Mega-motors will liquefy Norway`s natural gas

Two of the world`s largest motors are being commissioned at a gas liquefaction plant on an island off the northern tip of Norway. The 65MW high-speed synchronous motors will be used to liquefy natural gas brought ashore through a 143km-long pipeline from an array of remotely-controlled production wells located on the seabed, 250-345m below the surface of the Barents Sea.

The £5bn Snøvit (Snow White) project is the first of its type in Europe. The liquefied gas will be shipped from the plant by a fleet of specially built tankers to customers - mainly in the US, Spain and France. The liquefied gas deliveries are scheduled to start late next year.and there will be about 70 shipments a year, with each 290m-long tanker carrying the equivalent of 870 million kWh of energy.

The massive motors and their matching converters, built by Siemens Large Drives division, are part of the liquefaction plant located on Melkøya island, near Hammerfest in the Arctic Circle. They will be driving compressors built by GE Nuovo Pignone in Italy. The compressor plant was assembled in Spain before being shipped by barge to Norway last year. The picture above shows one of the motors during construction.

The first 65MW compressor will pre-cool the gas before it is liquefied by a 32MW compressor (also powered by a Siemens drive). The liquefied gas will then pass through the second 65MW "sub-cooling" system to produce the liquid gas at -163°C. During the compression process, the gas will undergo a 600-fold reduction in volume. The energy-intensive cooling process will require 1.5TWh of power to maintain production for 330 days a year. This power will be generated by five 45.8MW gas turbines.

Traditionally, large compressors have been driven by gas or steam turbines. But these turbines are available only in standard sizes, while variable-speed drives can be customised to suit the application. VSDs can also vary the throughput of the liquefaction plant to cope with changing demand or operating conditions. The electric drives are also easier to start up and shut down than turbines, and they are expected to be more reliable, and easier and cheaper to maintain.

The order for the compressor drives was worth about €18m to Siemens Large Machines, which built the motors at its Special Machines site in Berlin and assembled the frequency converters in Nuremberg. Siemens also supplied transformers, harmonic filters and digital closed-loop controllers for the project.