Boil-Off Gas Management on LNG Carriers: Recondensation Versus Compression
Heat gets into an LNG cargo tank through the insulation. You can't stop it. What you get for it is vapour — boil-off gas (BOG) — that builds pressure inside the cargo containment system and has to be dealt with continuously, every voyage, not just when it becomes a problem. Let the pressure climb and you vent, you lose cargo, and on modern tonnage you trip a regulatory non-compliance. On a laden passage the BOG flowrate follows the Boil-Off Rate (BOR):
BOR (kg/h) = [Q_total × (1 − x)] / H_vap
where Q_total is the total heat ingress to the cargo system (W), x is the liquid mass fraction, and H_vap is the latent heat of vaporisation of the LNG mixture (J/kg). Every kilogram per hour of unmanaged BOG is lost product, fuel you didn't burn, or compressor load you paid shaft power for. Pick the management route — recondensation against the send-out stream, direct compression to fuel, or onboard reliquefaction — and you have picked your operating cost, your voyage flexibility, and how well the ship fits the terminal.
The Three Management Routes
Route 1: BOG Compression to Propulsion Fuel or GCU
Oldest route, simplest route. Compress the vapour off the cargo tanks and send it to the main propulsion plant — steam turbines burning boil-off directly on the older ships, high-pressure or low-pressure gas injection engines on modern dual-fuel vessels. A Gas Combustion Unit (GCU) sits behind it as the safety backstop for when propulsion demand can't eat all the BOG being generated.
Compression alone does not recover the BOG as liquid cargo. Slow steam on a laden passage and propulsion demand can fall short of the BOG generation rate; the surplus goes to the GCU and that cargo is gone. LNG Industry notes that reciprocating compressors offer better turndown for variable BOG rates, while centrifugal machines suit high-throughput, steady-state operation but carry surge sensitivity that requires careful antisurge control.
Route 2: Recondensation Against the Send-Out Stream
At the receiving terminal, high-pressure LNG send-out pumps push liquid to the vaporisers. Recondensation puts a mixing drum between the cargo pumps and the high-pressure pumps: compress the BOG to an intermediate pressure, inject it into subcooled LNG coming off the cargo tanks, and the vapour condenses back to liquid. From there the combined stream goes on to the high-pressure send-out pumps.
Energy-efficient — as long as you have a large send-out flow to act as the cold sink. Recondenser drum sizing, and the ratio of BOG mass flow to LNG liquid flow, decides whether you get complete condensation. Drop the send-out rate — terminal start-up, partial unloading, slow offtake — and the cold sink shrinks. Recondensation goes incomplete. Now you need a bypass to the GCU, or you hold back cargo pump flow.
Route 3: Onboard Reliquefaction
Reliquefaction systems compress BOG to high pressure and cool it back to saturation temperature using a dedicated refrigeration cycle, returning liquid to the cargo tanks. Several cycle configurations have been studied.
A further configuration — direct reliquefaction using subcooled LNG as the cold source without a dedicated refrigeration loop — was examined in Energy (2020, vol. 199).
Comparative Assessment
| Criterion | BOG Compression + GCU | Recondensation (terminal) | Onboard Reliquefaction |
|---|---|---|---|
| Cargo preservation | None — BOG consumed or flared | Full, during unloading only | Full, throughout voyage |
| Voyage-length suitability | Short to medium laden passages | Unloading window only | Long laden passages |
| Terminal dependency | Independent | Requires high send-out rate | Independent |
| Power demand | Low (compression only) | Low to moderate | Highest (full refrigeration cycle) |
| Capital complexity | Low | Moderate (recondenser drum, controls) | High (refrigeration plant, cryogenic compressors) |
| Regulatory / emissions | GCU emissions on slow steam | No venting; clean route | No venting; clean route |
| Flexibility at part load | GCU absorbs surplus | Constrained by send-out rate | Broad operating envelope |
Numbers withheld from table cells where no attributed source value is available; qualitative descriptors used throughout.
Decision Framework by Voyage Length and Terminal Type
Short Laden Passages with Conventional Terminals
Short laden passage, terminal running a high and steady send-out rate — recondensation is the route you want. BOG made in transit goes to propulsion fuel; once the ship is alongside, the recondenser takes the residual vapour off the unloading. The terminal's send-out stream is your cold sink, and you carry no extra refrigeration machinery for it. Keep the GCU lined up for the stretches where send-out rate drops away.
Long Laden Passages or Spot-Charter Voyages
Long laden passages — transoceanic runs with a lot of sea time before you reach a terminal — generate BOG faster than propulsion can burn it, and slow steaming makes that worse. This is where onboard reliquefaction earns back its capital and power cost: it preserves cargo that the GCU would otherwise destroy.
Floating Storage and Regasification Units (FSRUs)
FSRUs are a hybrid problem. While sending out, recondensation against the high-pressure stream is the dominant route, exactly as at a land-based terminal. Idle or at low send-out, the recondenser has no cold sink, and BOG has to go to the GCU or to a supplementary reliquefaction capability. If you are specifying FSRU equipment, make sure the recondenser control system can ride rapid send-out rate transitions without pressure excursions in the cargo tanks.
Illustrative Scenario
The following is an illustrative example constructed to demonstrate the decision logic; it does not represent a named vessel or project.
A Q-Flex-class LNG carrier is nominated on a laden passage of approximately 21 days from a Middle East export terminal to a Northwest European import terminal operating at reduced send-out rate due to seasonal demand. The vessel is fitted with dual-fuel main engines and a GCU but no onboard reliquefaction plant.
Week one, propulsion demand takes the majority of the BOG. Then the ship closes on the delivery port and comes off speed for scheduling, and propulsion demand falls below the BOG generation rate. The GCU activates to consume the surplus vapour. On arrival the terminal's send-out rate is insufficient to provide a full recondensation cold sink, so a proportion of BOG during unloading is routed to the terminal's own fuel gas system rather than recondensed.
The outcome: measurable cargo loss versus a vessel equipped with partial reliquefaction capacity. The decision framework conclusion for this route and terminal combination is that an onboard reliquefaction plant — even one sized for partial BOG recovery rather than full reliquefaction — would reduce GCU consumption and preserve cargo on the slow-steam leg.
Pre-Voyage and Equipment Selection Checklist
- Get the receiving terminal's nominal and minimum send-out rate, put it against expected BOG generation rate during unloading, and size the recondenser drum and liquid-to-BOG ratio from that.
- Establish the laden passage duration under the contracted speed profile; if GCU operating hours under slow-steam conditions exceed an acceptable threshold, evaluate onboard reliquefaction.
- Check compressor turndown against the minimum expected BOG rate (ballast passage, cooled-down tanks) so you don't sit in surge or recycle excessively.
- For reliquefaction plant selection, compare normal-temperature compressor mixed-refrigerant cycles against subcooled-LNG direct condensation routes based on expected cargo temperature at mid-voyage.
- Confirm that the GCU capacity covers the maximum credible BOG rate (high ambient temperature, full cargo, maximum heat ingress) as the safety backstop regardless of primary management route chosen.
- Review the recondenser control philosophy for rapid send-out rate changes: the control system must prevent BOG breakthrough into the high-pressure send-out line.
- For any maintenance intervention on BOG compressors or reliquefaction plant: apply full isolation of suction and discharge, depressurise to verified zero pressure, confirm zero energy state, apply LOTO, verify absence of flammable atmosphere with calibrated gas detection, and ensure safe venting to an approved vent riser or flare header before breaking containment.
- Align BOG management strategy with the applicable IMO requirements for gas carriers and the vessel's approved Ship Energy Efficiency Management Plan (SEEMP).
Conclusion
No single BOG management route is optimal across all operating conditions. Recondensation is the correct solution at terminals with sustained high send-out rates and for short-haul trades where unloading windows dominate the BOG budget. Compression to propulsion fuel plus GCU backstop is adequate for medium-duration passages on vessels where the propulsion demand profile closely tracks BOG generation. Onboard reliquefaction becomes economically and operationally justified on long laden passages, slow-steaming contracts, and where cargo preservation is commercially critical.
The next step for any project team is to model the expected BOG generation curve across the full voyage — accounting for ambient temperature, loading port temperature, and speed profile — and overlay it against the propulsion fuel demand curve and the terminal's send-out capacity. That model, rather than a generic equipment specification, should drive the BOG management system design and the compressor and reliquefaction plant sizing.
