Bringing Forward the Next-Generation Multiphase Compressor
Authors: Fløisand John Olav , Torkildsen Bernt Helge , Almqvist Joakim , Lindøen-Kjellnes Hans Fredrik 
Published: April 2019

Abstract
The world's energy demand is continuously increasing, and natural gas will play a vital role in covering the future need for energy as part of a shift toward a cleaner carbon fuel mix. Offshore reserves constitute a considerable part of the world's recoverable gas. Accordingly, viable development of these reserves is instrumental for future socially responsible energy production and meeting the commitments of the Paris agreement.
The competitive marketplace for natural gas is challenging the subsea project economics now more than ever. This is driving the innovation for field enabling subsea technology solutions, targeting reduced capital and operational costs while increasing recovery of reserves compared with conventional offshore extraction.
In 2015, the world's first subsea multiphase gas compression system was installed offshore Norway. The system comprises two-off 5-MW machines configurable for serial or parallel compression. This system has now gained considerable and valuable operational experience. One of the most noticeable learnings from the field operation is the way the multiphase compressor has been utilized to unlock abandoned liquid reserves. In addition to the gas produced, a cyclic production of more than 5,000 bbl/d has been documented. Operation of the system has also shown how the subsea compressors regulates the wells’ backpressure and thus constitutes an effective pressure filter toward topside. This allows the operators to be more flexible with well operation without disturbing topside pressures.
To effectively produce and improve ultimate recovery in large offshore gas fields, the next-step requirements for volumetric flow capacity and drawdown pressure become substantial for multiphase compressors. Accordingly, this also applies to the required shaft power. State-of-the-art computer modeling and aerodynamic testing has been applied to improve the compressor design and throughput capacity. The differential pressure capability of the multiphase compressor has, up until now, been limited by the ultimate load capability of the axial thrust bearing. A thrust-balancing solution is now being included, and detailed design work is ongoing as part of a larger technology collaboration with major operators. Enhancements of the motor technology to larger outputs is part of this program as well. Combined, these improvements are fundamental for the ongoing qualification of the 8 MW and later 12 MW multiphase compressors while adding flexibility to the associated system design.
Shifting focus from compressor to system is a key factor to ensure the life-of-field return on investment. As tieback and power rating increases, minimizing the power system cost and complexity can entail rethinking of the compressor topology. This further justifies this focus shift in terms of field development planning. Ensuring an effective fit and compatibility with the subsea power system key units currently in qualification with world-leading powerhouses is a competitive advantage. The multiphase compressor, with its two-motor contrarotating design, ensures not only efficient power system compatibility but can contribute to game changing step-out topologies due to the low transmission frequency required for the power supply. Minimizing the complexity of both process and power architecture is crucial in terms of cost, robustness, and system reliability.