LNG tank pressure rise after loading cargoes of different density
The 145,000 m³ membrane LNG carrier Pacific Aurora has loaded No. 2 tank from two shore sources. Shore tank A supplied LNG at 445 kg/m³ and shore tank B at 453 kg/m³. Loading was completed without incident, and the ship sailed with No. 2 tank pressure at 108 mbar. Eight hours later the pressure has risen to 142 mbar, the boil-off rate has increased from 0.15% to 0.30% per day, and the tank gauge shows a stable level but unusual temperature spread between upper and lower sensors. The relief valve setting is 250 mbar and the gas combustion unit is available. The duty engineer proposes opening the vapour header to another tank to equalise pressure. The master is concerned about losing cargo and asks for an assessment before any venting.
1. Diagnose the likely cause of the pressure rise and explain why it is not simply a normal boil-off increase.
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The pattern points to rollover caused by density stratification. Two LNG parcels of different density were loaded into the same tank, and the heavier parcel can remain below the lighter parcel if mixing is insufficient. The interface is initially stable, but heat ingress and ship motion can cause the lower layer to become unstable and mix rapidly with the upper layer. That mixing releases a large amount of vapour and produces a rapid pressure rise, not the steady increase expected from normal boil-off.
The increased boil-off rate, pressure rise over a short period and unusual vertical temperature spread support this diagnosis. Normal boil-off would not usually double within eight hours unless ambient conditions or cargo operations changed markedly. The stable level does not exclude rollover. The duty engineer's proposal to equalise with another tank may reduce pressure temporarily but could spread the problem and does not address the stratification. Venting should be avoided unless pressure approaches the relief setting, because methane release has safety and environmental consequences.
2. State the immediate actions you would take as master or duty officer.
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First, stop any cargo operation that could add heat or disturb the tank, and increase monitoring of pressure, temperature profile and boil-off rate. Inform the master, chief officer and engine room, and keep the gas combustion unit available. If the ship has cargo recirculation or spray arrangements suitable for breaking stratification, use them according to the manufacturer's procedure, carefully and without causing a sudden pressure surge. Do not open the tank to atmosphere or vent as a first response.
If pressure continues toward the relief valve setting, reduce pressure using the approved means, such as increasing boil-off consumption through the GCU, and prepare for controlled venting only as a last resort under the ship's procedures. Notify the company and charterer, and record all actions and readings. The officer should avoid transferring vapour to another tank without assessing the effect on that tank and the overall cargo system. The priority is controlled mixing and pressure management, with no ignition sources on deck and no unnecessary personnel exposure.
3. What loading controls would have prevented this situation?
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The loading plan should have assessed the density difference between the two parcels and the loading sequence. Where parcels of different density must go into one tank, the plan should specify adequate mixing, such as controlled recirculation or top filling with a suitable rate, and should avoid loading a heavy parcel beneath a light parcel without mixing. The ship and terminal should agree density, temperature and loading rate before starting, and the cargo control room should monitor the tank temperature profile during and after loading.
A post-loading settling period should not be used as a substitute for mixing when densities differ significantly. The plan should include a limit for density difference, a requirement to monitor pressure and boil-off after loading, and a contingency if stratification is suspected. The chief officer should record the densities and loading sequence, and the crew should be briefed on rollover symptoms. Good planning would have identified the 8 kg/m³ difference as a risk and required either separate tanks or a verified mixing procedure before the ship sailed.