Subsea power grids undergo final testing

Shallow-water test of a subsea power-distribution system. Credit: Siemens

Electrical equipment and automation provider ABB and industrial manufacturing company Siemens are in the final stages of testing their subsea power-distribution stations. The stations connect to a topside or above-water generator, such as a wind turbine or power plant, and redistribute electricity to underwater equipment. Both companies have been working on these projects for the past five years. Siemens completed the final round of testing its subsea power grid in Norway last November. ABB expects to complete the final round of testing for its variable-speed drive (VSD) in June 2019.

The Siemens model consists of a transformer, a medium-voltage switchgear, and a VSD. Its distribution voltage is about 30 kV, while its VSD puts out 6.6 kV. The system can provide electricity to devices with power ratings between 1 and 15 MW. The cabling that connects it to a generation station includes an embedded fibre-optic cable so operators can run it from onshore.

ABB’s VSD is designed for subsea gas compression, with its main aim to move power distribution from onshore to the seabed and an estimated 100 MW capacity to be transmitted over 600 km, the VSD will create valuable space currently unavailable on topside installations. The VSD, like Siemens’s subsea power grid, will connect the product through one cable further reducing operational costs.

Both systems are able to withstand the high-water pressure from the seafloor, as they are expected to work at depths of up to 3,000 m for 30 years with minimal maintenance. This is possible because the electronics in the devices are being flooded with a synthetic fluid called Midel.

This biodegradable fluid maintains the same pressure as the seawater, which alleviates the stress. The fluid will also cool the device by transferring the heat produced by the equipment to the seawater.

Siemens has worked with Eni Norge, Equinor, and ExxonMobil to develop the project, while ABB has teamed up with Statoil, Total, and Chevron on their VSD equipment development. Siemens and ABB now need to deliver their first models for installation in an active production site.

Brian Skeel, professor of subsea engineering at the University of Houston, has commented that subsea technologies should be developed with caution. A common problem that previous subsea technology has encountered is that if water does not continuously flow around the devices, marine life will grow on the equipment and shorten its lifespan.