HOUSTON—When it comes to improving field economics, industry leaders say subsea boosting should be considered a “default solution” when developing hydrocarbon resources offshore.

Among the economic drivers are the potential to increase oil recovery, improve flow assurance and lower production time while increasing net present value (NPV) for greenfield and brownfield developments, according to Aker Solutions’ Hans Christian Nilsen, who spoke during this month’s Offshore Technology Conference (OTC) on how new subsea boosting products can grow profits.

To improve economics Aker Solutions, working with joint industry project partners, is developing new multiphase subsea pump technology powered by a stronger motor. Talk of the new boosting technology, which is expected to be released later this year, comes as operators go farther and deeper offshore in search of oil and gas with subsea technology gaining focus amid a wealth of tieback opportunities.

Tapping into the need for speed and power could lead to improvements, which is where Aker Solutions and partners took aim. As described by Nilsen during a technical session at OTC, the project involved doubling the motor’s power to 6 MW and increasing the speed from 4,000 rpm to 6,000 rpm, suitable for a range of pressures, temperatures and gas volume fractions.

Benefits are that fewer pumps may be needed to perform the job faster, equating to time and cost savings. A key feature of the multiphase pump is mixed-flow impellers that are placed in opposite directions, eliminating the need for a balance piston,which can cause vibrations.

“Up to now the helico-axial principle has been used for multiphase rotodynamic pumps,” Nilsen added. “We have developed an extended multiphase technology called helico-mixed flow,” which generates more pressure from its use of centrifugal action. It’s designed to reduce backflow.

Using a plug-and-play approach, the faster motor with twice the power has many of the same components of the 3-MW pump with 80% of the parts being reusable, he said. The impellers have the same geometry and are interchangeable, which enables the pump to be reconfigured as operations change through the lifetime of the field.

New cables capable of withstanding higher temperatures also are being qualified.

Advancing Technology

Advancing subsea pumping technology has been a mainstay for Aker Solutions, which has several subsea pumps based on “centrifugal pump technology that utilizes various stages of impellers to provide the required pressure increase,” according to the company.

While the MultiBooster subsea multiphase pump—armed with the 6-MW, 6,000-rpm subsea motor—targets hydrocarbon flows with a high gas volume fraction, the HybridBooster with mixed-flow impellers to compress gas is “ideal for low to medium gas content,” the company said on its website. Aker Solutions’ toolbox also includes a subsea single-phase pump, called LiquidBooster, which uses radial pump technology for liquid hydrocarbon/condensate boosting or water injection.

Such boosting technology has the potential to grow profits. Nilsen pointed to a subsea boosting case that compared using gas lift at riser base with 2-MW, 4-MW and 6-MW motors. The case centered on a subsea field at a water depth of 1,500 m (492 ft) with a 30-km (19-mile) step-out from the host facility. The field had four production wells and two water injection wells along with a high water cut.

The 2-MW boosting increased total recovery by 22%, while the 6-MW boosting power increased recovery by 30%. The NPV jumped, while production time fell.

Nilsen concluded that subsea boosting and multiphase pumps are well-proven concepts with ongoing development to expand their potential with greater pressures and flows. In most cases, subsea boosting will increase profit, and it should be the default choice for any offshore field, he added.

Jack/St. Malo’s Story

Subsea boosting is not a new concept, but innovation is pushing the technology to new heights.

Chevron turned to subsea boosting to get hydrocarbons from the Jack and St. Malo fields in the U.S. Gulf of Mexico to the production platform when initial reservoir pressure fell.

During the OTC subsea processing technical session, Mads Hjelmeland of OneSubsea (now part of Schlumberger)—which worked on the development—spoke about subsea boosting’s impact on the two fields being jointly developed. Discovered in 2003 to 2004 about 40 km (25 miles) apart, Hjelmeland said successful artificial lift is essential for the fields.

The ultradeepwater fields are located in the Lower Tertitary Trend and face production challenges that include low permeability, low gas-oil ratio, pressures ranging from 17,000 psi to 24,000 psi and temperatures as high as 132 C (270 F).

The fields, which are tied to the Walker Ridge Regional Platform, used single-phase boosting—the first for operator Chevron. The option was selected over others, which included multiphase pumps, electric submersible pumps, waterflood and gas lift. Among the advantages of the subsea boosting was the ability to produce oil at a lower wellhead pressure, which increased production and ultimately the project’s NPV. This includes the life-cycle costs, Hjelmeland said.

The fields have three identical single-phase pumps, which went online in 2016. It came following work that started in 2009 that led to the qualification of a 3-MW pump and motor housing rated for 13,000 psi along with a transformer and other components—the world’s first high-pressure subsea boosting system.

“The actual performance was very much in line with what we predicted,” Hjelmeland said, later adding the technology qualification process lasted three years. It was followed in 2011 by engineering, procurement and construction with the equipment deployed in 2014. The system was wet parked that year because the pressure was so high that production occurred by itself, he added. But the system has been in continuous operation since April 2016 working in automatic mode.

According to Chevron, daily production from the first development stage is expected to rise to 94,000 bbl of oil and 594 Mcm (21 MMcf) of gas over the next several years. Subsea boosting is expected to play a role in elevating production rates for the next 30 years. Production at the fields, with total recoverable resources at an estimated 500 MMbbl, started in 2014.

Hjelmeland called subsea boosting a key enabling technology and a natural concept for subsea tiebacks.

—Velda Addison

Offshore