Collaboration between Norwegian and Italian companies could see the installation of the world's first subsea gas compressor towards the end of the decade.

Last August Norway's Aker Kværner and Italy's GE Oil and Gas announced they had completed preliminary work on development of a prototype centrifugal subsea compression system, Blue C, intended for deepwater applications under a joint development agreement.

A prototype 850 kilowatt system has been tested and 2.5 and 5 megawatt (MW) power rated-systems are being designed and developed as a result of the collaboration, with varying gas compression performance characteristics. Previously, the two companies had a joint development agreement, but with the signing of another collaboration deal, last July, a framework in place to develop a larger unit targeting commercialization by 2008.

Up to 600 hours of endurance testing has been completed on the prototype in Italy. In the longer term, the two companies are looking to develop a significantly larger compression module, rated at up to 12.5 MW, allowing step-outs from shore or from a host platform of up to 94 miles (150 km).

"As of today subsea compressions does not exist commercially," declared Luca Passaleva, regional general manager for Pignone Engineering, within GE Power Systems' oil and gas group.

"There is subsea pumping, and multiphase pumping, but just subsea compression which is used for gas export or gas injection in the well does not exist. This will be the first."

Blue C evolved from collaboration between the Norwegian and Italian groups with backing from the previously mentioned Demo 2000 project financed by the Norwegian government. Consequently the first use of this technology could well be in deepwater on the Norwegian Continental Shelf, conceivably on either the Ormen Lange or Snøhvit gas projects - or offshore Italy.

The technology under development by the two companies - which has so far accounted for up to $20 million of investment - is designed to make it possible to economically develop deep gas fields at water depths of 1,640 ft (500 m) or more.

Although the original collaboration was between Kværner Eureka and GE, the project recently made a major move forward with a new co-operation agreement between GE and Kværner Oilfield Product to develop the 12.5 MW system.

One of the key components of this technology is providing reliability, such that the compression unit can operate without maintenance on the sea floor for 3 years or more. Issues such as gas sealing, dynamic stability and cooling need to be assessed, tested and proven, Passeleva said.

Consequently the compressors under development are designed to operate at sea floor pressures - eliminating the need for gas-tight seals.

"There is no need for gas seals because we have equal pressure inside and outside," he said.
Magnetic bearings are being used to seat the rotating components within the compression module. "That is so that they eliminate the need for [lubrication] oil requirements," Passeleva said, addressing the issue of component reliability.

Working to API engineering standards for compressor design criteria, the joint team is working on "extremely stiff" rotors within the compression chamber. A vertical design is being used, utilizing Norwegian processing expertise, and Italian control and power systems technology.

"There are three or four potential fields where it will be installed," Passeleva said. Several oil company operators are backing the research program and a commercial installation of the technology is expected by 2008, alongside surface compression equipment to provide a parallel comparison.
Qualification testing of the 12.5 MW unit is expected by 2005.

Low vibration is a key to ensuring the compressor works - high vibrations will damage seals leading to equipment failure. A good deal of attention has been paid to ensuring rotor stiffness is maximized, with few stages and a large diameter rotor shaft. Impellers on the rotor are close to 9-in. long and rated for 12,000 rotations per minute.

Collaboration between the two companies on subsea applications dates back to 1992 when Kværner Eureka and Nuovo Pignone worked on an 850 kilowatt low speed motor for subsea compression unit which was tested for 200 hours in a water filled tank.

That research was left untouched for 6 years until Demo 2000, launched in 1999, to qualify deepwater technology, provided new impetus.

Essentially the system comprises a centrifugal compressor train with an optional separator, a planetary gearbox and a variable speed electric motor.

Separately, Kværner has been working on a subsea version of its Compact Electric Coalescer (CEC), already field proven in a topsides process train on the Petrojarl I floating production, storage and offloading vessel operating for Statoil on the Glitne field in the Norwegian sector of the North Sea, where it began operating July 2002 and has clocked up more than a year of service since.