Aker Solutions is developing subsea technology that the company says will decarbonise natural gas used to produce reliable and affordable power.

The magic behind the Zeus project — for "zero-emissions underwater power station" — is that it all happens on the seabed, says Rune Vesterkjaer, Aker Solutions’ vice president for production, compression, power and pumps.

“If you look at what the concept of Zeus is, it’s to produce gas, to produce power without emitting anything. We’re basically doing instant carbon capture," he tells Upstream.

“We are getting CO2 and water out of the combustion process on the seabed that we can directly inject for storage. We can use it for EOR [enhanced oil recovery] but storage is the base case.”

Gas is the cleanest fossil fuel but, with the drive to net zero gaining traction for so many nations and companies, CO2 challenges will need to be overcome.

Two-thirds of the world's gas reserves have yet to be exploited, according to Aker Solutions, but there is a real risk that they will remain undeveloped unless carbon emissions can be curbed or — even better — eliminated.

Injecting CO2 associated with offshore gas production can reduce carbon dioxide emissions by 70%, says Vesterkjaer, but the Zeus system takes the process a massive step forward by generating power on the seabed.

The CO2 is condensed and immediately injected back into the reservoir for permanent storage, with the power typically delivered to shore via subsea cable.

The base case Zeus facility envisages 1 million cubic metres per day of gas production, with 2300 tonnes per day — 885,000 tonnes per annum — of CO2 captured and reinjected.

The carbon dioxide would be stored in a supercritical liquid state, meaning it could be pumped without the need for subsea compression.

Around 60 megawatts of the power generated on the seabed would be used by the air separation unit (ASU) housed on an offshore platform to produce the oxygen required for the unit — some 3200 tonnes per day for a 1 MMcmd-size Zeus facility.

Meanwhile, 100 MW of emissions-free electricity would be sent via wire either to customers onshore or to offshore installations, where it could replace power generated by gas turbines.

Problem gas is no problem

Vesterkjaer says Zeus can use any gas as feedstock, including associated gas, methane hydrates, CO2-rich gas and stranded gas.

“The beauty is that you don’t need export-quality gas to generate power. We can deal with ‘problem’ gas with up to 70% CO2 content,” he says.

Unlike power produced by renewables such as wind and solar, electricity generated by Zeus would be available at a constant rate, around the clock and 365 days a year.

Field economics could even be improved, Vesterkjaer says, by using the CO2 in an EOR application. Injection could also continue after production stops, potentially for third party users who require permanent CO2 storage.

Zeus could also be used as a clean power source for subsea oil and gas field developments, delivering power to manifolds, wellheads and other power consuming equipment.

The Zeus system builds on the company’s pioneering subsea compression technology in use at Equinor’s Aasgard field offshore Norway and slated for Chevron’s Jansz-Io project offshore Western Australia.

Born on a napkin sketch

Aker Solutions started working on subsea compression back in 1985, when the technology “was born on a napkin sketch”, says Vesterkjaer.

The contractor aims to leverage subsequent refinements and experience gained for the maintenance-free and remotely controlled Zeus project.

Aasgard's subsea compression system, delivered in 2015, has been promoted as part of a range of technologies that can lower emissions for oil and gas operations.

The patented high-pressure oxy-fuel combustion power plant in Zeus expands that toolkit.

Oxy-fuel combustion, which involves burning the gas with nearly pure oxygen rather than air, has been performed before, but on land and in low-pressure conditions.

Vesterkjaer says of the Zeus concept: “It’s really doing a marinisation. The great thing here is that we can work with high pressures and that is really our patent on it. It’s a game-changer, I think, this solution.”

He adds that the high pressure gives a very compact solution saving 80% weight on the processing equipment while allowing direct liquefaction of the exhaust. The latter simplifies the post-processing of the exhaust, providing significant savings on costly equipment and making the solution more reliable.

“Of course, one of the challenges when we go to the seabed and want to do things is that we want to have compact equipment with high reliability. Another challenge is that it takes some time for the market to connect to it — that’s what we learned on subsea compression. But here we actually have a pull from the political side as well.”

He reveals that the most complex part [of the Zeus system] when it comes to making equipment compact, unmanned and reliable is not what is happening on the seabed.

“That is the air separation process, and we are looking into different solutions for that now. We have not decided on the technology that will be used. We are talking with different suppliers on that,” explains Vesterkjaer.

Although Aker Solutions is currently focusing more on the subsea part of Zeus, Vesterkjaer notes that the system could potentially use oxygen supplied by sources other than an ASU, such as oxygen separated from air using nascent membrane technology, or oxygen by-product from a hydrogen production facility that could be piped from shore.

Zeus could be implemented in offshore coastal areas with large onshore hydrogen plants based on power from wind. If these plants have a certain/suitable scale the contractor can use the O2 “exhaust” from hydrogen production to eliminate the ASU.

While the base case for Zeus is immediate injection of the carbon dioxide produced with the gas, the contractor is currently in discussions with an unnamed partner in the US “that needs CO2 for EOR”, he says.

Aker Solutions has also developed a novel CO2 pump for a project in Brazil, for which the contractor is currently tendering — technology that it has demonstrated at its test facilities in Norway.

“We have proven… that we can pump CO2 in a dense phase,” says Vesterkjaer.