Baseload LNG plants

A gas field 200 km offshore in the Carnarvon Basin has no customers nearby. Piping gas to Japan or Korea isn't possible. The only way to monetise it is to liquefy it and ship it. A baseload plant is sized to the reserves of the field — the aim is to run at full rate for decades so the enormous capital cost gets paid back. "Baseload" literally means the plant is the base of the production profile: it doesn't flex with demand, it just runs.

Because the plant is designed around one field's gas, the front-end (gas treatment) is tuned to that gas. Gorgon's feed is about 14% CO₂, which is why it has the world's largest CO₂ injection project bolted on — the acid gas removal unit strips it out, then it's compressed and injected 2 km underground into the Dupuy formation instead of being vented. North West Shelf gas has far less CO₂ and a different NGL slate, so its front end looks different.

A single train is a chain of processing units. Everything happens in this order.

1. Receiving. Gas from the offshore trunkline enters a slug catcher — a bank of large pipes on a slope — that separates liquid slugs from gas. Condensate is stabilised (flashed to remove light ends) and sold separately. Monoethylene glycol used offshore for hydrate control is recovered and regenerated.
2. Acid gas removal (AGRU). Gas bubbles up through an absorber column against an amine solution (usually activated MDEA) flowing down. The amine grabs CO₂ and H₂S. Rich amine goes to a regenerator where heat strips the acid gas back out, and the lean amine is cooled and recycled. Target is under 50 ppm CO₂.
3. Dehydration. Two or three molecular sieve vessels run in a cycle — one adsorbing water while the others regenerate with hot gas. Water out below 0.1 ppm.
4. Mercury removal. A non-regenerable bed of sulphur-impregnated activated carbon. Changed out every few years. Mercury would otherwise amalgamate with the aluminium in the main exchanger and crack it.
5. Pre-cooling and scrub column. Gas is pre-cooled (in C3MR, against propane) to about −35 °C and fed to a scrub column. Heavier hydrocarbons and aromatics drop out as liquid and go to fractionation; the overhead methane-rich gas continues.
6. Fractionation. The scrub column bottoms pass through a deethaniser, depropaniser and debutaniser. Ethane goes to fuel or refrigerant make-up, propane and butane to LPG storage or refrigerant make-up, C5+ to condensate.
7. Main liquefaction. In C3MR, the gas enters the bottom of the main cryogenic heat exchanger (MCHE) — a spiral-wound aluminium tube bundle inside a steel shell up to 50 m tall — and comes out the top at about −150 °C as liquid. Mixed refrigerant is let down through Joule-Thomson valves and sprayed over the outside of the tubes, evaporating and absorbing heat. In Cascade, the gas passes through plate-fin cold boxes chilled successively by propane, ethylene and methane loops.
8. End flash and nitrogen rejection. The sub-cooled LNG is let down to near-atmospheric pressure in a flash drum or through a hydraulic turbine. Nitrogen concentrates in the flash gas, which is compressed and used as fuel gas. This keeps LNG nitrogen under about 1%.
9. Storage and loading. LNG is pumped by in-tank submerged pumps to the jetty through insulated lines, loaded at 10,000–14,000 m³/h through articulated loading arms into ships of 125,000–266,000 m³. Vapour displaced from the ship returns to the plant.

Day to day, the plant is about staying at rate. Operators watch a handful of things that decide whether the train makes its tonnes:

• Ambient temperature. Air-cooled plants lose 3–5% capacity on a 40 °C day versus a 20 °C day because the refrigerant condenses at a higher pressure. Karratha in February is the hardest place in the world to run a C3MR train.
• Mixed refrigerant composition. Too heavy and the exchanger pinches at the warm end; too light and it pinches at the cold end. Adjusting MR make-up is a constant tuning job.
• Molecular sieve cycle timing. A bed that breaks through sends water to the cold end. Freeze-ups show up as rising differential pressure across the MCHE.
• Fuel gas and flaring. Every tonne flared is a tonne not shipped. Plants track flaring in real time.
• Tank inventory and ship scheduling. With 2–3 tanks and a ship every 2–4 days, the tank levels are a rolling puzzle.

Planned turnarounds happen every 4–6 years per train and last 4–8 weeks, with 1,000–3,000 extra workers on site. This is when the carbon beds are changed, mol sieves replaced, compressors overhauled and vessels inspected.

• Electric drives. Replacing gas turbines with motors fed by renewables or efficient combined-cycle power cuts emissions per tonne by 30% or more. Every new project studies it.
• Backfill. Existing plants run out of their original gas and get new fields tied in (Scarborough to Pluto, Barossa to Darwin, Browse proposed for NWS). This is cheaper than new trains and keeps jobs going.
• Carbon capture. Gorgon's reservoir injection is the template, and the lessons — slow start-up, water management in the injection formation — are shaping the next round.
• Bigger trains, fewer of them. Qatar's expansion is all 8 mtpa trains. Economies of scale still win when the field is big enough.