Design Case Study for a 4 MTPA FLNG System for Severe Metocean Conditions

Abstract Floating LNG is gaining acceptance as a viable alternative to conventional land based LNG plants. In the past few years, several FLNG projects have become commercial and reached FID. However, most FLNG systems designed to date remain complex, costly and operationally limited to areas with benign to moderate metocean conditions that allow side-by-side offloading of the LNG. This paper describes a "Split Process" FLNG design where primary production and gas treatment functions are provided on a host platform while liquefaction occurs on a separate vessel(s). The arrangement works in moderate to severe metocean operating conditions, such as offshore Western Australia or the Grand Banks of Eastern Canada, and is scalable from 1 – 6 MTPA. The design and features of a 4 MTPA, deepwater case study are described. Salient features of the concept include: Segregation on separate facilities of gas processing/condensate storage from liquefaction/LNG storageImproved safety, constructability and overall project scheduleUse of existing, standard hull designs for host facility and liquefaction vesselsPre-cooled Dual N2 expansion liquefaction for safety, simplicity and operabilityDual fuel diesel electric (DFDE) power to improve fuel efficiencyNo offloading of LNG in open water; all offloading takes place in sheltered water Process improvements have been made to the Dual N2 expansion liquefaction (DNX) process to reduce the unit liquefaction power. Fuel consumption for liquefaction is further reduced by the higher efficiency DFDE prime movers over gas turbines. The system offers lower capital cost and improved operability over conventional, integrated designs. Eliminating offshore LNG transfer and achieving fuel efficiency on N2 liquefaction similar to mixed refrigerant (MR) processes contribute to reliable/cost effective operations. The liquefaction vessel is derived from a standard DFDE Moss LNG carrier which is extended 42 ft. at the bow to add a liquefaction module, switchgear and disconnectable turret mooring. Additional power generation is added to drive the 2 MTPA liquefaction unit. The liquefaction vessel disconnects when full and transits to a transfer jetty in sheltered water to offload. A fleet of 3 × 2 MTPA dedicated liquefaction vessels provides continuous operation and total system capacity of 4 MTPA. A host platform, determined by site specific considerations, handles primary production, condensate storage/offloading and treats the gas to an LNG inlet specification. The host may be an FPSO, TLP or fixed platform/FSO depending on upstream field development requirements. The FLNG concept described offers both schedule and cost advantages through the use of standardized hull forms and repetitive, standardized design.

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