25 July 2018
Some folks like their free water knockouts horizontal. Some like them vertical. And others, mostly Canadians, like them inclined. Not too inclined, only about 8 degrees...
Some folks like their free water knockouts horizontal. Some like them vertical. And others, mostly Canadians, like them inclined. Not too inclined, only about 8 degrees. The inclination encourages gas and oil to rise away from the water for better separation. And the Canadians have built over 150 of these inclined free water knockouts, Foremost Energy Equipment, branded them SKUD’s® because they resemble SCUD missiles. And like Canadians, they didn’t want to offend, so asked that we not refer to them as SKUD’s® to an American audience.
The units are designed to take advantage of well pressure so they are located at the production pad. Shear reduction is the biggest advantage of placing the units upstream of choke valves and pumps. Low shear equals bigger oil droplet sizes equals higher rise velocities equals better separation.
Because they are 54” in diameter, they can be economically pressure rated up to 720 psi. They are usually used in higher operating pressures. Low operating pressures for the SKUD® are in the 30 to 50 psi range.
SKUD’s are meant to be a rough cut, either two or three phase, to address high water cuts, greater than 90%. The effluent oil in water and flow capacity will vary according to the API grade. A standard SKUD®, which is 54” in diameter and 55 feet long, handles about 40,000 BPD. A smaller, 36 inch diameter SKUD is also available for lower flow rates.In most cases another oil/water separation step is required downstream.
Seabed Separations - Three-phase subsea oil/water separator
Another tilted pipe technology has been developed to allow subsea discharge. The Dual Pipe Separator (DPS) meets the demands of the subsea world with a pressure-rated for 300 bar (4,320 psi). Like the SKUD ®, the uni separates oil from water under the pressure of the well. This prevents the emulsifying effects of the choke valve and allows for effective separation without chemicals.
To further improve the efficiency as a three-phase separator, a manifold called a “gas harp” precedes the unit and allows most of the gas to be released from the stream and bypass tilted dual pipes. After the bulk of the gas is separated, the liquid stream enters an internal pipe that flares out to slow the velocity. At its far end, this pipe-in-pipe is strategically perforated to allow oil to rise through the top and force water to drop into a wider, slower velocity, secondary pipe. An oil layer accumulates at the top of the tilt, which is 15 degrees off horizontal.
Pressurized tests results show oil removal down to less than 20 mg/L. Seabed is also looking at the onshore market with a unit that handles lower pressures and uses less expensive alloys. After thorough full scale testing, Seabed separations is looking for a company to commercialize the technology.