Compression 2.0

After completing the world’s first wet gas compressor offshore Norway, OneSubsea looks to continue pushing the technology to new limits.

The multiphase compressor station during installation. Photos from OneSubsea.

Mid-October 2015, after more than 24 years’ worth of development work, the world’s first subsea wet gas compression facility was fired up 130m beneath the waves, 175km west of Bergen, offshore Norway.

The technology will help increase recovery by 22 MMboe, or from 62% to 74% of the field, and extend the production plateau from Statoil’s Gullfaks South Brent reservoir – figures that are hard to ignore.

The multiphase compressors on the seafloor at Gullfaks South are two, 5MW, WGC4000 units, with 21 stages each, and 6000 cu m/hr throughput capability. Installed in parallel, they provide a 32 bar initial pressure increase. When in series – achieved by operating two valves – that increases to 64 bar, which may be needed towards end of life.

It’s an achievement, but there’s more yet to come. Having increased the capabilities and performance of this technology over the past two decades, OneSubsea thinks there’s scope to take it even further – a job they’re working on already.

“We want to improve the capacity and the differential pressure capability,” says Arne B. Olsen, director of sales, OneSubsea. “That’s what we are working on. Together with customers, we are targeting at least differential pressure up to 80 bar with one single machine.” All while keeping the power requirement low.

OneSubsea’s work in subsea compression builds on the work of Framo Engineering, which was bought by Schlumberger in 2011, and subsequently wrapped into its Cameron-Schlumberger jointly owned company – OneSubsea.

The development work on subsea wet gas compression started back in the 1980s. But, at that time, the industry wasn’t quite ready for it, Olsen says. The unit capacity was also too small and unable to handle enough volume.

The design principle is based on a set of vertically oriented contra-rotating impellers, one set pointing outwards from an internal shaft, the second on an outer shaft facing inwards, and each driven by its own motor, installed at each end of the unit and supported by axial and radial bearings. Every second row, or stage, of the 21 impellers in the Gullfaks unit, rotates in the opposite direction at a maximum 4500rpm. The motors and power system are based on those OneSubsea uses in subsea booster pumps, which have chalked up more than 2.5 million operating hours.

“What we have developed is a hybrid between a multiphase pump and a compressor,” Olsen says. The OneSubsea multiphase compressor doesn’t have diffusers, it has two separate shafts of impellers, which contra-rotate, facing inwards towards each other. “This design makes the compressor more compact,” Olsen adds.

But while it’s taken on some of the characteristics of a compressor, it’s more like a multiphase pump in terms of its mechanical robustness, Olsen says. “The hydraulics, i.e. the impellers, are mechanically robust, which means it can handle any combination of gas and liquid and can even be started while filled with liquid.”

According to OneSubsea’s research, the WGC4000’s polytropic efficiency – a measure of compressor efficiency – is 85%. “This matches conventional compressors,” Olsen says. Of course this will change according to how much liquid is in the stream.

“It’s been a 24-year development project,” Olsen says. “The fundamental principles have always been the same. The earlier designs were all horizontal. In 2000, we changed to a vertical orientation, for subsea compactness and retrieveability. Yet the main change has been its performance in terms of capability handling larger volumes. Previously it was a bit too small for some of the bigger gas fields so we worked very hard to improve on that. Now, volume has increased by a factor of five compared to 2000, primarily by changing the geometry of the impellers. The capability today is much closer to what is needed for medium to large gas fields.”

So what next for wet gas compression? Technology-wise, OneSubsea is already working on the next generation multiphase compressor – the WGC6000, with yet more focus on the compressor capacities. Geography-wise, most point to East Africa and Australia.

“There is a big need for this technology in places like Mozambique, Tanzania, and Australia. There will be a big take up of these technologies, I’m quite sure,” Olsen says. There could even be scope offshore Norway, according to Kjetil Hove, senior vice president for Statoil’s operations west cluster. Speaking of when the Gullfaks multiphase compressor first started up, he said: “We see great opportunities for wet gas compression on the Norwegian continental shelf… for improved recovery on small and medium-sized fields. We are searching for more field opportunities that are suitable.”