A full subsea production system including the introduction of subsea compression has been designed for use on a case study of an offshore green-field application. The field is located at a water depth of 1600 m with a long tie-back of 60 km to an LNG plant onshore. A new design methodology that combines the complete production system including the compressor station into one model has been developed. The design methodology use a network model combining multiphase flow and process models from wells to topside, hence the complex interaction of the realistic constraints given by the reservoir, the fluid properties, the compressor and the onshore plant is captured. The simulator is fast and flexible and enables screening of different process design concepts. The main objective of the simulations has been to see if the Subsea Gas Compression system and power design can be optimized compared to a traditional design methodology. By a traditional design methodology prior to the case study it was concluded that compression for a power system designed for 100 MW was required to keep production at target rate for 20 years. Such a huge power supply seemed unrealistic.
The new design methodology of life-of-field has optimized the subsea compression station design with 50%–75%. Different parameter options have been screened based on operability, cost and optimal compressor performance combined with given system constraints. Production optimization has been compared to power consumption and compressor performance. It has been understood how the interaction between the compression, the reservoir and the LNG plant interacts with each other and how the subsea process station will operate in a life of field perspective. It has also been understood that a traditional design methodology end up with far too conservative estimates of every discipline, which adds up to a system that also will be more difficult to operate than an optimized system. The conclusion is that it is necessary to include the main disciplines from early design and the operability of life-of field.
