Case Study
A 170,100 m³ lean LNG cargo bound for an FSRU at the end of a long river approach. The sea passage looked fine — steady, inside warranty. It was already tracking 0.87 kPa above the terminal’s limit by the pilot station, and the river approach then added another 1.31 kPa. Three-fifths of the problem was made in 36 of the voyage’s 473 hours.
FSRU — a moored ship that takes in LNG and sends out gas, used in place of a shore terminal.
GCU — gas combustion unit, the shipboard burner that disposes of surplus boil-off.
Lean LNG — LNG that is almost pure methane, with little of the heavier hydrocarbons; lighter and more volatile than a rich cargo.
Pilot station — the point where the pilot boards and the river passage begins.
Manifold — the pipe connection where ship meets shore.
Days 1–11 are observed. Everything after day 11 is forecast — both the baseline and the plan. Representative case: the method and the physics come from monitored voyages; vessel identity, parties, ports and dates are withheld or composite.
What the state data saidLean LNG cargo loaded at −161.3 °C and 8.0 kPa, running a steady +0.19 kPa/day across the ocean passage. That alone tracks to about 10.9 kPa at the pilot station — 0.87 kPa over the 10.0 kPa limit before the river is even reached. On its own it reads like a small problem. It is not.
The last-mile spikeA long river transit at reduced speed, pilot boarding and a wait at anchor: gas use drops from about 120 to 30 m³/day while the tanks keep boiling. The rise goes from 0.19 to 0.87 kPa/day — 4.7 times, matching the surplus gas ratio (144.6 − 30) ÷ (144.6 − 120.3) = 4.7. That adds 1.31 kPa of the 2.18 kPa overshoot.
The governing balanceHeat into the cargo, minus the latent heat actually carried away as boil-off. Venting removes mass, not stored enthalpy, so after a one-off flash burst the curve resumes the same slope.
Why lean cargo is harder to controlLean LNG is almost pure methane, with very little of the heavier ethane and propane that steady a richer cargo. It is lighter and more volatile, so the same heat leak turns more of it into vapour and tank pressure reacts faster to every change in gas use. Any nitrogen boils off first, so the boil-off gas and the liquid left behind keep changing as the voyage goes on. Forecasting that drift, instead of assuming a fixed boil-off rate, is what lets the plan hold pressure to fractions of a kPa.
Method: tank state is rebuilt from the ship’s own noon reports and the lean-cargo profile — no extra sensors and nothing fitted on board.
01Cargo stateLiquid energy, boiling pressure and vapour superheat rebuilt from the ship’s own reports — no extra sensors, nothing fitted on board.
02The approach is modelledRiver speed, pilot boarding and the wait at anchor sit inside the pressure forecast instead of being bolted on at the end as an assumption.
03Speed re-profile, same ETA — 0.62 kPa13.5 kn split into a 15.1 / 11.9 kn profile: 4.2 % more gas through the engines, 81 m³ of surplus burned as propulsion instead of flared.
04Spray cooling — 0.65 kPaRecovers the vapour superheat and stratification share of the overshoot in the last days at sea. Pump power only; nothing leaves the tank.
05Vapour-header balancing — 0.20 kPaPressure evened out across the tanks before the pilot boards, so no single tank sets the manifold reading.
06Residual released at sea — 1.11 kPa146 m³ through the GCU before the pilot station, clear of laytime, against 503 m³ at anchor. Pilot-station target 8.3 kPa, manifold 9.6 kPa.
Every lever carries a trigger, a UTC stamp and a target. Full derivation, property basis and sensitivity analysis are published in the accompanying calculation note.
277 m³ of cargo kept ≈ 6,280 MMBtu ≈ US$ 82k at US$ 13/MMBtu, a deliberately low price. Plus 40 hours at US$ 75k/day ≈ US$ 126k. 82 + 126 = US$ 208k, published at US$200K after rounding down. The risk to the berth slot in a peak-demand window is not counted. Lines left of the marker are observed. Both lines right of it are forecast; the 9.6 kPa outcome is a modelled result, not a completed voyage.
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