Depletion of oil and gas reservoirs will gradually reduce the wellhead pressure. Consequently, the process equipment for separation and transport of oil and gas will move into off-design operation with reduced efficiency. Turbocompressors are critical equipment used for re-compression, injection or transport of natural gas. Off-design operation may result in a major compressor efficiency drop.

Statoil operates several offshore North Sea process platforms where reduction in wellhead pressure is a challenge. This paper describes the experience with compressor revamp on one onshore and one offshore process plant covering scope of work / supply, installation, testing, operation and cost/benefit analyses.

The processing facility on Statoil’s Troll A platform in the North Sea interacts with the installations at the Kollsnes gas treatment plant near Bergen, where natural gas is transported to the European Continent by the world’s largest variable speed electrical drive pipeline compressors. Changes in gas inlet conditions required revamping in order to adapt the compressor performance to existing process conditions. In addition, a high performance 3 dimensional rotor design was installed in the old compressor casing.

Sleipner East and Sleipner West lie in the Norwegian North Sea. The first of these gas and condensate fields came on stream in October 1993. After gradually reduction of the wellhead pressure on the Sleipner A processing platform, it was decided to modify the processing facilities and revamping of two gas turbine driven re-compressors were required.

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