FPSO Gas Handling: How BOG, Refrigeration and Reliquefaction Skids Work Together (2026)

In March 2026, SBM Offshore moved into FEED on ExxonMobil's Longtail FPSO offshore Guyana, and the P-81 and P-87 awards for Petrobras advanced toward signature offshore Brazil. After a quiet stretch, the FPSO order book is filling again. But there is a second thread running through this cycle that gets less attention than the hull contracts: a growing share of these floaters are being built to handle the gas, not just the oil. Associated gas that once would have been flared is now something operators are expected to capture, compress, treat and either reinject, export or liquefy — and that puts a gas-handling train on the topsides of vessels that, a decade ago, would have carried almost none.

That train is not one machine. It is a chain of modular process skids — boil-off and associated-gas compression, process refrigeration, and in the LNG cases reliquefaction — that has to behave as a single coherent system once it is bolted to the deck. When the gas-handling package gets split into skids and sent out for quotation, the most expensive thing an EPC can do is treat each skid as an independent box. They aren't. They share refrigerant, they share utilities, they share a control philosophy, and they share a safety case. The integration risk lives precisely in the seams between them.

We have built marine and offshore gas-handling skids for the better part of a decade — reliquefaction units delivered in series of six and twelve against single vessel programs, BOG compressor skids supplied in sets of eight across sister hulls, and process refrigeration / refrigerant-loop packages for offshore production — designed, fabricated, integrated, tested and documented in-house, certified across DNV, ABS, BV, CCS, LR, NK and KR. From inside that work, here is how the gas-handling chain actually fits together on an FPSO, and what an EPC should lock down before the package is carved up across vendors.

The Short Version

For procurement and process teams who want the conclusion first, an FPSO gas-handling package is a chain of skids, and the things that decide whether it works as one system are:

1. The BOG / associated-gas compression skid — what it has to swallow (rate, suction temperature, composition swing) and how its staging is set.
2. The process refrigeration / refrigerant-loop skid — the cold engine that serves cooling, condensation and reliquefaction duties; its refrigerant inventory and compression have to stay coherent across skid splits.
3. The reliquefaction skid — how it ties back to compression, refrigeration and the cargo/utility systems instead of being a standalone island.
4. Skid-to-skid interfaces and control integration — refrigerant lines, utilities, serial links, ESD and trips that have to line up at hook-up.
5. Ex-zone protection and materials — hazardous-area certification plus low-temperature toughness and sour-service (H2S/CO2) material selection.
6. Series and approval drawings — repeatability across sister units and a class-drawing timeline that doesn't put the vessel on the critical path.

Price and lead time matter, but the value of a gas-handling supplier is in whether the chain behaves as one. The six sections below are where that is decided.

1. The BOG / Associated-Gas Compression Skid

The compression skid is the front door of the gas-handling chain. On an LNG-carrying or FLNG-style unit it deals with boil-off gas (BOG) generated as cargo absorbs heat; on an associated-gas FPSO it takes the gas that comes up with the crude. Either way, the skid's job is to take a low-pressure, often cold, often composition-varying gas stream and raise it to the pressure the next stage needs — reinjection, export, fuel gas, or feed to reliquefaction.

What an experienced buyer reads in a compression skid quotation is whether the fabricator has sized the machine for the real envelope, not a single design point:

• Suction conditions that move. BOG temperature and rate swing with cargo level, sea state and ambient. Associated-gas composition drifts over a field's life. A compressor selected for one tidy point, with no turndown story, becomes a tripping problem in month two.
• Staging and intercooling matched to the discharge target — single- or multi-stage, with intercoolers and knock-out so liquids never reach the compressor.
• Driver and turndown — the staging, anti-surge and capacity-control philosophy that lets the skid follow a varying gas rate without recycling itself into inefficiency or surge.

We supply BOG compressor skids for LNG-fuelled vessels and have delivered them in sets of eight across sister hulls — which means the compression skid is not a one-off selection exercise for us but a repeatable package whose suction-to-discharge envelope is already proven. On a gas-handling FPSO, the compression skid's discharge is the refrigeration and reliquefaction stages' inlet, so getting its envelope right is the first domino in the chain.

2. The Process Refrigeration / Refrigerant-Loop Skid

If compression is the front door, the refrigeration loop is the cold engine that everything else borrows from. On a gas-handling FPSO the refrigerant loop does more than one job: it provides process cooling, it condenses heavy components, and on LNG units it supplies the cold duty that reliquefaction depends on. The reason this skid is so easy to get wrong across a multi-vendor split is that the refrigerant inventory and its compression are a single thermodynamic system pretending to be two boxes.

The questions that decide whether the loop holds together:

• Refrigerant selection and inventory. Single-component or mixed refrigerant, the charge, and where it lives. Split a loop across two skids without owning the total inventory and you get a system that is impossible to commission cleanly.
• Refrigerant compression staging matched to the evaporating and condensing duties — and matched to the BOG/process compression upstream so the two don't fight each other.
• Heat rejection. On a deck, you reject heat to seawater or to an intermediate loop with a finite envelope. The condensing side of the refrigerant loop is constrained by what the FPSO's cooling-water system can actually take, not by an onshore ideal.
• Off-design behaviour. Ambient and seawater temperatures swing; the loop has to hold its cold duty across that range, which is a turndown and control question, not just a sizing one.

Our offshore refrigeration and refrigerant-loop experience is exactly this coupled problem — keeping the refrigerant inventory, its compression and its heat rejection coherent so the cold duty is there when the condensation and reliquefaction stages call for it. When the refrigeration skid and the compression skid come from the same engineering logic, the loop closes; when they don't, the EPC inherits the gap.

3. The Reliquefaction Skid — Tied In, Not Standalone

On LNG-carrying and FLNG units, reliquefaction is where boil-off gas is turned back into liquid and returned to cargo or product. The temptation — and the mistake — is to treat the reliquefaction skid as a self-contained appliance. It isn't. It sits downstream of compression and draws cold from the refrigeration loop; its performance is only as good as the two stages feeding it.

A reliquefaction skid that integrates cleanly shows three things:

• A defined tie-back to compression. The reliquefaction process takes compressed BOG; its inlet conditions are the compression skid's discharge. Lock that interface and the cycle is predictable.
• A defined cold-source interface. Whether reliquefaction uses its own refrigerant cycle or draws from the process refrigeration loop, the cold duty has to be specified as an interface, not assumed.
• A return path to cargo/utility systems. The liquefied product goes back to a cargo or storage system; the non-condensables and any flash gas have a defined route. These are utility and process interfaces, and they belong in the interface register before the skid is built.

We have delivered reliquefaction skids in series — batches of six and twelve against single vessel programs — which is the strongest evidence that the reliquefaction stage has been made to behave as part of a chain rather than as an isolated machine. A reliquefaction skid that has only ever been built as a prototype hides exactly the tie-in problems that surface offshore.

4. Skid-to-Skid Interfaces and Control Integration

Here is where a gas-handling package stops being three good skids and starts being one good system — or doesn't. The seams between compression, refrigeration and reliquefaction carry refrigerant, utilities, power, and — most easily underestimated — control and safety signals. A package whose skids each arrive with their own self-sufficient control philosophy, no two of which agree on how to talk to the FPSO's integrated control and safety system (ICSS), turns hook-up into a re-engineering exercise.

What integration-ready looks like:

• A locked battery-limit and interface register for the whole train: every refrigerant and process tie-in, every utility (cooling water, instrument air, nitrogen, drains, vents), every electrical termination, mapped to the integrator's tag philosophy.
• A coherent control hand-off. Each skid PLC's serial/hardwired link to the ICSS defined the same way, so the topsides controls engineer integrates a consistent set, not three dialects.
• A train-level shutdown logic. ESD and process trips that account for the chain — a compression trip has to do the right thing to the refrigeration and reliquefaction stages downstream, not just stop its own machine. This is a safety-case question, and it has to be designed at the train level.
• Coordinated start-up and capacity control, so the three stages ramp together instead of fighting each other on the deck.

When the same supplier engineers compression, refrigeration and reliquefaction, the interfaces between them are internal — defined once, version-controlled, and handed to the integrator as one coherent register rather than three. That is the single biggest reason an EPC benefits from a gas-handling chain whose skids share an engineering logic.

Interface	What crosses the seam	Lock it before
Compression → refrigeration	Discharge gas conditions, cooling-water sharing	Fab drawings released
Refrigeration → reliquefaction	Cold duty / refrigerant supply, inventory	Refrigerant charge specified
Every skid → ICSS	Serial/hardwired control, ESD, trips	Controls integration starts
Every skid → utilities	Cooling water, air, N₂, drains, vents	Utility balance closed

5. Ex-Zone Protection and Materials (Cryogenic / Sour Service)

A gas-handling train sits in a hazardous-area environment and, unlike a water or oil skid, runs cold and sometimes sour. Two material and certification questions decide whether the package survives the class review with a clean spine or a list of holes.

Hazardous-area (Ex) certification. Motors, instruments, junction boxes and lighting in classified zones must carry the right Ex protection level for the zone — and on a gas-handling skid, with compressors and refrigerant equipment, the classified envelope is larger than on a utility skid. The buyer should see the Ex equipment certified to the project's required level, not "Ex-rated" as a vague assurance.

Low-temperature and sour-service materials. This is the trap specific to gas handling:

• Cryogenic / low-temperature service. BOG and refrigerant streams run cold. Pressure-containing materials need the right low-temperature toughness — impact-tested grades, the right stainless or nickel alloys where the temperature demands — so the steel doesn't become brittle at the minimum design metal temperature.
• Sour service. Associated gas can carry H2S and CO2. Where it does, materials and welds have to meet sour-service requirements, with the hardness and material limits that implies.
• Material traceability behind all of it — mill certs (EN 10204 3.1 / 3.2 as specified), heat-number traceability from plate to weld.

This is exactly why a fabricator's welding-procedure depth matters. We maintain 600+ PQR welding procedure qualifications because a single gas-handling job mixes carbon steel, stainless, duplex and low-temperature alloys, and each material and each service condition needs a qualified procedure behind it. A skid is only as certifiable as its least-documented weld, and on a cryogenic-and-sour gas train the documentation bar is at its highest.

6. Series Repeatability and Approval Drawings

The last dimension is the one that protects the schedule and the program. Modern FPSO and FLNG work is increasingly built in series — standardized hulls, sister units, repeated topsides modules — and a gas-handling chain is a prime candidate for series delivery. The question is no longer "can you build one?" but "can you build the Nth chain exactly like the first, and clear its class drawings without putting the vessel on the critical path?"

• Series repeatability. We have delivered reliquefaction units in batches of six and twelve, and BOG compressor skids in sets of eight, against single vessel programs — with frozen, version-controlled fabrication drawings so unit #8 matches unit #1, a QC process that catches drift between units, and an 8,000 m² assembly workshop to hold cadence without subcontracting quality away.
• Class approval-drawing timeline. On a classed FPSO the gas-handling skids' drawings must clear the class society before fabrication proceeds in earnest, and the vessel's certification cannot close until every module's documentation is in order. A supplier who has worked directly with DNV, ABS, BV, CCS, LR, NK and KR — and can commit a realistic, evidence-backed approval timeline — keeps the gas train off the critical path.
• Commissioning and FAT. A structured FAT against an agreed ITP, a complete Manufacturing Record Book (MRB) that closes the documentation loop, and engineers available for integration and start-up — for the whole chain, not three separate sign-offs.

How the Chain Fits Together

None of these stages is exotic on its own. The hard part is making compression, refrigeration and reliquefaction behave as one system, certified, repeatable, and integrated onto a deck that a multi-billion-dollar floater depends on. A package split across vendors with no one owning the seams flattens the chain into three prices and hides exactly the integration risk that costs the most.

Stage / dimension	The hidden risk if treated in isolation	The question to put in your RFQ
BOG / associated-gas compression	Tripping on the real suction envelope	"Show me the suction-to-discharge envelope and turndown story."
Process refrigeration loop	Refrigerant inventory split, uncommissionable	"Who owns the total refrigerant inventory and heat rejection?"
Reliquefaction	Standalone island, tie-ins surface offshore	"How does reliquefaction tie back to compression and the cold source?"
Skid-to-skid + control	Re-engineering at hook-up	"Give me a train-level interface register and ESD logic."
Ex + materials	Failed class review (cryo/sour)	"Provide Ex certs plus low-temp and sour-service material plan."
Series + drawings	Drift + vessel certification delay	"Show me a delivered series and a class-drawing timeline."

A supplier who engineers the chain as one — and can show a delivered series behind it — is not the cheapest line on your bid tab. They are the one whose seams you can stop worrying about, which on a gas-handling FPSO is most of the risk.

FAQ

What is "gas handling" on an FPSO, and why is it growing?
Gas handling is the train of equipment that captures, compresses, cools, treats and either reinjects, exports or liquefies the gas that comes with the oil — including boil-off gas on LNG-carrying units. It is growing because operators are increasingly expected to monetize or reinject associated gas rather than flare it, so floaters that once carried almost no gas equipment now carry a full compression-refrigeration-reliquefaction chain on the topsides.

Why shouldn't I just buy the BOG, refrigeration and reliquefaction skids from three different vendors?
You can, but someone has to own the seams between them — the shared refrigerant inventory, the cooling-water balance, the train-level shutdown logic, and the control hand-off to the ICSS. When three vendors each optimize their own box, those seams become the EPC's integration problem, and they surface at hook-up where they're most expensive. A chain engineered under one logic hands the integrator one coherent interface register instead of three dialects.

How does the refrigeration loop relate to reliquefaction?
Reliquefaction needs cold duty to turn boil-off gas back into liquid, and that cold comes from a refrigerant loop — either a dedicated cycle inside the reliquefaction skid or the process refrigeration loop serving the whole train. Either way the cold duty has to be specified as an interface, with the refrigerant inventory owned as a single quantity, or the loop becomes impossible to commission cleanly.

What materials issues are specific to FPSO gas-handling skids?
Two: low-temperature toughness, because BOG and refrigerant streams run cold and the steel must stay tough at the minimum design metal temperature; and sour service, because associated gas can carry H2S and CO2, which constrains material and weld hardness. Both sit on top of the usual hazardous-area (Ex) certification and full material traceability. It's why welding-procedure depth — qualified procedures for each material and service condition — matters more on a gas train than on a utility skid.

Further Reading

• BOG Compressor Skids for LNG-Fuelled Vessels
• Reliquefaction Skids: A Decade of Series Delivery
• Fuel Gas Supply System (FGSS) Skids
• Modular Skid Packages: Workshop to Sea Trial
• Lmart Marine & Offshore Project References

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Lmart (Suzhou Lmart Energy Equipment Co., Ltd.) designs, fabricates, integrates, tests and documents modular gas-handling skids — BOG compression, process refrigeration, refrigerant-loop and reliquefaction packages — for marine and offshore programs in-house, certified across DNV, ABS, BV, CCS, LR, NK and KR. If you're scoping a gas-handling chain for an FPSO or FLNG program, we're happy to walk through how compression, refrigeration and reliquefaction tie together for your case.