Seabed production boosting systems push the limits

As subsea development water depths and step-out lengths increase, operators require more technologically advanced and cost-effective methods to produce reserves over the life of deepwater fields. The objective, of course, is to maximize production and minimize costs, and ultimately to expand the economic development limits of subsea technology.

The challenge
In January 2007, Shell awarded the contract for two major deepwater seabed production boosting systems for subsea projects – Perdido in the Gulf of Mexico (GoM) and BC-10 offshore Brazil – to Baker Hughes. The Perdido system marks the first project using electrical submersible pumping (ESP) systems in seabed vertical booster stations in the GoM.

The vertical booster stations require installation of a large 36-in. conductor pipe on the seafloor by drilling or suction pile, if the seafloor is muddy. The ESP system is encased in a pressure vessel with a connection system on top. The system is lowered into the “dummy well” by a light construction vessel or rig. The booster station can be located at any point between the well and host facility. If more than one field is connected to the host production platform, the booster station may be closer to the platform and can boost production from several fields.

In developments where several wells are in one seabed location, the booster station may be needed closer to the wells. The vertical configuration of these installations makes gas separation easier, and the pump encapsulation in the pressure vessel onshore decreases installation costs. While several wells can be produced through one vertical booster station, this setup eliminates the ability to optimize flow from each well. The same pressure boost is applied to each well, which limits the system by the lowest-producing well.

The solution
Baker Hughes’ production solutions were chosen to boost fluids from deepwater subsea fields. The seabed booster

Caisson cross-section from Argonauta B-West, part of the BC-10 field, shows gas re-blending ports just below the ESP. (Photo courtesy of Shell)

systems minimize design complexity and offer higher efficiency and pressure boost capacity than alternative artificial lift methods. Flow rates up to 150,000 b/d and boost pressures in excess of 5,000 psi are achievable. High gas fractions also can be accommodated with a suite of multiphase pump designs.

In May 2010, Baker Hughes installed Centrilift XP enhanced run-life ESP systems in two vertical subsea boosting stations at Shell's Perdido field in the GoM. The setting in 8,000 ft (2,438 m) of water is a world-record water depth for ESP installation. The Perdido system is unique in that it has direct vertical access for installation and retrieval of the ESP systems.

The six systems installed in vertical subsea boosting stations at Shell's Parque das Conchas (BC-10) field in the Campos Basin offshore Brazil are located approximately five miles (8 km) from the floating production, storage, and offloading (FPSO) facility in 5,250 to 6,250 ft (1,600 to 1,905 m) of water and are designed to boost up to 100,000 b/d of fluid, which is the maximum capacity of the FPSO. The systems installed on BC-10 do not allow direct vertical access and will require a rig or a light intervention vessel for retrieval.

The Perdido vertical booster stations handle production from three subsea satellite fields (Great White, Silvertip, and Tobago) tied back to the spar host facility. This is the deepest spar production facility in the world, moored in approximately 8,000 ft of water. The booster stations are directly beneath the spar and are connected to the platform via top tensioned risers.

Pushing the ESP envelope
Centrilift standard ESP systems have higher pressure boost capabilities than most traditional surface systems. ESP systems, by design, are intended to be immersed in fluid, whether it is in the well or on the seabed. ESP motors are pressure balanced with the environment, whether that is downhole pressure or water pressure, in subsea applications. Basically, ESP systems are designed for the subsea environment, unlike traditional surface pumps that must be re-engineered to overcome pressure and penetration issues. A major issue for any field development is economics, and ESP seabed booster systems offer several advantages over other alternatives:
• Seabed ESP systems can be deployed with vessels of opportunity instead of via semisubmersible rigs, which reduces both the overall cost of installation and intervention and deferred production resulting from a waiting period for a rig;
• Seabed ESP systems can be configured to provide a backup system to maximize run-life and minimize deferred production. Some seabed ESP systems use existing infrastructure to house the systems, which also reduces overall development costs; and
• Seabed ESP booster systems are not as space-constricted as in-well systems. Production from several wells can be boosted with only one seabed ESP booster system.

Centrilift XP production systems extend the capabilities of ESP system technology to increase proven reserves. Historically, harsh fluid conditions, including high temperatures, CO2 levels, extreme abrasives, free gas content, and hydrogen sulfide levels limited the application range for ESP systems. The Centrilift Performance series pump designs maximize vane openings, optimize flow paths, and include patented particle swirl suppression technology as the first line of defense against abrasive downhole environments. The extra-wide vane openings boost performance in the presence of gas, sand, viscosity, and scale.

Baker Hughes installed ESPs in caisson on the seabed to boost several production wells in commingle with a single unit. (Photo courtesy of Baker Hughes)

The technology advancements built into the Centrilift XP systems also improve reliability in difficult operational challenges, such as hard starts and uncertain or changing downhole conditions. These tasks are achievable because the design uses carbide bearings, high torsion rated shafts, and a special motor insulation process. Nearly all of the features have been upgraded, such as mechanical seals, motor oil, elastomers, and couplings. The pumping systems at Perdido and BC-10 consist of two pumps that produce 700 and 1,000 gal/m at their best efficient points, and a tandem motor capable of generating 1,600 hp.

Centrilift Xtreme performance motors offer high-horsepower for technically challenging applications where downhole conditions and high intervention costs dictate robust, long-lasting solutions. These motors have been globally field-proven to increase ESP system run life in harsh well applications, allowing operators to increase production, reduce operating expenses, and decrease HSE incidents at the well site.

Baker Hughes project management teams consulted with Shell engineers regarding Perdido and BC-10 system details at all stages of production and delivery.

Throughout the process, from project award until installation and commissioning, Baker Hughes and Shell engineering staffs collaborated to ensure that the delivered project met expectations. After the units were installed, the companies worked together to ensure continued success by the use of monitoring and automation systems, both on the host vessels and at the Shell onshore offices. This teamwork will continue through the life of the project, as both Shell and Baker Hughes continue to apply operational learnings to the equipment as the field matures.