Ormen Lange Subsea-Compression-System Pilot
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 20028, "Ormen Lange Subsea-Compression Pilot System," by Bernt Bjerkreim, Snorre Frydelund, John Arild Lie, Knut Ola Staver, and Karl Olav Haram, StatoilHydro; Bjorn Nystad, Shell; Hakon Skofteland, AkerSolutions; Hans Gedde, VetcoGray; Alberto Tesei, GE Oil and Gas; and Michel Postic, ConverTeam., originally prepared for the 2009 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed.
Ormen Lange is a long-tieback gas field with gas-processing facilities onshore 120 km from the production wells. The development strategy is pressure depletion. To maintain the production plateau for as long as possible and recover the anticipated gas and condensate resources, offshore compression is required to maintain production. The full-length paper describes subsea compression as a cost-effective alternative to the platform-compression solution and the strategy for qualifying the subsea-compression system before offshore-compression concept selection.

Introduction
Ormen Lange is a gas field 120 km off the northwest coast of Norway on the Norwegian continental shelf, in water depths varying between 850 and 1100 m. The reservoir covers an area 40 km long, 8 to 10 km wide, and 3000 m below the surface. Recoverable reserves are estimated at approximately 397 billion std m3 of dry gas and 28.5 million m3 of condensate. Ormen Lange has been in production since October 2007.
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 were subsea compression (Fig. 1) or platform compression.
The date for the final decision on the offshore-compression concept is planned to be 2012, 4 years after sub-sea-production startup (Fig. 2). An evaluation of the technical maturity and cost for the two offshore alternatives will be the basis for concept selection. The current plan is to start off-shore compression in 2016. Onshore precompression may be used and may delay the investment in offshore compression by some 2 years.

Business Case
Subsea compression represents a number of significant advantages compared to a platform solution. The solution is capital-expenditure cost efficient. This can be illustrated easily by the weight difference. With the Ormen Lange core functionality, the subsea-compression station has a weight of 6500 tonnes, while a platform will have a total weight of 25 000 tonnes. Further, operating expenditure cost also will represent a significant advantage for the subsea-compression alternative. No additional operational manning will be required to operate the field from the onshore terminal, including reduced logistics and reduced offshore operations. Using subsea compression compared to platform compression also will contribute to increased safety by reducing helicopter flights that would be required for the platform alternative. Additionally, locating compression closer to the reservoir will enhance the ultimate recovery, and subsea compression can be optimized easily for tail-end production.