Emergency sump on offshore oil platform - Why is drain line connected to E. sump 120 ft below water?

[chemebabak]

Emergency sump on offshore oil platform - Why is drain line connected to E. sump 120 ft below water? See attached diagram.
Oil and water collect in the E. sump. The bottom of the sump is open to the ocean. Oil and emulsions float to the top of the E. sump. Twice a day the oils are suction pumped from the surface of the oil.
We have a 14 in drain line that is connected to the sump at 120 ft below water. Since this platform has been placed in service this drain line has filled with oil. It recently has developed a leak. One of our many repair options is to cut out that section of pipe and reconnect the overflow to the top of the E. sump. See page 2 of the attached diagram.
We do not know why the E. sump had been designed with the drain overflow connected 120 ft below water. It may have been designed as a seal leg.
What experience have you all had with this design? Why would it be designed this way?

 

[hydromarine]

How old is the platform ,and what area of the world

 

[chemebabak]

30 years and off the coast of California. How does that affect the answer?

 

[hydromarine]

Its a strange one,like you cant think why its connected at 120 feet ...only to say its in the air dive range ..but if so could have been connected at say 30 ft .My original question doesnt have any relation to the answer , but have come across some unexplained designs on older platforms over the years. Perhaps the constructors of the platform can shed some light

 

[hydromarine]

Have you considered a Plidco Sleeve for repairing the leak .... ?

 

[Compositepro]

The sump is designed to separate the oil and water that goes in, store the oil, and drain water to the ocean. Putting oil and water into the top of the sump, rather than the middle will tend to create emulsion rather than to separate it.

 

[chemebabak]

Hydromarine,
Yes, we have consulted the original design books and we have not found any information of the design philosophy. We are considering Plidco, several competitors and several other repair techniques.

Compositepro,
We thoroughly understand that sumps are designed to separate oil and water. We also thoroughly understand that an emulsion might form from the turbulence. We also understand that the residence time is sufficient for that emulsion to break.

What we do not understand is why the inlet to the E. sump is so deep underwater. This means that the oil and water is separated in the line itself. The oil in the line will not pass to the E. sump until the buoyancy of the oil is great enough to overcome the 120 ft of liquid head in the E. sump.

Thank you both for your answers.

 

[Compositepro]

If you understand, then what is the question? The design would be determined by the worst case flow rate and volumes in an emergency, not by daily drip rates. Perhaps you need a smaller separator in your drain line that won't disturb the function of your sump in an emergency. But your complaint is not about how the sump functions but the difficulty of repair. It seems your suggested modification will prevent oil separation under emergency conditions. Just don't have an emergency and everything will be fine.

 

[chemebabak]

No emergencies?! I wish I would have thought about that.

Compositepro, you are right. I am complaining about how difficult it is to repair.

Under further review, we believe that this is correctly designed for O/W separation, but it has a design flaw which allows oil to stagnate in the pipe. The correct design is for the major lines to drop deep below sea level, as per the drawing, to avoid emulsions in the sump. In addition to that, there should be a 1/4" connection between the major lines and the sump at sea level. This way, during normal operation, any oil floating in the pipe would slowly pass to the sump.