Nitrogen Commissioning of a Subsea Compression Station. An investigation of how process equipment intended for natural gas is affected by operating with nitrogen

Abstract

In some cases, 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. Unmanned subsea compression stations are low carbon solutions to natural gas extraction, where losses common to topside platform processing are mitigated, often with the added benefit of increasing well lifetime. After 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 a subsea compression station. Specific focus is placed on individual process equipment by investigating the thermodynamics at play using process simulations. The thesis content is limited to the commissioning activities related to a compressor, where it is operated in a closed loop. Nitrogen gas is compressed and recycled to a cooler and a scrubber before re-entering the compressor. In addition process gas is extracted from the compressor to provide compressor motor cooling.

Process simulations are carried out using the dynamic process simulator K-Spice. Results are discussed and analyzed, 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.