Troll Oil separation quality draws further floater orders

Key principles of Proser's gas dehydration and glycol regeneration technique.

Over the next five years, Proser's strategy will be oriented very much towards floating production, according to managing director Lionel Waintraub. This follows a string of orders for separation systems for new Norwegian field developments, which in turn has ignited interest in the UK and Far East.

Since 1985, this Paris-based subsidiary of Framatome has been developing new concepts for two and three-phase floating production separators. The contract which triggered the latest successes, awarded by Aker, was for Norsk Hydro's Troll Oil production semisubmersible. Proser designed and built the first and second-stage test separators, as well as the first, second and third-stage production separators and coalescers.

Since this platform came onstream, the performance of these duplex stainless steel systems - maximum water in oil content of 5% and maximum oil in water value of 0.1% - has helped raise crude production 50% to around 270,000b/d. Water flow rate is currently 156,000b/d.

Aker then contracted Proser for the test separator and three-phase production separators for Hydro's new Njord Field semisubmersible. Topsides completion will start shortly, in preparation for maximum oil production next year of 85,000b/d and water production of 17,000b/d. The Njord separators, made from carbon and duplex stainless steels, will achieve a maximum water in oil content of 0.5% and oil in water at 1,000ppm.

Now Proser is discussing further systems with Hydro for a new Norwegian sector development, where oil processing from the complex reservoir looks to be problematic. Also in Norway, Proser has supplied part of the separators for Esso's current Balder development, this one based around a monohull FPSO. And there are numerous other candidates for floating production separators in fields such as Troll C, Hermod and, south of Aasgard, Tyrihans, where a development plan is due to be submitted shortly.

In a concurrent project, Proser is working on the development of subsea separation technology for operations down to 1,000 meters of water

Separator basics

Although roll and pitch differ on a monohull from a semisubmersible, Proser has configured its basic system to dampen liquid motions on both vessel types. The separator also avoids resonances that may arise from even very small oscillations, in order to maintain liquid/liquid separation at the same efficiency level as on a fixed platform.

Key features of the floating production separator are:

a split flow inlet momentum device, which achieves a first separation between the liquid and gas phases and splits the inlet stream into two parts
primary separation, provided via a distribution plate that distributes the liquid along the vessel to each head
foam breaker gas/liquid separation sections
gas clean-up, achieved by a wire mesh mist eliminator or a vane pack section, which removes the small droplets of liquid dispersed in the gas phase and then drains to the liquid phase
packing sections that control liquid/liquid separation and also dampen the liquid movement induced by heave, roll and pitch of the platform or tanker
water and oil collectors, equipped with vortex breakers.

For Troll Oil, the design was verified using a 1/7 scale model with flows simulating real conditions - a liquid of similar viscosity, 14 cpo, as the actual oil and water - as well as the motion of the semisubmersible Troll platform. Tests were performed at Heriott Watt University's platform motion simulation facility in Edinburgh.

For floating production, Proser designs process modules in the form of two `pancakes' or two system packages with complete functional specification. Installation is also tailored according to the main piping systems and electrical/instrumentation layout onboard the vessel.

In the UK North Sea, Proser has held discussions concerning several new fields due to be produced through monohull FPSOs. Irrespective of the outcome of these talks, Proser is conducting parallel studies to reduce the size and weight of its FPSO separators.

Some FPSO separation contracts are also out to bid for South-east Asia fields. To date, Proser's main involvement in this region has been in gas processing and conditioning systems, including dehydration of gas to achieve glycol regeneration. A package is currently being delivered to Total Indonesia for the Tunu North development.

Back in the North Sea, the emphasis is on treating aromatics in gas which cause glycol to dissolve, adding to CO2 emissions. A new system has been developed, called PROGLY, which is being targeted at new gasfield developments such as Oseberg South, Aasgard and Elgin/Franklin.