Greener offshore power

Generating units that emit much less CO2 and NOx than traditional offshore power plants have been installed on board the Snorre B platform and two other Norwegian offshore installations. Photo: Hydro

Norway has quietly put three gas--fired power plants into operation – which offer undisputable environmental benefits

By Svein Tønseth

The plants are located on oil platforms on the continental shelf and produce much lower levels of emissions than other systems of offshore power generation, virtually all of which are also based on gas.
The new power plants are based in part on know-how produced by NTNU/SINTEF, and they emit 25 – 30 % less CO2 and NOx per kWh generated than the vast majority of the installations in use on the continental shelf today, according to Alstom Power. The multinational has supplied the equipment that has made it possible to reduce emissions by such a noticeable extent.
– Compared with normal offshore installations, these three plants produce a reduction in emissions equivalent to the CO2 produced by about 100,000 cars, says Alstom division director Pål Kloster. The offshore sector produces a quarter of Norway’s total CO2 emissions, and 75 % of the CO2 from the continental shelf is produced in the generation of electricity.

Uses same heat twice
The normal procedure today is to generate electricity for use offshore by burning natural gas in gas turbines on the platform itself. The gas is burnt in the turbine together with large quantities of compressed air. The airflow is expanded by the heat of combustion and drives a series of turbine wheels; these in turn drive a generator, which generates electricity. The turbines can also drive compressors that compress natural gas for transport to shore.
Gas turbines are also used in gas-fired power stations, but in this case the heat of combustion is utilized twice. The heat from the gas turbine’s exhaust is used to generate steam in a boiler. This drives a steam turbine, which runs yet another generator and/or compressors. Combined cycle plants of this type extract much more power from gas than a gas turbine so that they use less gas to generate each kilowatt of electricity. Thus they produce lower CO2 emissions than single cycle gas turbines.
Alstom’s technology has made it possible to fit combined cycle plants of this type on platform decks, with the weight and space limitations involved. Electricity is now being generated on the Oseberg Field Centre, Eldfisk and Snorre B offshore installations according to the same principle as used in onshore gas-fuelled power stations. Snorre B started operating this summer. Each of these three plants has a generating capacity of around 10 % of that of the 400 mW power stations that are being planned for Kårstø and Kollsnes gas terminals.

World first
– To the best of our knowledge, generating plants of this type do not exist anywhere else in the world, says Pål Kloster. Alstom Power supplied the equipment that is used after the gas-turbine stage in the three combined-cycle plants.
The technology includes know-how “generated” from Norway’s 1987-1993 national gas research programme SPUNG. This programme funded the work of NTNU/SINTEF researchers Olav Bolland and Sturla Sæther which made it possible to develop compact, lightweight steam boilers.
ABB Miljø, which is now called Alstom Power, adopted this technology, utilized its own heat exchanger for gas turbines – equipment with a low weight to performance ratio and received funding from the Research Council of Norway, Norsk Hydro and Saga Petroleum for the development of these combined-cycle plants.
Six former students at NTNU’s Department of Thermal Energy and Hydropower are currently working on this technology at Alstom. Pål Kloster explains that the technology can be incorporated in new platforms, and can also be utilized when electricity generation capacity on existing platforms needs to be expanded.
Kloster points out that such combined-cycle plants are still somewhat more expensive than pure gas-turbine plants, but that the extra investment is outweighed by the lower costs of operating combined-cycle equipment. Gas which would otherwise have been used for power production can be sold instead. Lower gas consumption for electricity generation also means that less CO2 tax has to be paid per kilowatt generated. According to Kloster, the combined-cycle system also involves lower maintenance costs.

Contact at NTNU: Olav Bolland
Tel: +47 73 59 16 04
Email: Olav.Bolland@tev.ntnu.no