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07/12/2009
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A report on a special solution to a difficult sensing problem that may also have uses elsewhere.
 A staff with a multiplicity of pressure sensors along its length is being used to detect the interface between gas and oil, oil and water and the thickness of oil water emulsion layers in oil water separators.
The problem arises when crude oil is pumped out of wells as as it is a mixture of gas, water, mud and sands. The oil, water and gas need to be separated before pumping the oil elsewhere or processing. So it is necessary to gauge the various levels, in order that the separation is accomplished efficiently.
The latest solution, devised by engineers at the StatoilHydro oil company, working with the Norwegian independent research institute, Sintef, is in the form of a vertically mounted, 3m long staff, with a hundred pressures sensors along its length, each 30mm long.
This is immersed in the oil water separator, and the positions of the various fluid levels deduced from the kinks in the pressure profile along its length. Software routines underlie the measurement process.
None of the commercially available solutions for sensing the levels of liquids in a container are suitable as Magnus Hjelstuen, a research scientist with Sintef explains: "The measuring problem can be solved with capacitive sensors and there are units that are commercially available. Water level can also be measured using density sensors based on radiation. Here also, there are units that are commercially available. But both of these measuring principles have one big weakness: they measure incorrectly if there is a scaling deposit on the sensors. This is a well known problem in the oil industry. We hope these new sensors are not so sensitive to scaling.
"Ultrasonic sensors are also an option for measuring water levels. Such sensors must usually be mounted in the bottom of the separation tank where sand and other heavy particles deposit so they are therefore not 100% reliable."
Sintef has so far made two staff gauges. StatoilHydro is currently piloting them on the Oseberg Field Centre in the North Sea. The Centre includes three platforms, Oseberg A, B and D, connected to one another with bridges, in the southern part of the Oseberg field, and the Oseberg C platform, which lies 14 km to the North. The centre processes both oil and gas.
Oil-water separation is additionally required in the treatment of waste water effluents from oil refineries, petrochemical plants, and chemical plants. They are also often used in electrical substations using oil for cooling purposes. Moats are constructed to catch leaked oil but they also catch rain water. Oil water separators are required in each case to recover oil and produce water, free of oil, that can be safely discharged. There are also many other instances in the food and chemical industries where separations between oil and water based phases need to be monitored.
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Author
Tom Shelley
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