Much has been written in recent years about subsea production technology and its application not only for deepwater and harsh environment projects but also for shallow-water and brownfield developments.

Several scenarios are currently playing out, with oil companies having to balance the advantages of applying seabed processing solutions—driven by the need to enhance reservoir recovery rates from their relatively low present levels—while also being fully immersed in a major cost-reduction cycle.

This is no easy balancing act. Every offshore operator today is fully focused on lowering costs, but anyone who works in the upstream business knows that it must at all times retain a long-term view for each and every field it develops. If developing a new or advanced subsea processing solution means it can be applied to a greenfield or brownfield development to increase the overall recovery rate—while also potentially removing the need for surface facilities—then according to those criteria there are cost advantages that can only be achieved by near-term and greater investment.

The basic elements of subsea processing are well known—booster pumps, compression and separation equipment. According to a report by DNV GL earlier this year for Norway’s Petroleum Safety Authority, “The motivation for subsea processing has changed, from reducing topside weight to being an enabler for late-life production till today, where subsea process facilities have been installed on greenfield developments. Increasing the oil recovery is a key driver.”

Producing fields with heavy oils and/or low reservoir pressures might also become feasible if installing subsea processing equipment, it continued.

Seabed Experience Growing

There is a growing record of industry experience in this sector, according to DNV GL’s report. It highlighted the Kværner Booster Station (KBS) in the 1990s that was built and tested although never actually used as well as the subsea separation projects Troll Pilot and (a decade later in 2007) Tordis, both installed on the Norwegian Continental Shelf.

Tordis was the world’s first full-scale commercial subsea separation, boosting and injection system, removing water and sand from the wellstream and reinjecting it into a nearby formation. A multiphase pump was installed to assist in transporting the oil and gas to the topside facility.

More recent flagships have included Shell’s Perdido and Total’s Pazflor projects in the Gulf of Mexico (GoM) and Angola, respectively, which were the first full-field subsea separation and pumping systems in their regions. Both use vertical gas/liquid separation units, whereby the gas free-flows to the topside host and the liquid mixture is boosted by means of subsea pumping.

Petrobras’ Marlim Field (2011)—the world’s first system for deepwater subsea separation of heavy oil and water—installed a horizontal pipe separator to separate oil from water, with the latter reinjected for reservoir pressure support. The oil and gas, meanwhile, are commingled downstream of the separator station and free-flow to the topside facility.

Compression Complexities

This year there are three major projects in progress involving the addition of subsea compressor stations—or at least, there were. Two are progressing as planned and are in the implementation phase: Gullfaks South, scheduled for the second quarter of 2015, and Åsgard just before it in the first quarter. The third, Ormen Lange, however, was delayed in a high-profile decision in April by operator Shell and supported by its partners (with the exception of the Norwegian state company Petoro).

Acting as a reminder that subsea processing is, despite its growing track record, very much still a work in progress, the Ormen Lange Management Committee decided the project was simply not viable at this time.

Ormen Lange faces greater challenges than the compression projects on Åsgard and Gullfaks, not only because of the deeper waters--Ormen Lange sits in approximately 900 m (2,953 ft) of water, while Åsgard is in 340 m (1,115 ft) and Gullfaks 135 m (443 ft)—but also because of the step-out distance. The gas field has been producing subsea-to-beach since 2007 over a distance of 120 km (75 miles). The subsea compression solution on Ormen Lange needs more electrical components and systems to be installed on the seabed than Åsgard or Gullfaks due to that larger step-out distance for the electrical power transmission.

Ormen Lange Alternatives

Committee Chairman Odin Estensen said in a public statement at the time of the decision: “The oil and gas industry has a cost challenge. This, in combination with the maturity and complexity of the concepts and the production volume uncertainty, makes the project no longer economically feasible. The Ormen Lange license remains committed to the ambition of maximizing the ultimate recovery from Ormen Lange in a sustainable manner. Significant new information both on reservoir behavior and technology developments will become available in the next few years and provide basis to [evaluate] new options.”

The partners clearly are not giving up on the solution, however, with the chairman continuing, “The Ormen Lange License group believes in the subsea compression technology and still regards the qualification of this technology to be an important stepping stone for the Ormen Lange future development alternatives. Subsea compression technology is a key contributor for ongoing and future field developments on the Norwegian Continental Shelf.”

Essentially this is a project that now will sit on the shelf for a number of years while the operator and its partners wait for both the technology and economics to add up.

Instances such as this are likely to happen whenever the limits are pushed in terms of the complexity of equipment and systems.

Pump It

When the term “subsea processing” first came on the scene as a concept, the focus was expected to be on seabed separation, with the other elements such as gas compression, boosting and raw water injection somewhat trailing behind.

However, the emphasis has firmly shifted in recent years toward rotating machinery, not only on the seabed but also in the well.

The Subsea Production Alliance (SPA), formed earlier this year between Baker Hughes and Aker Solutions, has talked about offering “reservoir development services”—like their rival OneSubsea established in 2012—to get the most oil out of the ground. The focus is very much on improved and new boosting systems, including what is now known as “dual boosting,” i.e. seabed and downhole pumping working in parallel.

Baker Hughes told E&P’s sister publication Subsea Engineering News (SEN) at the 2014 Offshore Technology Conference in Houston that it has been working for some time on improving the reliability of its Centrilift brand electric submersible pumps (ESPs). Its through-tubing design is aiming at a run-life of 10 years—a major advance from the 90 days of old. Another big element will be a rigless intervention system, an aspect that will be of significant interest to a growing number of operators.

Rigless Intervention

Related to this, earlier this year Baker Hughes confirmed that its seabed-installed ESP, supplied to Petrobras for the deepwater Cascade Field in the GoM, came into operation. This is just one example of how an ESP can be deployed in such a way that intervention would be rigless. Petrobras has also used this configuration in its domestic waters.

The other half of the SPA “dual” system will be a seabed multiphase pump that Aker Solutions has been working on for several years. The design is called a “semi-axial” hybrid pump.

SPA is not the only one working on pumps, with OneSubsea already a market leader for seabed pumping based on its Framo Engineering legacy designs. According to SEN, there also will be work to increase the power of both the Hi-Boost pump and its wet gas compressor, one of which is being supplied for Statoil’s Gullfaks South enhancement. The latter unit is 5MW, and the aim is to slowly increase the power to 6MW, then 8MW and eventually 10MW.

Contact the author, Mark Thomas, at mthomas@hartenergy.com. The photo on the home page is courtesy of Aker Solutions.

Offshore