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Courtesy of Natco Group Inc.
Floating offshore platforms are used around the world for the production,
storage, and off-loading of crude oil and derivative products. Separation,
one component of the production phase, poses a unique challenge on a floating
platform because of the inevitable wave motion to be expected at sea,
which is present even in calm weather conditions. By simulating the flow
inside a gas-oil separator on a production platform, engineers at Natco
Group Inc. have been able to understand and improve its performance under
different sea conditions.
The oil-water (blue) and gas-oil (red) interfaces are shown about 14 seconds
after the start of the simulation of the improved separator design
A floating platform can experience six degrees of wave motion. These
are known as surge, sway, and heave (axial motions), and pitch, roll,
and yaw (twisting motions). When combined, these motions act to mix the
gas, oil and water in the separator, which is counter-productive to separation.
Since separation occurs as the gas, oil, and water flow through the unit,
reduced separation efficiency (resulting from mixing) means that the separator
has to be larger in order to allow enough time for optimum separation
to occur.
The sloshing (shown on a vertical plane) is much worse
without internal baffles (top) than with them (bottom)
Using the volume of fluid (VOF) model in FLUENT, Natco engineers were
able to simulate the transient sloshing motion of oil, gas, and water
inside a separator. Both normal and storm sea states were studied. The
six degrees of motion were incorporated through user-defined functions.
A custom panel was developed that allowed the engineers to input the characteristics
of the wave motion using the graphical user interface.
The panels created through user-defined subroutines allow the user to
input the six degrees of motion
Baffles are normally used inside this type of separator to dampen the
sloshing that arises from the complex wave motion, and thereby reduce
the mixing of the fluids. CFD simulations were run with and without baffles
to study their effect on sloshing and mixing. When the separator was simulated
without baffles, severe sloshing was predicted, as expected. When the
separator was simulated with a conventional baffle design, it was found
that the baffles were not able to substantially suppress the sloshing
motion and resultant oil-gas mixing. In fact, if not properly placed,
the baffles could aggravate fluid short-circuiting in the separator.
A new baffle system was then proposed that included strategically placed
perforated plates. When this was modeled, the results showed that the
modified design led to a considerable performance improvement for the
separator. The improvement was good enough that the separator design could
be based entirely on the use of baffles. Other more complex and expensive
internals were not required to achieve the goal of suppression of wave-motion
induced sloshing. The cost savings resulting from this redesign are now
being passed on to Natco's customers.
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