The idea
Some gas fields are inconveniently placed. They sit far offshore, or off a coast with no infrastructure, or in a country where building a large industrial plant is slow or politically fraught. The volumes may not justify a pipeline to shore plus an onshore terminal.
FLNG answers that by moving the plant to the gas. A hull is moored over or near the field. Gas comes up, is treated, chilled and liquefied on deck, stored in tanks in the hull, and loaded directly onto carriers that come alongside. Nothing is built ashore.
What makes it hard
Putting a chemical plant on a floating structure creates problems an onshore plant never has.
Space. An onshore plant spreads out, with distance between hazardous units as a safety measure. A hull cannot. Everything is stacked on a deck a few hundred metres long, with hydrocarbon processing, cryogenic storage, accommodation and utilities close together. That drives the choice of a simpler refrigeration cycle: fewer machines, less pipework, less to go wrong in a confined space.
Motion. The hull moves. Distillation and separation equipment that relies on liquid sitting level has to tolerate roll and pitch, and cryogenic liquid in partly filled tanks sloshes. That pushes designers towards containment that handles motion well and towards processes that are less sensitive to it.
Offloading at sea. An onshore terminal loads at a fixed jetty. An FLNG unit transfers cargo to a carrier moored alongside or in tandem, in open water, through cryogenic hoses or arms. Sea state limits when that can happen, and it is one of the operational realities that determines how much a unit actually exports against its nameplate.
Maintenance. You cannot bring a crane and a thousand contractors to an offshore hull as easily as to a site on land. Everything has to be maintainable in place, which raises the premium on reliability and on equipment that can be swapped out in modules.
How the technology differs
The refrigeration cycles used offshore skew towards single and dual mixed refrigerant designs. Black & Veatch’s PRICO process, a single mixed refrigerant cycle, has been used on several floating units. Shell’s DMR, a dual mixed refrigerant process, is used on the largest FLNG hull built to date and is now being offered more widely for floating projects. The common thread is a smaller equipment count than a propane pre-cooled train.
Containment on FLNG hulls follows the same logic as on ships, and both membrane and free-standing systems appear, with sloshing behaviour a stronger consideration than it would be onshore. Our containment explainer covers why that pushes some projects towards tanks with internal structure.
Two different animals
The term FLNG covers two quite different things, and conflating them causes confusion.
Purpose-built hulls are new, large and designed as one unit around a specific field, with capacities in the range of 2 to 4 million tonnes a year and, in one case, considerably more.
Conversions take an existing LNG carrier and rebuild it with liquefaction topsides. This is cheaper and faster, and the resulting units are typically smaller. Several of the operating floating liquefaction vessels in service are conversions of this kind.
Our data treats floating liquefaction in both places it appears. It is a terminal in the terminal tracker, with capacity in Mtpa, and often also a vessel in the carrier tracker, with a cargo capacity in cubic metres and a shipyard. The floating liquefaction hub on this page’s related list links both views.
When it makes sense
FLNG is not a cheaper way to build liquefaction. Per tonne of capacity it usually costs more than an equivalent onshore train, because everything must be compact, motion-tolerant and marine-classified.
Its case rests on what it avoids: an export pipeline to shore, a coastal industrial site, the associated permitting, and in some cases a decade of political negotiation. It also offers redeployment. A hull whose field is depleted can in principle be moved to another, which no onshore plant can do.
That combination suits stranded offshore gas, smaller fields that cannot carry the cost of onshore infrastructure, and countries wanting export capacity without a large domestic construction programme. Where a big field sits near an existing industrial coast, an onshore train remains the cheaper answer, and that is still where most of the world’s capacity is built.