Kvaerner's interest in subsea water separation flows naturally from its previous involvement in subsea separation technology. This goes back some 12 years, and was originally directed at developing the separation/booster system known as the Kvaerner Booster Station (KBS).
The KBS was concerned with gas/liquids separation, rather than water/hydrocarbons, but nevertheless provides a valuable experience on which the current development effort draws, according to Harald Wiken, Kvaerner Oilfield Products' manager, mechanical and process.
In Wiken's view, his company has a head start on its rivals. "Kvaerner has worked the longest and spent the most on subsea separation, including boosting and water knock-out," Wiken says. "We have an extensive experience on the process side, with many separation systems in operation in the North Sea, and a lot of subsea experience too."
It was KOP which won the CoSWaSS contract last year, and which, having performed concept definition, was busy earlier this year with the second phase...
The Kvaerner system is already available for smaller flows and shallower waters, Wiken says. More work needs to be done for larger flows and deeper waters.
The system comprises three main parts: separator unit, pumping unit, and control system. A protection structure including pull-in and connection facilities is also included. Assuming the water is to be re-injected, a xmas tree can either be incorporated in the system or located alongside it. The system is designed to be installed and maintained without the use of divers.
For a 60,000 b/d flow, a typical system including the protection structure will weigh some 200 tonnes and have a height of some 6.5 meters and a footprint of some 6.5 x 26 meters including the flowline protection ramps.
The unit can be installed downstream of the commingling manifold as an integrated part of the production flowline. Hence the design must allow for possible pigging operations of the flowline.
The wellstream is routed into the separator, where the rich hydrocarbon phase flows over the top of the separator and back into the flowline for transport to the host platform, while the water-rich phase flows out from the bottom of the separator to the water injection pump for re-injection. Typically some 90% of the water is separated out; with a residual oil content of less than 1,000ppm, it is sufficiently clean for re-injection.
Any solids in the wellstream are directed to the water outlet to avoid potential blockages, a feature also incorporated in the KBS. This assumes that the production strategy is based on choking of the wellstream to obtain a sand-free flow. If required, a sand removal module could be included for production regimes which permitted a significant volume of sand in the flow.
The separated water is re-injected by means of a centrifugal pump unit driven by an electric frequency motor. The motor is set at a slight over-pressure with respect to the pump unit to avoid contamination of the motor coolant system with process fluids. In one study of an application on a Norwegian field, the motor had a rated power output of 2,500 kW while the pump design power was 1,800 kW.
A high-voltage wet mutable connector provides the connection between the motor and the dedicated power cable. On the CoSWaSS project, GEC Alsthom is providing the power expertise.
The control system is a conventional electro-hydraulic one, with a level sensing system for tracking the oil/water interface in the separator, pressure monitors upstream and downstream of the pump, temperature sensors for the motor cooling fluid and accelerometers for pump monitoring.
The oil/water level is the principal control parameter as re-injection of produced hydrocarbons must be avoided, Wiken says. The interface signal is coupled to the frequency converter which controls the pump speed so as to maintain the interface within the desired parameters. The use of frequency conversion subsea is new, Wiken says.
He is confident of seeing the system in action before the turn of the century, but this will not be the end of the story. At a later stage more complex systems will be developed, with additional facilities such as sand removal modules, water treatment modules and pig launchers.
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