Building the ultimate machine

The Gullfaks subsea multiphase compression project made waves last year, becoming the first of its kind. In June, four people  involved in the project won the Underwater Technology Foundation’s Subsea Award at the Underwater Technology Conference (UTC) in Bergen for work on the project: Bernt Helge Torkildsen and Simon Kalgraff, from OneSubsea, a Schlumberger company, and Jarle Ottar Hella and Caroline Bøe, from Statoil. One has been working on the technology at the center of this project, OneSubsea’s subsea multiphase compressor, since work on it started, nearly 30 years ago. Elaine Maslin had an exclusive interview with him.   

Gullfaks wet gas compressor. Photo by Harald Pettersen/Statoil.

For Bernt Helge Torkildsen, the UTF Subsea Award is recognition of nearly 30 years’ work on an idea some thought would never work and, at times, only he was working on. Last year, the work culminated in the installation and startup of two OneSubsea subsea multiphase compressors at the Gullfaks field on the Norwegian Continental Shelf.

Torkildsen is proud, yet he’s quick to praise and recognize others involved in this multi-decade-long journey. Without the foresight of Frank Mohn to set up a dedicated research company, Framo Engineering, without Mohn’s successor Trond Mohn’s ongoing support, and without support from Statoil and Schlumberger – which took a share in Framo before buying it outright, and then OneSubsea, Schlumberger’s joint venture with Cameron, which it also now owns outright – plus the many staff and engineers involved, the technology might not have made it this far.

But, just as Torkildsen is quick to shrug off holding the fastest 3000m track record for 29 years, from 1978, his input, and thinking, based on an aeronautical background, shouldn’t be underplayed.

It all started in 1983, with the formation of Framo Engineering. In an attempt to assess industry requirements, the team started reviewing North Sea infrastructure.

“On a map of the North Sea, circles of 50km around existing infrastructure were drawn. Although at that time, 80% of known reserves in the area were within these circles, new platforms had to be built for every new field. If the known reserves could be transported to the existing platforms, you could extract that 80% without building new platforms. One of the missing elements to do this was multiphase pumps,” Torkildsen says.

Back then, pumping was Frank Mohn’s “normal business,” but mostly to ship cargo and for water lifting. The firm was starting to get into the offshore market and saw an opportunity to use its pumping know-how in that market.

Early on, Framo had had an agreement with Shell to develop a subsea booster station. At the time, Norwegian oil firm Statoil, French research center IFP, and oil firm Total were working on a hydraulic multiphase pump, under the Poseidon project, launched in 1984. Two licenses were issued to use the Poseidon technology, one to Framo Engineering and one to Swiss engineering firm Sulzer. But, Framo decided it wanted its own, and by leveraging technologies it already had, the contra-rotating concept (CR) and later wet gas compressor (WGC) concepts were developed. “The obvious benefit was that we didn’t need the diffuser part [of a normal compressor] so you can have more impellers on the same shaft,” Torkildsen says.

The concept proved to be particularly suitable and efficient for subsea wet gas compression. The first prototype was quite small, but performed well, and it was decided that a bigger unit should be built, and a new iteration was envisioned.

In 1990, a CR 400 (250Am3/h, 500kW) design was tested on the NAM-operated De Leer field, an onshore gas field in the Netherlands. Until then, all the development work related to the gas market was towards land-based applications. “This new design also performed well. But, it was still clear that the robustness of the design needed to be increased again to fulfill the desired requirement,” Torkildsen says.

OneSubsea subsea multiphase compressor. Photo from Statoil.

The next iteration was the CRA compressor (1000Am3/h, 1000kW). “The size was starting to get more sensible and closer to what was needed for real application,” Torkildsen says. “This unit was equipped with advanced instrumentation, including neutron back scatter technology, which was used in combination with radioactive tracers to show how gas and liquid were transported through the compressor internals.” Previously, such information had not been readily available even in larger research organizations or in universities.

By 1995, despite the increase in knowledge, many in the market and within the engineering group remained skeptical that such a device could work. The need to deliver the multiphase compressor was also less urgent and so there was something of a fallow period.

“But, it was never shelved, and in 2000, with a drop in the oil price, we won some Demo 2000 funding, with support from the Norwegian Research Council and the Ormen Lange License,” Torkildsen says. “At that time, it was for products to realize economics even for low oil prices. There was some reluctance, but we went ahead and developed a new WGC2000 and that was the first time we aimed it towards the subsea market. Before, we had aimed for topsides and the land market.” Statoil was also now supporting the technology, alongside Shell and Norsk Hydro.

There were some who were predicting this would not work on real hydrocarbons and that its performance – the physics – could not be predicted. But, in 2002-2003, a WGC2000 machine was taken to Statoil’s K-Lab for performance testing on real hydrocarbon wet gas. “It was a success,” Torkildsen says, “but, again its capacity was too small for the likes of an Ormen Lange project.” And again, the market fell a little quiet.

Torkildsen then brought his aeronautical engineering perspective into play. Torkildsen holds an MSc in aeronautical engineering and a pilots’ license. He thought it would be possible to increase capacity at load from the 3.6MW, 1400 Am3/h WGC2000 unit to 5MW 6000 Am3/h. The Demo 2000 machine (WGC2000) was used, fitted and tested with new impellers and it worked with the target capacity being demonstrated.

“Traditionally, we were considered a pumping house,” he says. “But I had a slightly different background and brought in some knowledge from other industries and looked at it a little differently from pumping,” Torkildsen says.

Torkildsen had spent some time in the European Space Research and Technology Centre (ESTEC) in the Netherlands and had always retained a high interest in aeronautics. As a result, he looked at the impellers as a series of airfoils, which needed to be improved in a way that hadn’t been considered before. “It was a quite simple thing, but meant being able to increase capacity and efficiency,” he says. “Standard compressors are not designed for wet gas or gas and liquids. They are designed and optimized for dry gas duty only. This thinking – how to be robust in handling both gas and liquid – had already been applied to multiphase pumps and could also be applied to the multiphase compressor,” Torkildsen says.

During the whole WGC2000 development period, Framo was also developing multiphase pumps and working on increasing performance, making sure liquid mixes well with the gas through the pump. Controlling this process, stopping the liquid or gas separating out, is key.

“The contra-rotating concept has a good feature in that respect. It contradicts the separation effect,” Torkildsen says. “Also, the inlet arrangement has a built in flow mixer to help condition the unprocessed well stream and any flow regime entering the compressor. If a normal compressor had this duty, the different impeller stages or impeller parts might experience very different loads resulting from gas and liquid separation compromising both performance and mechanical integrity. We make a 21-stage impeller, which means each is very lightly loaded, and because it is contra-rotating, each impeller provides twice the head of a conventional single rotating impeller. Specific loading is very low and hence the multiphase fluid is treated very gently as it moves through the compressor.”

The step up in capacity came just before Norsk Hydro and Statoil merged. The team had already approached Norsk Hydro and were working on a topside, 3000Am3/h concept for the Tune field. However, in early 2007, following the merger, Statoil came up with the Gullfaks project.

This put Tune on the back burner, but created the opportunity to develop a unit for subsea use, at a time when it would have to compete with topsides alternatives. A technology qualification program was launched, this time with Framo Engineering in close cooperation with Statoil.

“The important thing was that we had made this increased capacity machine and tested it. Then we took the figures to Statoil,” Torkildsen says. That is what got the Gullfaks process started, he says.

The next step was qualification of the whole Gullfaks compression system. This time Framo, now part-owned by Schlumberger, invested in its own test rig with 100-bar live hydrocarbon wet gas capability.

Statoil’s Gullfaks’ B platform. Photo from Statoil.

Framo Engineering also had to step up as a company, moving into full systems engineering. A cooling system would also be needed and the compressor, cooler, monitoring systems, etc., all qualified. The team working on the wet gas compressor suddenly grew. In 2011, Schlumberger bought out the remaining shares in Framo that it didn’t own, before rolling the company into its OneSubsea business.

In 2015, two OneSubsea multiphase compressors were put onstream and became the first multiphase compressors with no requirements for an upstream separation facility or an anti-surge system, helping to simplify the subsea system requirements.

They are targeting an increase in recovery rate from 62% to 74% for the Gullfaks field. This will see some 22 MMboe increase in recovery from the Gullfaks South Brent reservoir.

Statoil has been keen to implement subsea compression, due to it having a bigger impact on recovery rates than conventional platform-based compression. It is also seen as an advantage that is does not take up space on platforms, as well as being a step towards Statoil’s goal for the subsea factory.

In making their award, the UTF jury recognized the removal of the need for upstream separation facilities and the ability for light vessel intervention as being among innovative steps contributing towards cost reduction in subsea technology.

While the compressors have since been temporarily removed, due to a leak in a utility cable not related to the compressor stations, Statoil has confirmed that the units had been operating to its satisfaction.

Fast forward to today and the focus once more is increasing performance and capacity. “Internal work could yield yet another giant step in capacity,” says Torkildsen, and there’s no reason to disbelieve him.

The key with the Gullfaks compressor is that it is very compact, light, small and simple. It can be installed using small vessels and that was very important for the customer. These OneSubsea multiphase compressors, should never be too heavy as there is a sensible limit— in the 100-150-tonne range. If it is more than that, it may become too big and heavy for easy and efficient intervention.”

Interestingly, the system could go full circle and be used onshore, Torkildsen says. “We are seeing what I think is the ultimate machine, taking into account all the constraints we need to consider. In the early days we focused on land; subsea compression was never mentioned.”

It’s been a long road. Torkildsen is keen to credit the foresight of all those involved through the years. For OneSubsea, the multiphase subsea compressor is a huge achievement.