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As you’ve likely noticed over the past few months, the concept of producing, processing, liquefying, storing, and transferring LNG via an offshore floating (FLNG) facility has caught on in a big way. For many locations, it’s a cheaper and more flexible option than a shore-based facility.
When it comes to choosing an FLNG facility, there are many options and considerations. Nearly every offshore engineering firm in the world is coming up with their own concept designs based around the storage capacity, production capacity, type of field (dry/wet gas), and liquefaction process. There are even concept designs being evaluated for vessels of up to 520 meters in length!
That’s a shocking amount of steel. In many offshore fields, a vessel of that size would not be the economic option.
Tokyo-based engineering firm and FPSO operator MODEC, in conjunction with Toyo Engineering and ClassNK, has come up with a very interesting design that could be the right solution for a number of operators.
Their LiBro FLNG is based off of a Capesize bulk carrier hull that has been converted into an LNG storage container with topside processing facilities. It’s a smart idea considering the fact that Capesize bulkers are one of the most over-built class of vessel in the world and are being scrapped left and right. It’s a fairly inexpensive option, relatively speaking, one that can be converted quickly, and is large enough to store 160,000 cubic meters of LNG.
More importantly, MODEC’s LiBro FLNG concept utilizes a highly optimized and non-flammable refrigeration process.
In the case of Shell’s Prelude FLNG, and for nearly all shore-based LNG liquefaction plants, the AP-C3MR process is used. This process incorporates a Coil-Wound Heat Exchanger (CWHE) coupled with propane as a refrigerant, which is both highly volatile and heavier than air. On board a ship this is a potentially hazardous situation; however to crank out 3.6 million metric tons of LNG per year, it’s really the only option for Shell.
MODEC has taken a different path, using a nitrogen-based refrigeration process within its CWHE coupled with a Lithium Bromide Heat Exchanger.
As an inert gas, nitrogen is completely safe for use at sea, however it’s roughly 25 percent less efficient than propane as a refrigerant. MODEC has worked around this issue extremely successfully and managed to achieve efficiency rates of 96 percent while cranking out 2 million tons per year of LNG.
FLNG plants need a lot of electricity and are powered by gas turbines, and turbines like the GE LM2500 are highly sensitive when it comes to air inlet temperatures. In cold environments, the dense air makes them more efficient, and the opposite is true in hot environments. By cooling the air entering their engines, MODEC notes they have seen gains of 9 percent in process efficiency.
Using hot exhaust air to cool inlet air
Gas turbines crank out a lot of hot air, and thus wasted energy. The LiBro FLNG uses this heat to provide the needed thermal energy for a Lithium Bromide heat exchanger, which is then used to pre-cool the natural gas and for cooling the nitrogen refrigerant in the CWHE. It’s a rather complicated process, however it results in a further 7 percent total gain in efficiency.
In summary, MODEC has worked around the efficiency issue inherent with a nitrogen-based refrigerant and optimized its system to achive a 93 percent efficiency rating on their system. Although this process does not allow the same volume, or the top-shelf efficiency of propane, it makes up for this with extremely high efficiency, lower cost and increased safety.
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