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.

 

[itsmoked]

Throw this into your mystery soup:

If saltwater is electrolyzed it produces chlorine. If chlorine evolves in the dark it is highly unstable and will detonate with any introduction of light.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[krisys]

Keith,

In fact I am trying to solve the mystery. If you are familiar with the oil and gas facilities, you will appreciate the gravity of the such incidents. It has a large hidden consequences.

The WHT is unmanned. When the mechanical maintenance crew visited the site, they have noticed that the cover is hanging in one bolt. The 6.6kV live terminal is exposed to the hazardous environment.

The JB has no visible mark of burns, hence people suspect that it h=could be the explosion of trapped hydrogen gas inside the JB.

For your argument of chlorine release:
The salt would help the solution to conduct. During the electrolysis process, I understand that the Oxygen and Hydrogen will be released. Not the chlorine.

Now please throw some light.

 

[Hoxton]

Any chance of photos?

 

[itsmoked]

Hi krisys. Not hydrogen and oxygen when electrolyzing salt water but chlorine and hydrogen. The chlorine could provide the ignition.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BJC]

How tight was the box? Smaller "Explosion proof" fittings usually have vents and drains.

 

[ScottyUK]

It's an Ex 'e' box - it's weatherproof, probably to IP56 / IP66. Very different to a flameproof (Ex 'd') design which is loosely equivalent to explosion-proof in the US.

 

[roydm]

My chart just describes EX e as increased safety, it doesn't really describe the construction. is it waterproof like a NEMA 4 or not like an EX d without a gasket.

I assume some sort of explosive gas built up and was ignited by a spark to blow the lid off.
Presumably the environment outside the box is not explosive so it must have been generated internally, perhaps inside the cable, is there a gas seal at the box?

 

[controlsdude]

Cant you just run a gas spectrum analysis on a swipe of the dust or the contained air in the junction box?
Maybe enclose it again for a predetermined amount of time maybe using a plastic bag to isolate the box, since you dont know the reason for the box explosion.

 

[ScottyUK]

From the standard:

4.9 Degrees of protection provided by enclosures
4.9.1 The degrees of protection as defined in IEC 60034-5 and IEC 60529 shall be as
prescribed in a) or b), unless otherwise specified in 4.9.2, 4.9.3, or Clause 5.
a) Enclosures containing bare conductive live parts shall provide at least the degree of
protection IP54.
b) Enclosures containing only insulated conductive live parts as in 4.5 shall provide at least
the degree of protection IP44.
4.9.2 The enclosure of an electrical apparatus may be provided with drain holes or
ventilation openings to prevent the accumulation of condensation. The requirements are
dependent upon the apparatus grouping as follows.
a) Apparatus group I – compliance with 4.9.1 is required.
b) Apparatus group II – the inclusion of the drain holes or ventilation openings may reduce
the degree of protection provided by the enclosure according to 4.9.1 , but shall not be
below IP44 in item a) of 4.9.1 or IP44 in item b) of 4.9.1.
When the presence of drain holes or ventilation openings reduces the degree of protection
below the requirements of 4.9.1, the details of the drain holes or ventilation openings,
including position and dimensions, shall be stated by the manufacturer and included in the
descriptive documents according to IEC 60079-0. The marking of apparatus with drain holes
and ventilation openings that reduce the degree of protection shall include the symbol "X" in
accordance with item i) of 29.2 of IEC 60079-0 and the reduced degree(s) of protection
provided by enclosure shall be shown on the certificate.

Most Ex 'e' enclosures exceed the minimum IP54 requirement, as I noted earlier. They generally don't look much different to a good-quality outdoor-grade enclosure.

 

[BJC]

Are there any bulges in the sides or back?
Is the core of the cable filled>
If the door is big enough you don't need much pressure to blow it off. Water in the cable and a high load ( for what ever reason) could generate enough "steam" pressure to push a door off.

 

[davidbeach]

Steam - good point. Was there a significant change in load prior to this event?

 

[krisys]

I thank for the overwhelming responses.

The JB is IP66, manufactured as per IEC. The installation is in the tropical region offshore where the relative humidity is almost 99%.

I have attached a typical photo for the understanding.

There is not a considerable bulges on the sides and back.
There is a breather for the JB.

It is stainless steel with corrosion-proof paining.

David,
The cable size is 3 core 240 sq mm and the load current is about 20A. So there is no question of the termination/connection heating up in this case. The cable connection is still intact and no charred mark. Only the door got ruptured.

 

[waross]

Is the drain in good shape?
I've boiled away several thousand gallons of salt water doing loading to ensure that the piston rings were well seated in new diesel generators.
Never got a whiff of chlorine, but lots of steam.
It is almost impossible to keep an electrical junction box dry in outside, humid conditions without a drain.
If the drain was plugged you may have had a steam pressure event.
Depending on several factors it is a possibility.

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

 

[ScottyUK]

Where are the other end of the cables terminated? Is there any possibility of gas propagation down the cable? Should one of the remote ends be fitted with a barrier gland?

 

[itsmoked]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[krisys]

Other end of the cable is connected to another similar WHT from where the feeder is coming to this WHT.

 

[roydm]

That box is EX e, you can't be serious

 

[ScottyUK]

roydm,

Why?

 

[EdStainless]

I would bet on gas evolution or propagation through the cable.
I doubt that there was a real explosion (with combustibles) but rather 'just' over pressure event.
There is obviously no vent on this, or not a functional one.
Venting these is very problematic. Every day as the box heats the air expands and vents. At night it cools and sucks in humid, salty air which then condenses inside the box. The next day the process repeats. I have seen the insides of 'sealed' boxes heavily corroded by this process.
And then with corrosion more heat is generated, and steam, and then all sorts of things go wrong.

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P.E. Metallurgy, Plymouth Tube