Centrifuge cleans water discharges on Alba FSU

Chevron U.K. Ltd.'s Alba development includes a floating storage unit (FSU) for offloading produced crude to tankers (Fig. 1).

North Sea Discharges (Fig. 2) [74,992 bytes]
A centrifuge system replaced coalescing filters for cleaning discharged water on Chevron U.K. Ltd.'s Alba floating storage unit (FSU), in the North Sea (Fig. 1).

According to the centrifuge manufacturer, Westfalia Separator Ltd., Aberdeen, one main problem with the coalescing system was that filters had to be replaced twice per day, mainly due to clogging. About 160 filters/day were required to maintain the system. Also, some produced water was stored and shipped in tankers to onshore treatment plants.

Westfalia Separator indicates the centrifuge cleans produced water to well below the Oslo-Paris (Ospar) Convention limits.

Alba application

Crude oil from the Alba platform is piped directly to the FSU for storage and for conditioning the oil to meet offloading specifications.

Westfalia Separator describes crude oil from Alba as generally containing water that can be as much as 8% of the produced stream during water-based chemical squeeze operations for stopping downhole scale buildup.

This water must be eliminated to meet product specifications of less than 0.5% water. The resulting water phase contains residual oil that has to be removed before the water is discharged overboard.

In the new system, oil from the Alba platform is piped directly into a settling tank on the FSU. After 3-4 days settling, the water phase is discharged overboard downstream of the centrifuge system.

Centrifuge operations

Westfalia Separator indicates the centrifuges can handle a throughput in excess of 40 cu m/hr, enough to cope with the entire water production from the platform. Discharges average 18-22 ppm oil in water at normal operating volumes. This is below the average for discharges in the North Sea (Fig. 2). The current average discharge in the North Sea contains 25 ppm of oil, which is lower than the high of 36 ppm in 1992.

At a lower throughput rate of around 30 cu m/hr, Westfalia Separator estimates that centrifuges clean the discharged stream to about 12 ppm oil in water.

Westfalia Separator indicates that similar centrifuges are being installed in Elf Exploration U.K. plc's Elgin and Franklin field facilities for handling produced water and in the Enterprise Oil plc Pierce field for both produced and drain water.

The Alba system for removing the oil from the water was supplied by Brown & Root AOC. Westfalia Separator provided two WSC50 centifuges as part of the system.