Philip Jonsson presents his master’s thesis

Student: Philip Jonsson
Main Supervisor: Andrew Grant (Aker Solutions)
Examiner: Henrik Ström
Opponent: David Norsund

Abstract

Offshore natural gas production requires compression to transport the gas onshore. To increase resource efficiency, the compression can be done on the seabed instead of on topside platforms. Unmanned subsea compression stations are low carbon solutions to natural gas extraction, where losses due to topside platform processing are mitigated with the added benefit of increasing well lifetime. During installation on the seabed, the station is commissioned using nitrogen to test process equipment and process safety systems, prior to introducing hydrocarbons. As the system is designed for hydrocarbons, operating conditions with nitrogen may be different with regards to compressor performance, cooling etc. Incorrect operation with N2 might cause catastrophic damage of equipment.

This work, carried out in cooperation with Aker Solutions, aims to investigate the effects of nitrogen on the operation of the compression station. With a focus on the individual process equipment by investigating the thermodynamics at play by using process simulations. Limitations of the thesis are the commissioning activities related to the compressor, where it is operated in a closed loop. Nitrogen gas is compressed where it is recycled to a cooler and a scrubber before re-entering the compressor. Extraction of gas in the compressor is done where the compressor motor is cooled by process gas.

Process simulations are carried out using the dynamic process simulator K-Spice. Results are discussed and analysed, where limiting factors and recommendations of commissioning activities with nitrogen are presented. It was shown that nitrogen will significantly limit the operating envelope of the compression station.