Components that make up the subsea raw seawater injection system include three subsea centrifugal pump modules qualified to 2.7 MW, power system, control system, structures, and manifold. All components are meant to function without maintenance through the Tyrihans lifecycle. (All images courtesy Aker Solutions)

StatoilHydro is on the verge of installing in the Norwegian Sea’s Tyrihans field the world’s biggest, most powerful subsea raw seawater injection system (SRSWI). Installed in 885 ft (270 m) of water, the SRSWI system is expected to boost oil production from the field 10% over current recoverable reserves, equaling approximately 19 million bbl of oil.

The Tyrihans field is the largest StatoilHydro-operated field awaiting development offshore Norway. Tyrihans encompasses two structures: Tyrihans South, discovered in 1983, is an oil field with a rich gas cap; Tyrihans North, discovered in 1984, is a gas and condensate field with a thin oil zone. Combined recoverable reserves from the entire field are estimated to be 182 million bbl of oil and condensate and 34.8 billion cf of rich gas — before installation of the SRSWI.

Tyrihans lies 22 miles (35 km) southeast of the Kristin field. Once brought into production — targeted for late 2009 — Tyrihans will set a world record also for the longest step-out for high-duty subsea pumps, with a 19.3-mile (31-km) power umbilical from the Kristin platform to the SRSWI.

StatoilHydro owns 58.8% interest in the Tyrihans field and serves as operator. Its partners are Total, with 26.51%, Eni with 7.9%, and ExxonMobil with 6.75%.

In mid-2006, StatoilHydro awarded Aker Solutions the US $40 million contract for design and production of new-generation subsea pumps for Tyrihans seawater injection.

Not first, but biggest

“Tyrihans is due to come onstream in 2009,” said Stein Vegar Larsen, Aker Solutions’ project manager for the Tyrihans pump project. “Opting to do this subsea brings advantages in cost and energy reduction. Aker Solutions’ technology certainly can deliver this objective.”

Tyrihans is not the first time subsea water injection has been used. But, said Larsen, “with each pump requiring 2.5 MW of shaft power to achieve the high-injection pressures and flow rates required, these will be the most powerful pumps installed subsea to date.”

Injection system components include three subsea centrifugal pump modules qualified to 2.7 MW (including one spare), power system, control system (including comprehensive condition monitoring), structures, and manifold. All components are meant to function without maintenance throughout the Tyrihans lifecycle; all have passed rigorous mechanical tests and stringent environmental regulations.

The pumps will suck in 89,700 bbl of untreated seawater daily and inject it back into the reservoir, driving the water into the ground between Tyrihans North and Tyrihans South. When seawater injection is combined with gas injection from the Aasgard B platform, optimal reservoir pressure support is gained.

A 26.7-mile (43-km) electrically heated pipeline — the world’s longest — will carry the Tyrihans wellstream to the Kristin platform for separation. The pipeline includes an outer carbon pipe and an inner stainless steel pipe. Rich gas from both Tyrihans North and South will be exported for processing via the Aasgard transport system to Kaarstoe, north of Stavanger, Norway. Crude and condensate will be stabilized with liquids production from Kristin output and piped to the Aasgard C storage ship for tanker export.

Pumping power

The two SRSWI pumps will operate in parallel. Each is driven by a variable speed drive (VSD) system, including step-down transformer, frequency converter, and step-up transformers topside on the platform. The power umbilical encompasses six high-voltage conductors, two barrier fluid lines, and one fiber-optic cable for future applications. The umbilical is one dynamic part from platform to seabed and one static part, which splits in two 1,640 ft (500 m) before reaching the SRSWI, where the two ends terminate in a transformer module.

The pumps run in parallel, feeding high-pressure seawater to a common manifold that facilitates a common, single connection between the pump units and water-injection christmas tree. Normally, both the hydraulically actuated 6-in. discharge isolation valves are open while the minimum-flow 2-in. valves are closed.

The minimum flow system protects the pumps from overheating and excessive vibration. A hydraulic valve controls water discharging directly from the pump outlet to the sea. Discharged water is environmentally friendly because barrier fluid used in the motor is a 60/40 water/glycol mixture, and fluid leakage from the motor to the pump medium is about 4 cups (1 liter) per day.

The pumps can operate independently, for a high degree of flexibility and to increase system availability. The pump module is arranged vertically with a freshwater/MPG-filled 2.5 MW/6.6k V motor on top of an Aker Solutions Liquid-Booster eight-stage centrifugal pump.

The LiquidBooster pump features a cylindrical outer barrel enclosing an axially split inner volute case. The outer barrel serves as a pressure vessel, while the inner case encloses waterways, shaft, and impellers. Impellers are mounted back-to-back to maintain axial thrust force balance. A stiff coupling connects the pump and motor shafts.

Thrust force generated by the pump impellers is absorbed in the electric motor thrust bearing. The axial split of the inner volute case enables complete pump rotor balancing in its final assembled condition. The pump radial tilting pad sleeve bearings are lubricated by the process medium, i.e., seawater. Raw seawater is drawn into the pump first-stage impeller through a simple inlet strainer. All materials are seawater resistant.

Audun Grynning, Aker Solutions’ system engineering manager for the Tyrihans subsea pump project, said, “An important difference between a subsea pumping system and a topside installed system is the need to protect subsea equipment against seawater intrusion, especially the motor and pump parts. Optimizing reliability of a subsea pumping system is critically important. Intervention operations to access equipment subsea entail a high cost. Although replacing a worn mechanical seal or bearing may be a simple task, bringing the pump module to the surface, then re-installing it subsea is not. The cost is compounded by downtime during mobilization, installation vessel hire, and operational risks.”

Aker Solutions’ barrier fluid system helps maximize pump module lifecycle and uses overpressure to avoid seawater intrusion.

Power to the pump

Each subsea pump is supplied with 2.5 MW of shaft power in continuous operation at frequencies up to 67.7 Hz (approximately 4,000 rpm). Power comes from the 11 kV Kristin platform grid, where two VSDs control and feed power through the umbilical to the subsea transformers. The step-down transformer supplies the frequency converter with its operating/switching voltage of 1.9 kV for 12-pulse operation. The frequency converter output forms the step-up (3.3/25 kV) transformer input and is integrated in the same unit as the step-down transformer. Combined step-up and step-down transformer design offers advantages during offshore installation. Size and weight is significantly reduced, as both transformer cores are immersed in a common liquid tank.

Two subsea transformers step the 22 kV power umbilical voltage down to the rated operating motor voltage of 6.6 kV. Maximum current during continuous operation is 320 A.

The wet mateable connection system performs a vital role in subsea water injection. System modularization helps avoid costly interventions in the event of equipment failure or wear. In the SRSWI system, the connector receptacle part is mounted directly on the transformer, while the connector plug part is mounted on a jumper cable terminating in the motor.

At the 2007 Offshore Technology Conference in Houston. Aker Solutions received the OTC Spotlight on New Technology Award for the achievements associated with its subsea pump design. Reflecting on that milestone, Larsen said, “We believe our pump design incorporates many advantages that represent a breakthrough for subsea water injection — a technology that offers the oil and gas industry a new solution that may encourage development of marginal fields by enhancing oil recovery today and in the future.”