Subsea compact gas compression with high-speed VSDs and very long step-out cables

The unexpected shrinking gas production forecast in the Asgard field on the Norwegian Continental Shelf was the motivation for a leading European Oil & Gas company to start a Technology Qualification Program (TQP) for Subsea gas compression using High-speed Variable Speed Drives (VSD) in order to keep the actual gas production volume at the initially forecasted levels. The goal is to minimize big investments and operational expenses in new off-shore production units by utilizing existing production units as a hub and have a significant part of the gas recompression equipment installed on the sea-bed by means of long step-out cables. This paper is predominantly focusing on the design and testing of a High-speed Variable Speed Drive (VSD) & long step-out power system at the customer´s Technology Laboratory K-Lab in Karsto, Norway. Hereby, several technical challenges will be addressed: 1. World longest umbilical cable length between frequency converter and machine/compressor of up to 47 km. 2. Biggest power rating for step-out system so far, rated for 6-8MW compressor power. 3. Highest motor supply frequencies of up to 200 Hz 4. Design and qualification of oil filled transformers (both topside and subsea) for rating up to 15 MVA, voltage up to 72,5 kV and supply frequencies up to 200 Hz The paper will also present measurement results of the high-speed VSD system, which recently has successfully been put into operation in the K-Lab in order to qualify several machine/compressor solutions.