside the box due to the exposed 6.6kV terminal thus releasing the hydrogen. The accumulated hydrogen must have resulted in an explosion, thus rupturing the JB door.

There is another school of thought which argues that there could be some chemical reaction inside the cable insulation thus generating the hydrogen and which must have reached and accumulated in the JB cavity.

Now the question:
Does anyone has such experience of hydrogen gas generation and accumulation in the cavities in the oil producing towers?

If you can share your experience, it would be very helpful to me.

 

[jraef]

Cable insulation breaking down into an explosive level of any volatile gas would be a problem so huge that it would have stymied the development of the entire oil and gas industry. Yes, under the right circumstances the plastics in the insulation can be broken down during a Partial Discharge (PD) event, but generally that occurs more easily in gas /air filled electrical environments. Cable insulation is designed at 50-100 times the dielectric of even the worst gasses, making PD in cables require an extremely higher voltage before it occurs. So, what could cause that here, lightning? But how could that take place without leaving visible evidence, like charring? In my opinion, that’s highly unlikely.

Much MORE likely to me is a moisture intrusion into the cable installation, resulting in a relatively minor discharge somewhere along a large section of the cable that instantly produced steam pressure that blew out your doors. I’ve seen that with underground utility cables, where over time, moisture seeps in through micro cracks in the outer insulation. Right in front of my house, it blew up and ruptured about a 20ft section of my sidewalk, from 4ft under ground. I watched the utility crew dig it all up and the only place that showed any carbon indication of the fault was a 1/4 inch hole where the initial failure took place. But the jacket was split open for 10 ft in either direction, and the steam pressure release was like a bomb going off under ground, breaking raising the section of 6” thick sidewalk concrete about 10” above grade.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[EdStainless]

Jraef, Yes, I wasn't talking about breakdown of the insulation, but rather out-gassing (either from mfg chemicals or moisture) or simply allowing gas to pass through the cable.
I have seen both H2S and water in the junction box for a submersible pump, that had traveled up 12,000' of cable from the bottom of the well. Needless to say with H2S we actually just found pieces of the box......

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[BJC]

Water in the interior of a power cable is rare but it happens. Unless the cable is cut and water runs out like it was a garden hose you may never know.
At least two cable manufactures and two utilities have procedures for removing water. They all involve purging the cable with dry air or nitrogen and monitoring the humidity of the outflow.
When water is discovered the search for why and where the water got in the cable starts. I have had to send samples of the water in the cable and the local domestic water to independent labs where is analyzed and compared to water at the cable manufactures etc.
By then it's in the hands of lawyers and people above my pay grade.

 

[marks1080]

Has anyone else in the utility business come across the 'green goo'? Sounds like something similar.

AC magnetic fields in moist environments is what us in the utility world blame this on. Sounds like adding saltwater to the mix isn't going to help.

So if the box is completely weather proof, does that mean it's humidity controlled? If the box is completely sealed and air-tight you wouldn't need ignition to blow the door open, just accumulation.

My money would be on some kind of chemical reaction off gassing in the box due to the environment.

How do you ground the box, or do you ground the box?

 

[aussiejohn2]

Hi Krisys,

I know of at least two other instances similar to yours on offshore facilities that were attributed to hydrogen. In one of them gas detection was installed post event & confirmed development of flammable gas in the JB. Some attempts were made to confirm which gas but I think inconclusive (It could be argued that that in itself suggests hydrogen, which escapes on the way to the lab).

Hydrogen from corrosion of conductor or (particularly) galvanized armour is a recognised problem for subsea cables and umbilicals (also fibre optics where it darkens the fibre). Permeation of hydrogen dissolved in the seawater is also discussed - I had thought only applicable to deep water but have seen recent references for shallow. Hydrogen-producing bacteria sometimes occur subsea. Hydrogen is very small (especially if dissociated to ions) as those who keep H2 inside generator stators, or specify pressure instruments exposed to process hydrogen, will confirm.

http://www.oedigital.com/subsea/item/269-umbilicals-under-pressure

http://ripr.org/post/case-closed-hydrogen-gas-combustion-caused-salty-brine-beach-blast#stream/0

http://www.jdrcables.com/case-studies/van-gogh-project-offshore-western-australia/

marks: green goo is degraded plasticiser. It is not explosive.

John.

 

[waross]

Will an alternating electric current flowing through water or moisture produce "Brown's Gas"?
Brown's Gas: (A mixture of oxygen and hydrogen gases in the same proportion as in water?)


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

 

[krisys]

I thank all for your active participation.

Hi aussiejohn2,
Your case appears to be more similar to this case. Can you please throw some more light on this?
I was not able to download the JRD Cables case study. Is it possible to attach the case study?

I remember in my previous assignments, we used to specify and use the lead sheathed cables in the oil and gas facilities. This is specifically important, when the cables are buried where the soil could be contaminated with the petroleum product.
The XLPE and PVC are vulnerable to the chemical attack of petroleum product when installed in the oil and gas facilities.

Please click the Link for more details.

 

[aussiejohn2]

Hi Krisys,
I could swear I had some notes from a presentation that mentioned one of the incidents, but have looked through my notebooks and not turned anything up. My memory is that it was a small unmanned platform, perhaps similar to yours. But my mental image of the photo of JB was it was small - too small to be tied to an umbilical. I've reached out to some old colleagues and will let you know if anything comes back.
The JRD reference was just to this sentence from the web page: "All electrical cable quads included special low permeation outer jackets to mitigate the impact of hydrogen gas." The issue is well recognised in small corners of the oil and gas industry; i think if you contact any of the umbilical makers they will be able to give you some details (Interestingly the offshore wind industry seems unaware, and i have not heard of any incidents even as they move into deeper water).

Nowadays there is a recognised alternative to lead sheath for oil resistance - plastic sheaths with hdpe for inorganics followed by polyamide for oils. Often there is a metalic water barrier. Lighter and without the environmental (and installer) questions around lead.

Regards
John.