A holy separation grail

In the technology maturity stakes, subsea separation could perhaps be described as one of the ugly sisters in the subsea processing world. But, perhaps Cinderella would be more appropriate. Elaine Maslin reports.

Subsea infrastructure at Tordis. Image from Statoil.

Subsea pumping has been around for decades. As of February, there were 24 subsea pumping systems in operation and a further five being manufactured, says Intecsea, with focus now on increasing system size. Subsea separation hasn’t had quite so much luck. Despite having been around for decades, as of February, there were just six subsea separation systems operating, and only one more being manufactured. Despite the lack of widespread adoption to date, however, many are still working on separation technologies and once this technology is cracked it could have significant potential, not least in unlocking fields that are otherwise uneconomic.

Separation spans a wide breadth of field possibilities. Gas-liquids separation would help unlock deepwater, long step-out gas fields, and could prove beneficial if used with subsea compression systems, of which only two projects have been installed to date –Åsgard, a dry gas compression system, and the smaller Gullfaks wet gas compression system – both by Statoil, offshore Norway. A further four subsea compression systems are in the concept stage, says Intecsea. Oil-water separation, meanwhile, would be a boon for fields where water cut is high and topsides space and weight constrained.

Some companies are working on both. Saipem is developing its SpoolSep oil-water separation technology, and Multipipe, a gas-liquids separation technology, for example. Sulzer, through its Dutch businesses Ascom and Prolab, is working with ExxonMobil to provide a flexible subsea separation solution qualified for a range of applications to avoid the costly need to qualify technologies for each application (OE: May 2016), using electrostatic coalescence plus gravity-based separation.

FMC, now part of TechnipFMC, has long been a player in this space, alongside OneSubsea (legacy Framo), part of oilfield services group Schlumberger. GE Oil & Gas and Aker Solutions are also players.

Most recently, Aker Solutions has been working on a solution for mature North Sea fields where water cut is as high as 90-95%. Using a gravity separator, the system is designed to separate water to 500-1000ppm oil in water. Depending on the water quality requirement, a second stage could be added, potentially a compact floatation unit (CFU). These are typically used topside, but in this case, it would be marinized for subsea, says Marco Gabelloni, business development director, Aker Solutions. Gas is injected into the CFU, making bubbles, which trap oil droplets and separate them from the water – achieving 100-150ppm. A project to marinize a CFU will start this year. Additional stages can improve the water quality. Hydrocyclones, as used on Marlim, could also be used, Gabelloni says.

Aker Solutions is also working on a deepwater separator, which would use pipe-in-pipe technology, a direction others, including Norwegian firm Seabed Separation (Read: Separation simple) and Saipem are going in.

Marlim subsea separation system. Image from FMC Technologies, now TechnipFMC.

Taking separation technologies into deeper waters offers greater challenges, however. Here, gravity separation – the traditional method used – becomes limited because of increased hydrostatic pressure and the associated wall thickness requirements. One option is a spherical system, which is able to withstand increased external pressure, but that means getting the internals to work, says Mac McKee, Intecsea’s director of strategy and planning.

The holy grail, however, would be for subsea water treatment and subsea disposal, where subsea separated water no longer needs to be pumped topside to be treated before either being sent overboard or back down to a well for re-injection (which itself can cause issues around reservoir souring). It’s also a stepping stone to the full subsea factory and would be an ideal combination with boosting or compression, depending on the application.

“There is a very fine line with what you can and cannot do with that water,” McKee says. “Being able to deal with it subsea is a bit like the holy grail of subsea processing. Without it, some fields will be un-producible and in the end, one of the main drivers for the size and cost of some of the topside facilities is the water treatment and water handling systems on the host platform or vessel,” McKee adds.

“Being able to eliminate those systems and combine subsea separation with subsea boosting (or subsea compression) would be revolutionary in terms of how you could redesign, or even eliminate, the offshore facility,” he says. “But, to do that, you have to be able to separate out the water and treat it to a level regulatory agencies will accept and dispose of subsea without bringing it to the surface.”

The challenges for water disposal, however, are not insignificant, from being able to achieve bulk filtration subsea to the current lack of guidance as to what’s acceptable for subsea disposal, because it has not been done before.

One of the attractions for separation is it makes boosting technologies more efficient. Subsea boosting performs well, but it isn’t the most efficient technology, especially multiphase, says Mika Tienhaara, head of upstream Americas, Sulzer Chemtech USA. With subsea separation, subsea processing could be more efficient.

So why has separation lagged? Probably due to inferior technology in the past. “At the same time, the design challenges [for separation] are immense when, deployed on the seafloor, you consider the start, or early versus peak production,” Tienhaara says.

The number of concepts, and maybe the resulting fragmentation, hasn’t helped, Gabelloni says. “Separation also hasn’t proven itself as such a benefit, where boosting has. However, separation with boosting could offer larger benefits than boosting alone by combining the benefits,” he says.

Cost has been a challenge also and this often relates to technology qualification. In the work with ExxonMobil, Sulzer has taken learnings from projects like Pazflor (gravity separation), Tordis and Marlim (long-pipe configuration gravity separation). “We took a systematic approach and while we didn’t standardize, we at least have solutions for a variety of potential field applications, all available and ready to go ahead (i.e. pre-qualified),” Tienhaara says, that should reduce costs.

Sulzer and ExxonMobil’s work notwithstanding, who will lead the charge in further subsea separation technology development and adoption is another challenge.

“Historically, the operators would come up with challenges and vendors would spend money to develop pumps, separators, and even compressors for the client,” McKee says. “That worked on a few pilot projects and a few designs were put forward to be more or less ‘off the shelf’ products. But, then, fields got more difficult, more complicated, and you got to a point where you had to have the operator funding development and testing, and they also wanted to be involved in it, too, because they wanted to make sure it was fit for purpose. Now we are in a situation where neither side has the money to do that, to produce these more difficult fields and meet these challenges. It has to be a meeting in the middle, but I don’t think we have really sorted that out yet.”

But, Tienhaara says: “Availability and capability of [separation] technology is better today than five years ago. The subsea community should be ready and able to embrace subsea separation and at least assess what it could bring, and how it could impact field architecture. I expect more studies that will involve subsea separation, for liquids and relating to compression stations. It could lower the complexity of the solutions typically implemented.”

McKee has a slightly different view. He says oil companies should now be screening all their subsea projects to see what benefit subsea separation would have. “Subsea separation is a must for subsea production systems to really achieve their potential and compete in this highly cost competitive market,” he says.