Maximizing the volume of oil recovered from hydrocarbon reservoirs is, understandably, high on the agenda for all oil companies. But oil production
is almost always associated with water coming from the reservoir too, requiring the
two liquids to be separated to achieve a desired specification for final oil quality before this is exported from the field.
Bulk separation of the oil and water is normally achieved by gravity separation in several stages in enclosed horizontal pressure vessels, each successive stage incrementally dewatering the oil. The final stage of separation – where it starts to get tougher to get water-in-oil content down to just a few percent in a reasonable time
period – frequently demands the inclusion of an electrostatic coalescer to coax the separation process along.
“The phenomenon of electrostatic coalescence has been known for almost a hundred
years,” says Mika Tienhaara, Aker Kvaerner’s Vice President for Brownfield Development and Services. “The presence of an electric field causes the water droplets – which are only microns in diameter – to be quickly attracted to one another. On contact, they coalesce into larger droplets, an effect which promotes the subsequent gravity separation of the oil and water.”
Conventional electrostatic coalescers
Conventional electrostatic coalescers use a grid of high-voltage electrodes suspended in the water-containing oil layer within the separation vessel, creating the electric field needed for water droplet coalescence. But such vessels, often tens of feet long and of large diameter, are renowned in the industry for being difficult to
operate continuously at best efficiency and often require the addition of emulsion-breaking chemicals. Furthermore, as time goes on, increasing volumes of produced water come to the surface from the reservoir, subjecting the separation equipment – initially installed in the field – to added liquid loads that make it harder to maintain performance levels and deliver acceptable oil quality. And on floating production
platforms, large horizontal pressure vessels respond to ship motions, which can cause the liquids inside to slosh around, sometimes causing short-circuiting of the electrodes and also reducing the overall effectiveness of the process.
The Petrojarl 1 production ship, left, has hosted a four-year trial of the CEC™ technology in Statoil's Glitne field offshore Norway. This photo shows the base of the unit.
An alternative solution
An alternative solution, CEC™ technology, has recently become available commerciallywhich promises not only to overcome these unwanted effects, but also to provide a more compact, efficient and reliable separation process.
Compact and Continuous
Statoil is the owner of the patented CEC™ technology and has issued an exclusive commercial license to Aker Kvaerner. Continuing on from original research in the 1980s at the University of Southampton in the UK, Aker Kvaerner has been working with oil company partners on the development and testing of CEC™ technology
for the past ten years. Support for the development work has come from leading oil companies – such as Statoil, BP and Chevron – in joint industry projects and from Norway’s Demo 2000 program.
“The result is a robust and very compact coalescer,” explains Tienhaara. “CEC™ units
have been proven onshore and offshore on floating platforms and on difficult-to-treat oils such as heavy crudes, and it continuously and reliably delivers a final export oil of improved quality.”At the heart of the CEC™ technology is a series of concentric circular electrodes, housed within a coalescing section inside a 14-18-ft (4.2-
5.5-m) tall, vertical vessel. Voltages of up to several thousand volts can be applied to the electrodes to create a more intense electrical field in the annular channels and thus a more intense coalescing process than is achieved in conventional grid units. Water droplets merge several times within a matter of seconds and increase their size around ten times in the coalescing section. The water droplets and oil then
enter a gravity separator for separation, but one with much reduced dimensions compared to a normal electrostatic coalescer.
Shorter overall residence time
Extensive testing carried out to date shows that the process requires a much shorter overall residence time than the traditional approach. The result is a combined CEC™ unit and its separator requiring about half the space and weighing around one third of the conventional alternative – attractive qualities, particularly for offshore applications. Perhaps even more advantageous are the operating characteristics of CEC™ technology.
“The vertical configuration with concentric circular electrodes and small liquid hold-up
renders it insensitive to vessel motions, and it is not prone to being plugged by solids in the wellfluids,” explains Morten Hana, Product and Technology Manager for the CEC™ technology.
“This enables the process to maintain high performance levels, while using less emulsionbreaking chemicals. It can also tolerate higher water cuts of up to 40 percent or more, giving it a wider operating window than conventional systems. The fact that CEC™ units are external to the separation vessel also brings important benefits
in terms of faster and easier maintenance.”
Optimal Operation
CEC™ units, targeted at both brownfield applications – particularly for flow debottlenecking – and greenfield projects, have been put through their paces in several locations. Notable among these is the multiphase flow test loop located at Hydro’s research center at Porsgrunn in Norway. Here, CEC™ operation
was verified on real crude oils in several joint industry projects and the subsequent offshore pilot trail onboard the Petrojarl 1 floating production vessel. Petrojarl 1 is on the Glitne field offshore Norway and is operated by PGS Production on behalf of field partners Statoil, Total, DNO and Pelican. First oil from Glitne came onboard Petrojarl 1 in 2001, with the CEC™ unit being compared to a conventional electrostatic coalescer from July 2002 at flow rates of some 47,000 bpd (310 m3/hr).
“The CEC™ unit has been running for more than three years with trouble-free continuous operation in year-round North Sea conditions,” says Frithjof Amundsen, Manager of Topsides Development at PGS Production. “The CEC™ unit has normally been operated in series with the conventional coalescer, but regularly tested for performance as a standalone unit as the water cut has increased. The resulting
export oil quality is better than 0.3 percent basic sediment and water – the industry's way of measuring export oil quality – for a variety of feed conditions with up to 20 percent water cut. Meeting oil export quality is a strict requirement for the Glitne field.”
Work is continuing at Porsgrunn – together with Hydro and Petrobras – to evaluate the
CEC™ technology on additional heavy crudes and for subsea applications in cooperation with Aker Kvaerner Subsea. Aker Kvaerner is also participating in a new joint industry study being led by Norway’s scientific research organization SINTEF into the basic science underlying the coalescence phenomenon.
Commercial Success
Significant commercial breakthroughs for the CEC™ technology have been made by Aker Kvaerner recently, with three offshore installations of the unit. In the UK’s Captain field, operator Chevron is replacing a conventional coalescer on its floating production facility for treatment of 130,000 bpd (860 m3/hr) of liquids. Operator Maersk is installing a 38,000-bpd (250-m3/hr) unit on the Al Shaheen fixed platform
in the Arabian Gulf. And for the Alvheim field, a new greenfield development offshore
Norway, operator Marathon has opted for the CEC™ technology to treat up to 130,000 bpd (860 m3/hr) of liquids on its new floating production vessel.
“We believe the CEC™ technology is a unique process unit that will rapidly find applications right across the oil and gas industry,” concludes Tienhaara. “It fits well with the old saying that small is beautiful.”
The CEC™ unit ready for shipment to Chevron's Captain FPSO in the UK sector of the North Sea.
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