Floating LNG (FLNG)

Plenty of gas sits in fields that are too small, too far offshore, or too far from any coast to justify a pipeline and an onshore plant. Prelude's field is about 475 km north-east of Broome — a pipeline to shore plus a greenfield plant on the Kimberley coast was studied and rejected on cost and environmental grounds. FLNG avoids the pipeline, the onshore land acquisition and approvals, the jetty and the dredging. When the field is depleted the vessel can, in principle, be moved to another one.

It also compresses the schedule. The hull and topsides are built in a shipyard (Prelude at Samsung Heavy Industries in Korea, Coral Sul at Samsung too), which is the most controlled construction environment in the industry. The whole plant is commissioned as far as possible at the yard before the tow.

1. Subsea and risers. Wells on the seabed produce through flowlines to the turret. Flexible risers carry gas, condensate and MEG up to the vessel. At Prelude the turret is 93 m tall and the vessel weathervanes around it, always facing into the weather.
2. Inlet and separation. Gas, condensate and water are separated on deck. Condensate is stabilised and stored in the hull. Water is treated and discharged. Everything a baseload plant does in its slug catcher and stabiliser happens here, in a fraction of the space.
3. Gas treatment. Amine CO₂ removal, mol sieve dehydration, mercury removal — the same chemistry as onshore, but the columns are designed to work with the vessel tilted and rolling. Structured packing replaces trays because trays malfunction when the liquid sloshes.
4. Liquefaction. Prelude uses Shell's DMR process with two mixed refrigerant loops and a spiral-wound exchanger; there's no propane, so no large pressurised LPG inventory on deck. Nitrogen-expander designs are chosen by others precisely because the refrigerant is inert — on a vessel where you can't walk away from a fire, that matters. Heat exchangers are designed and tested for tilt; spiral-wound units tolerate motion better than plate-fin.
5. Storage in the hull. LNG goes down into cryogenic tanks inside the hull — membrane tanks (like an LNG carrier) or self-supporting prismatic (SPB) tanks that resist sloshing at any fill level. Sloshing is a real structural load: partly-filled membrane tanks in a seaway can hammer the insulation, so fill levels and sea states are managed together.
6. Offloading. An LNG carrier moors alongside (side-by-side) every week or so and loads through articulated arms that can follow the relative motion of two vessels. In harsher seas, tandem offloading with the carrier trailing behind uses floating cryogenic hoses. Condensate is offloaded to tankers separately.
7. Utilities and people. Power generation (Prelude: steam turbines and gas turbines producing well over 100 MW), seawater cooling drawn from depth through risers, flare tower, living quarters, helideck, lifeboats. The vessel is a town and a refinery at once.

Life on board is offshore platform life with a refinery attached. Crews rotate FIFO by helicopter on a roster measured in weeks. The operating challenges that don't exist onshore: managing tank fill levels against sea state to control sloshing; scheduling offloading around weather windows; keeping the marine systems (ballast, mooring chain tension, hull inspection) in step with the process plant; and the fact that any significant repair means bringing people and parts across hundreds of kilometres of ocean. Prelude's early years showed that power system reliability is the crux — when the vessel loses power, every system from process to hotel goes dark, and the 2021 event that left the crew without full power for days led to a lengthy regulator-directed shutdown.