Ormen Lange is a long tieback gas field developed with gas processing facilities onshore 120 km from the production wells. The development strategy is to deplete the reservoir. In order to maintain the production plateau for as long as possible and recover the anticipated gas and condensate resources, offshore compression is required at a later stage.
This paper describes subsea compression as a cost effective alternative to the platform compression solution and the strategy for qualifying subsea compression system at the time of offshore compression concept selection.
This paper further describes the subsea compression technical solution.
Ormen Lange is a gas field located 100 km off the northwest coast of Norway on the Norwegian continental shelf, in water depths varying between 850 and 1,100 meters.
The reservoir covers an area 40 km long, 8 to 10 km wide and 3,000 meters below the surface. Recoverable reserves are estimated to approximately 397 billion Sm3 dry gas and 28.5 million m3 condensate.
Ormen Lange is under development and production will start on October 1, 2007.
Fig. 1. Ormen Lange initial development without offshore Compression (available in full paper)
The main drive mechanism for the Ormen Lange reservoir is pressure depletion. The estimated recoverable reserves are based on the use of offshore compression facilities for pressure boosting upstream from the onshore plant, as the reservoir pressure declines.
The Ormen Lange subsea production system is designed to accommodate the flexibility to decide the type of offshore compression concept at a later stage. The options considered are:
Subsea compression
Platform compression
The date for the final decision on the offshore compression concept is planned to be in 2011, four years after subsea production start-up. An evaluation of the technical maturity and cost for the two alternatives will be the basis for the concept selection. The current plan is to start offshore compression in 2015. Onshore pre-compression might be utilized to postpone the investment in offshore compression by 1-2 years.
The design basis describes the purpose of the offshore compression facility as follows:
"The main drive mechanism is pressure depletion. The estimated recoverable reserves are based on use of offshore compression facilities for pressure boosting upstream of the onshore plant, as the reservoir pressure declines.
The production can be described in terms of the following production phases:
Early production, production prior to installation of offshore compression
Flowing wellhead pressure of typically 155 bara which is required to meet the onshore arrival pressure of 90 bara and the corresponding sales gas export rate of ~70 MSm3/sd
A flowing wellhead pressure of typically 140 bara required to meet the onshore arrival pressure of 75 bara and the corresponding sales gas export rate of ~60 MSm3/sd will determine the point at which installation of offshore compression is needed
Production after installation of offshore compression:
Onshore plant arrival pressure is to be maintained at 75 bara
