Nick Terdre, Contributing Editor
The recent delivery of subsea booster pumps to BP’s King field in the Gulf of Mexico marks a breakthrough for Aker Kvaerner Subsea in multiphase pumping. Two pumps (a third was also supplied as a spare) were installed in August 2007. Following commissioning, both have been operational since November.
In BP’s words, “The two pumps will enhance production from the King field by an average of 20%...In addition to the increase in production, the project will allow a 7% increase in recovery, extending the economic life of the field by five years.”
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Aker Kvaerner Subsea’s first delivery of a subsea multiphase booster pump was to Canadian Natural Resources’ Lyell field in the UK sector in 2005/2006, as a pilot project under the aegis of the Norwegian government’s Demo 2000 program. This pump has been in operation for more than 18 months without a single pump failure. However, the King contract represents the company’s first commercial delivery of the technology.
“This is a great milestone for Aker Kvaerner Subsea’s boosting business,” says Knut Nyborg, the company’s vice president for processing and boosting. Previously, only one other vendor had supplied subsea multiphase boosting systems.
The delivery also significantly extends the operating envelope for the technology:
- At a water depth of 5,500 ft (1,675 m), it is the deepest installation to date
- At more than 15 mi (24 km), it is the longest tieback
- At 50 bar over the pump, these units provide the highest pressure differential achieved to date.
The King development is also the first application of this technology in the GoM. An essential part of the project was to qualify a deepwater 5,000 psi pump and motor casing, including high-voltage penetrators, for GoM application.
The benefits claimed for subsea multiphase pumping are well known. As a field is produced, reservoir pressure drops naturally, so that production progressively falls. Subsea boosting can reduce the back-pressure on the wells, potentially increasing the flow and improving recovery. Aker Kvaerner therefore expects the technology to both boost the production rate and extend the producing lifetime of an oil field.
Multiphase pumping also increases the distance over which the wellstream can be transported, thereby extending potential step-out distances. The technology is applicable in all water depths, but brings particular benefits in deepwaters where alternative solutions are limited.
In addition to the three MultiBooster subsea pumps, the King delivery included variable-speed drives, high-voltage subsea connectors and jumpers, topside and subsea control systems, a topside lube oil hydraulic power unit, manifolds, and tie-in systems.
For installation on the seabed, the pumps were located in pump stations, comprising a skid placed inside a manifold. Each station measures 10 x 6 m (32.8 x 19.69 ft) and weighs about 90 metric tons (99 tons), of which the pump module accounts for 50 metric tons (55 tons). The stations were installed in single lifts.
Should maintenance or repairs become necessary, parts of the system – including the pump module and subsea control pod – can be retrieved separately, Nyborg says.
In addition, a condition monitoring system is built into the control system, employing data on vibrations, pressure and temperature gathered by sensors installed on each pump. This control system makes it possible to detect deviations from normal performance at an early stage before they develop into a malfunction.
The pump internals are 1 MW Bornemann MPC-335 twin-screw units, with a screw diameter of 335 mm (13.19 in.). Bornemann has experience in supplying multiphase pumps for surface applications, both onshore and offshore, having delivered more than 300 units.
One prime benefit of the twin-screw pump is its ability to pump oil and gas streams with a wide range of gas content, or more specifically gas void fractions (GVFs) from 0% to 98%, Nyborg says. This is particularly relevant to fields that contain a high proportion of gas. The pump is also self-priming, which means it can work with a low inlet pressure and does not have problems starting up in the presence of high gas content.
Furthermore, it runs at a relatively low rate, typically up to 1,800 rpm, which means the rotating parts should suffer relatively low wear and tear compared with, for example, a centrifugal pump running at twice that rate.
All of the King pumps underwent extensive testing underwater in Aker Kvaerner’s new pump test facility at Tranby, Norway. The program included pressure testing, performance testing through a full spectrum of gas fractions, and endurance testing at full power.
Development of the power system included extensive simulation work by the Sintef research institute in Trondheim. Power is supplied to the field – allowing for small transmission losses – at 6.6 kV, the level required for running the pumps. There is no need for a subsea transformer to step down the voltage.
Aker Kvaerner Subsea is involved in two other leading-edge projects, both in the Norwegian Sea – a raw sea-water subsea injection system for Statoil’s Tyrihans development, and a subsea compression pilot project for StatoilHydro/Shell’s Ormen Lange.
