The Swedish Södermanland class of diesel-electric submarines consist of the HMS Södermanland and HMS Östergötland. These two submarines were originally launched as Västergötland-class submarines in 1987 and 1990, and have been relaunched as a new class after extensive modernization 2003 and 2004 by Kockums AB. The pressure hull had been cut in two after the sail and a long new section with an air-independent propulsion system was inserted. It contains two Stirling engines which are coupled to electric generators and heated by burning diesel fuel with liquid oxygen stored in cryogenic tanks. The AIP system can provide electric energy to extend the submarine's submerged time from days to weeks.