Evolution. Floating LNG (FLNG) is an attractive concept for several reasons:
Several decades of reliable track records with LNG onshore plant development and floating production, storage and offloading (FPSO) have given gas field developers the confidence to build FLNG vessels.
LNG transportation by ship began in 1959. Now over 100 LNG liquefaction trains have been constructed in 20 LNG exporting countries, and over a dozen multi-train LNG projects are under construction. FPSOs began with Shell Castillion in 1977, now there are close to 300 FPO/FPSOs deployed around the globe.
FLNG projects to date include several that are being completed and commissioned, Shell Prelude for Australia and Petronas in Malaysia. Golar have successfully converted LNG carrier vessel Hilli, with an option contract to convert the Gimi. One announced project encountered a delay, Exmar Pacific Rubiales for Columbia.
Design. In designing FLNG, consideration must be paid to topsides and ocean motion impacts. Topsides require reducing weight and footprint while accommodating ocean impacts. Added safety issues include eliminate ignition sources and cryogenic spills, reduce flammable inventories, and provide safe havens for personnel. Impacts of FLNG being in an ocean include accommodating motion modes, particularly turrets and mooring systems, and equipment internals design to control motion effects. With vapor-liquid contact devices, structured or random packing is typical, with special attention to bed height, increased liquid distribution and novel distributors. For the storage portion of the vessel, design focuses on control of sloshing.
Process. FIG. 1 is a block schematic of an FLNG processing scheme, starting with feed gas to the vessel, to producing LNG, LPG and stabilized condensate products. In this section, process options for mercury removal, acid gas removal (AGR) and dehydration are covered, along with technology options for liquefaction.
Fig. 1 FLNG process overview.
Mercury is a metallurgical and personnel problem. When condensed, mercury causes embrittlement; if released, personnel exposure should not exceed 10 ng/Nm3. Fortunately, there are no marinization concerns for FLNG, with the choice of technologies being vapor phase and static solid phase. Choices include non-regenerable metal oxide/metal sulfide or sulfide-activated carbon, or regenerable silver-doped molecular sieves.
In the non-regenerable choices, the sulfide-activated carbon has a lower fill cost, but disposal is a concern: metal oxide/meta sulfide has lower sensitivity to liquids and a smaller bed size. The regenerable choice has the advantage of a potential reduction in required space—i.e., via integration with molecular sieve beds—and it enables higher pressure to liquefaction. However, the disadvantage is more complex dehydration. Mercury removal can be located upstream of the AGR unit, which offers more protection to personnel and equipment, with no dryout required. Locating it downstream of dehydration means slightly smaller beds and flexibility to use non-regenerable technologies.
Credit is given to KBR for their presentations to AIChE South Texas Section in March 2016 and Topsides Conference in February 2015 that are the basis for this article. Access to the full article is available here.
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Featured image: Over the next decade, the floating liquefied national gas (FLNG) sector will see significant growth in both investment and activity. (Image courtesy of Keppel Offshore & Marine Ltd.)
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