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Seabed compression in a smaller package

Less than 18 months after pioneering subsea gas compression technology debuted at Statoil’s Aasgard field, the system’s engineers are ready to roll out a second-generation version that provides the same capacity and functionality in a much smaller package.

Officials are fond of comparing the performance of the subsea compression system at the Aasgard field in the Norwegian Sea to that of an especially reliable timepiece.

“We are very proud to say that Aasgard subsea compression has been running like a Swiss watch, as we’ve put it,” says Knut Nyborg, head of Front End Norway at Aker Solutions, echoing public comments made by colleagues at both Aker and Statoil.

“We are very pleased to see that the system we have delivered has maintained a regularity of close to 100% since the start-up in September 2015.”

The equipment had only been up and running for a few weeks when Aker Solutions and MAN Diesel & Turbo, the German compressor manufacturer, set to work on a second-generation concept, drawing on lessons learned during their Aasgard collaboration to help make the technology practical for wider use (a subsea compression system for wet gas delivered by OneSubsea came online at Statoil’s Gullfaks South field in October 2015).

Its advocates say the industry has much to gain. Recovery rates improve considerably if compression is located on the seafloor near the subsea wells — Statoil has said the technology raised the recovery rate from the Midgard and Mikkel reservoirs on Aasgard from 67% to 87% and from 59% to 84%, respectively.

KnutNyborg.jpg Knut Nyborg, head of Front End Norway at Aker Solutions
While the seabed offers generous space for machinery, there are advantages to scaling back the size of subsea compression equipment. The modules that compose Aasgard’s twin trains, each driven by a MAN Diesel & Turbo 11.5 MW high-speed, oil-free integrated motor (HOFIM) compressor unit, are housed in a template structure 20 metres tall with a footprint of 75 metres by 45 metres and a total weight of about 4800 tonnes. The size was necessary to meet the operator’s schedule and to make sure the system functioned as planned, says Nyborg.

Aasgard “was a schedule-driven project” involving many industry firsts and several technology qualification programmes. “This made it challenging to implement improvements once work was under way. You might say in retrospect that it looks bigger than it needs to be. But I don’t think we could have saved anything during the project because there was never an option to fail. It had to operate smoothly from the first installation and during normal operation,” says Nyborg.

The strategy, agreed with Statoil, was the correct one, he insists. “It was the only right solution to the task we were given, and it has proven to be a safe system. It has been working well all the way through testing, commissioning, start-up and continuous operation.”

Scaling back

The Aasgard team recognised early on that some of the requirements for the first-ever system would not be needed for subsequent installations. To challenge some of the requirements and propose improvements, engineers made detailed notes throughout the project and compiled an extensive list of what Nyborg calls “improvement opportunities”.

“During the project we had to identify the challenges and solve them as we went along. There were no such things as a subsea compression encyclopedia or industry standards. So we had to identify all the potential pitfalls and find solutions to them.

“We saw during the project that, if you had time, you were able to implement improvements. In the next project, that could be reducing cost and size and weight. So we instructed our lead engineers to identify improvement opportunities, and we created an improvement opportunity register where they noted down lessons learned and the requirements they’d like to challenge.”

"We were looking for some simple improvements."
Knut Nyborg, Aker Solutions
 

Aker and MAN Diesel & Turbo engineers used the list as a basis for a simplified subsea compression system concept, which the companies say will be less than half the size and weight of the first system and considerably less costly to install. The second generation will accomplish that “with exactly the same functionality”, Nyborg says.

“Our focus has been what we call a risk-free optimisation. It was a clear instruction to the lead engineers to select the opportunities that did not impose any new risk. This means you can claim the same qualification status as the system that is in operation today. You keep all the functionality of Aasgard, implement only those opportunities that pose no risk, and by doing that we are able to make an optimised system that is suitable for a new project.”

The new configuration fits all the compression system modules in a standard four-slot production template, which removes the need to deploy a heavy-lift vessel for installation.

The design reduces the number of modules per train from 13 to seven and greatly simplifies the hydrate prevention system, optimises the process piping and uses Aker Solutions’ new Vectus control system and their SIS2 compliant electrical actuators to cut the number of jumpers from 155 to less than 50 for each train.

Proven technology

Marinising the compressor at the heart of the subsea system was a decade-long project, says Basil Zweifel, head of oil and gas upstream at MAN Diesel & Turbo.

“The biggest challenge was making the internal parts robust enough to handle wellstream gas,” he says. MAN has more than 100 compressors based on the company’s HOFIM technology in the field. “But most of them run on pipeline gas, which is nice and clean and dry. So the marinisation process was really to make it fit for the typical wellstream gas quality.”

The HOFIM compressor system uses a high-speed induction motor and active magnetic bearings, and eliminates components such as dry gas seals, lube oil system and gearbox. This makes it a simpler system and therefore more reliable, says Zweifel.

“By not having these systems you start from a good point,” he says. “Then when you design for a very long time between interventions, it’s about quality assurance, robustness in design, and simplifying the system by eliminating components that are not needed. There has been a lot of effort spent on Aasgard to guarantee that we have maximum availability and reliability.”

While the second-generation subsea compression system will be much smaller and have fewer components than Aasgard’s, the compressor itself will remain mostly unchanged, he says.

“In principle, we don’t want to change a lot with the compressor. We will take the proven technology that we know works from Aasgard, and our changes are more about optimisation of the interfaces and the integration of the subsea HOFIM compressor into the wellstream gas system.”

Since Aasgard’s compression system came on stream, he says, MAN has supplied a similar HOFIM compressor unit for a platform-based system. “So the technology we developed for subsea has also been applied for a topsides project, because it was also necessary to handle less-treated gas quality.”

The signs so far are encouraging, he says. “We learn more and more about the machine's behaviour in not-very-good gas quality. We have learned in real operation how different gas compositions affect the performance of the machine. This knowledge, not from the lab but from the real world, allows us to enhance our wellstream gas compression capabilities. So we get more confident in what our machine is actually able to do.”

Simple solutions

Aker has worked with MAN to optimise power supply to the new compressor system. Power distribution can add substantial cost, particularly for high-voltage subsea cables in long step-out developments.

“We were looking for some simple improvements,” Nyborg explains. For example, the original plans for the Aasgard system called for all modules to be mounted on a cassette base frame and installed all at once. The heavy-lift contractor, however, opted to lift the modules one by one. Aker has retained the practice of lifting the modules independently in the second-generation concept, thereby eliminating the need for the 400-tonne base frame per train.

Likewise, the Aasgard project required that each individual module — pump, compressor, separator, and so on — should be independently retrievable. By doing away with that requirement, engineers were able to group the modules differently and reduce by half the number needed per train.

“None of these changes have any negative impact on the functionality,” Nyborg says. “It’s just how you stack things together. It’s actually a positive because you have less piping and fewer connectors and so on.” This will in turn reduce pressure drop and simplify installation and intervention operations, he adds.

Nyborg says work is already under way on a third generation of subsea compression — a high capacity wellstream compression system that will further simplify the process by removing the subsea pump and its power supply.

“As a major pump supplier, it might seem a little bit strange, but the third step in developing subsea compression further is to remove the pump and take the gas and whatever comes out of the well directly into the compressor.”

For now, however, Aker Solutions is betting that the business case for subsea compression and the optimised design will lead to more projects off Norway and beyond.

“What we have managed to do with this second generation is reduce the size by 50%, and reduce the cost,” Nyborg says. “We have been able to reduce the complexity and number of components by about 50%. This makes the overall business case better. Yes, it works, and it performs very well, and yes, it can be dramatically smaller and lighter and cheaper. And it can be installed anywhere in the world because you don’t need heavy-lift vessels.

“The business case is quite simple,” he continues. “There are a lot of large to medium-size gas fields and they normally need compression as the pressure starts to deplete in the reservoir. The closer you get to the well, the more effect you get out of each megawatt of compression. Subsea compression can help reduce carbon dioxide, make the environmental footprint smaller, and it’s inherently safer if there’s no platform.”

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