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LGTCO/LGTF exit exam questions and answers — Set 17 — Liquefied Gas Tanker Cargo Operations

Set 17 of the Liquefied Gas Tanker Cargo Operations (LGTCO/LGTF) exit exam bank — 30 of 599 questions, with the correct option marked and a one-line explanation for every one. Read it end to end, jump to a single set, or search it — then sit the same bank as a timed paper. The real DG Shipping exit exam is 30 marks with 15 to pass.

These are the bank's original groupings. Practising deals the whole bank out again every time you reshuffle, so a practice “Set 3” is a different 30 questions each time — the set numbers below are the source file's, and are only used here so the key stays navigable. Options are listed in their original order with fixed letters; when you practise, those move too.

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Set 17 — 30 LGTCO/LGTF exit exam questions with answers

1. A cargo tank could physically hold 100% of its volume, yet the Gas Codes cap the filling limit near 98%. What is that ullage reserved for?

  • A. Space for an inert-gas blanket above the cargo
  • B. Tolerance for the level-gauge float to travel
  • C. A safety margin in case the gauge reads low
  • D. Room for the liquid to expand as the cargo warms during the voyage
Answer: D — Room for the liquid to expand as the cargo warms during the voyage
The limit must never exceed 98% at the highest temperature reached on the voyage, because the liquid expands as it warms and, with nowhere to go, the tank becomes liquid-full and pressure runs away against an incompressible liquid. There is no inert-gas blanket above a loaded cargo (option 0) — the space is cargo vapour.

2. During loading the incoming liquid pushes vapour out of each tank. Why is the high-duty compressor usually not run from the very start of loading?

  • A. The compressor is used only for cool-down and never during loading
  • B. While tank pressure is low the vapour free-flows to shore; the HD compressor is brought in only as pressure rises, to hold the vapour header down
  • C. The compressor delivers liquid, not vapour, to the shore tanks
  • D. Running it early would draw air into the cargo tank
Answer: B — While tank pressure is low the vapour free-flows to shore; the HD compressor is brought in only as pressure rises, to hold the vapour header down
Early in loading the displaced vapour free-flows to shore through the vapour crossover, and the compressor is only needed later, once rising tank pressure calls for extra gas flow to hold the header — and hence the tanks — down. It handles vapour, not liquid (option 2), and it does not draw air into a positive-pressure tank.

3. Loading is finished and the loading arm is still full of liquid. What is the correct order of operations before the ship/shore connection is broken?

  • A. Drain the lines, then depressurise, then purge with nitrogen, and only then disconnect
  • B. Disconnect quickly, then drain the arm down on deck
  • C. Purge with air, disconnect, and drain afterwards
  • D. Depressurise and disconnect, then drain the lines
Answer: A — Drain the lines, then depressurise, then purge with nitrogen, and only then disconnect
The order is the safety: drain the liquid inboard, depressurise every deck line and purge with N₂ before the flange is ever opened, because nitrogen will not condense or form a flammable mixture with the vapour. Disconnecting first (options 1–3) leaves cryogenic liquid waiting for someone to open a flange.

4. A large refrigerated gas carrier is discharging her cargo. Which is the normal, efficient method used for the bulk of the discharge?

  • A. Pressurising the vapour space with shore vapour to push the liquid out
  • B. Discharge by the ship's main cargo pumps
  • C. Displacing the liquid with boil-off gas only
  • D. Allowing the cargo to gravity-drain ashore
Answer: B — Discharge by the ship's main cargo pumps
Pumped discharge is the fast, normal method; the submerged main pumps do the bulk work while the ship ballasts concurrently. Pressure-displacement (option 0) is slow, inefficient and limited to small Type C ships, used only where pumps are unavailable.

5. The terminal asks you to slow the discharge. How is the discharge rate of a submerged cargo pump correctly regulated?

  • A. By throttling the pump suction valve
  • B. By the vapour-return line alone
  • C. By raising the cargo temperature with the heater
  • D. By adjusting the discharge valve and the pump speed — never the suction, which would cavitate the pump
Answer: D — By adjusting the discharge valve and the pump speed — never the suction, which would cavitate the pump
The rate is controlled at the discharge valve and by pump speed, keeping the machine inside its safe envelope. Throttling the suction (option 0) starves the pump: the liquid, already at its boiling point, flashes at the impeller and cavitation destroys it.

6. The submerged main cargo pumps move roughly 1,650 m³/h each; the small stripping/spray pump about 35 m³/h. Why is running any of them dry, even briefly, so destructive?

  • A. The motor windings are the first thing to overheat and burn out
  • B. A centrifugal pump loses suction and cannot re-prime afterwards
  • C. The pumped cargo lubricates and cools the bearings — run dry, they seize within seconds
  • D. Vapour in the impeller over-pressurises the discharge line
Answer: C — The pumped cargo lubricates and cools the bearings — run dry, they seize within seconds
Nothing else lubricates these pumps: the very liquid being pumped cools and lubricates the bearings, so dry running seizes them almost at once and the damage runs up the shaft. Winding overheat (option 0) is a real secondary effect, but the bearings fail first.

7. Mid-voyage the reliquefaction plant's compressor discharge pressure and temperature are creeping up and it is condensing less cargo vapour. What is the most likely cause and cure?

  • A. Incondensable nitrogen has collected at the top of the condenser and must be vented off
  • B. The seawater is too cold and should be warmed before it enters the condenser
  • C. The expansion valve should be shut to raise the condensing pressure
  • D. The compressor should be run against a closed discharge valve
Answer: A — Incondensable nitrogen has collected at the top of the condenser and must be vented off
Nitrogen left over from inerting cannot condense; it gathers at the top of the condenser, raises the condensing pressure and temperature and progressively kills the plant, so it is vented from the condenser top as required. Running a compressor against a shut discharge valve (option 3) is never done — it damages the machine.

8. A section of liquid line is isolated with cold cargo shut inside it, a valve closed at each end. The sun comes out and the line warms. What happens, and what protects against it?

  • A. Nothing — the liquid is incompressible so the pressure stays constant
  • B. The trapped liquid warms, boils and can burst the line — every isolatable liquid section needs a relief valve
  • C. The liquid contracts and pulls a vacuum that collapses the pipe
  • D. The cold cargo keeps the metal below ambient so pressure cannot rise
Answer: B — The trapped liquid warms, boils and can burst the line — every isolatable liquid section needs a relief valve
Trapped liquefied gas has nowhere to expand, so it takes up heat, boils and the pressure runs away — a hydraulic lock that will burst the line, which is why every section that can be isolated with liquid in it carries a thermal relief valve back to a tank or the vapour line. Calling the liquid incompressible (option 0) misses that it boils to vapour as it warms.

9. The tank relief/venting arrangement must be able to cope with the worst case during loading. Its required capacity is based on the maximum liquid volume entering the tank PLUS what?

  • A. The volume of inert gas remaining in the tank
  • B. The swept volume of the cargo pumps
  • C. The volume of ballast being displaced at the same time
  • D. The vapour generated in the tank during loading — the flash and boil-off
Answer: D — The vapour generated in the tank during loading — the flash and boil-off
The relief system must pass both the incoming liquid's displaced volume and the extra vapour that liquid flashes off as it enters, or the tank over-pressures. Sizing on the inert-gas volume (option 0) is wrong — the tank has been gassed up and holds cargo vapour, not inert gas, by loading time.

10. ESD-1 has tripped: pumps are stopped and the ESD valves are shut, but the loading arm is still bolted to your manifold, full of LNG. What does ESD-2 add?

  • A. It closes the individual cargo tank filling valves as well
  • B. It drains and purges the arm with nitrogen before release
  • C. It fires the Emergency Release System so the PERC parts the arm from the manifold
  • D. It sounds the general alarm and musters the fire party
Answer: C — It fires the Emergency Release System so the PERC parts the arm from the manifold
ESD-1 makes the transfer safe and static; ESD-2 separates ship from shore by firing the ERS, and the powered emergency release coupling parts the arm — always after the flow has stopped and the valves are shut. Closing the tank filling valves (option 0) is done by the overfill sensor, not by ESD-2.

11. The earliest LNG terminals coupled ship and shore ESD systems with pneumatic links; nearly all now use electric or fibre-optic links. What drove that change?

  • A. Pneumatic links are slow, and dirt and moisture in the air line make them unreliable
  • B. Fibre-optic cable is far cheaper to install than pneumatic hose
  • C. The IGC Code now prohibits pneumatic links at LNG terminals
  • D. Pneumatic lines always freeze solid in cold climates and cannot be used
Answer: A — Pneumatic links are slow, and dirt and moisture in the air line make them unreliable
A pneumatic signal must propagate as a pressure change down a hose, so it is inherently slow, and dirt and moisture foul the line — speed and reliability drove the move to electric and fibre links. Cost (option 1) is wrong: fibre systems with their reels, connectors and enclosures are not cheap, and pneumatic links are not banned.

12. An ESD valve that closes faster stops the cargo sooner, yet SIGTTO and terminals insist on a minimum closing time for the ship's ESD valve. Why?

  • A. The valve actuator cannot deliver full torque that quickly
  • B. A fast closure turns the moving liquid's momentum into a surge pressure wave that travels up the shore line
  • C. The ship/shore link needs the extra time to signal the terminal
  • D. The IGC Code fixes every ESD valve to close in exactly 30 seconds
Answer: B — A fast closure turns the moving liquid's momentum into a surge pressure wave that travels up the shore line
Stopping a large moving column of liquid quickly converts its momentum into pressure; the faster the valve shuts, the higher the peak surge, and that wave runs back up the jetty line to burst pipework, lift reliefs or damage the arms — so pumps trip first and the closing time is agreed in advance. The Code sets a maximum of about 30 s (option 3), not an exact fixed value.

13. A bunker barge comes alongside and wants to deliver fuel while your ship is loading cargo. What governs whether these simultaneous operations may go ahead?

  • A. They are always permitted, as bunkering is a routine operation
  • B. The bunker barge master decides on his own authority
  • C. They are allowed only under a specific written risk assessment agreed by ship and terminal, with separate teams and watches
  • D. They are prohibited outright by the IGC Code in every case
Answer: C — They are allowed only under a specific written risk assessment agreed by ship and terminal, with separate teams and watches
There is no blanket ban on simultaneous operations, but they multiply the sources of spill and ignition and split the deck watch's attention, so they proceed only where company, terminal and Master authorise them under a specific written risk assessment with separate watches. An outright IGC prohibition (option 3) does not exist.

14. Control air to the ESD valve actuators is lost during a transfer. On a properly designed fail-safe system, what happens to the ESD valve, and why is it built that way?

  • A. It locks in its last position so the cargo transfer is not interrupted
  • B. It can only be closed by hand from the deck if the actuator fails
  • C. A duplicate valve takes over and the first one is ignored
  • D. It closes automatically — a system that cannot prove it is healthy must not keep transferring cargo
Answer: D — It closes automatically — a system that cannot prove it is healthy must not keep transferring cargo
Loss of control air, hydraulic pressure or electrical power is itself an ESD initiator, and the valve closes rather than freezing in place, so a transfer is never left running with no means of stopping it. A valve that locked in position (option 0) would leave a full-rate flow uncontrollable.

15. Three of the following events will each, on their own, initiate an emergency shutdown of the ship's cargo transfer. Which one will NOT?

  • A. A cargo tank reaching 95% full during loading
  • B. Loss of control air pressure to the ESD valve actuators
  • C. A fusible element melting at the manifold in a fire
  • D. Total loss of electrical power to the cargo control system
Answer: A — A cargo tank reaching 95% full during loading
95% is the level at which you reduce the loading rate — an operational cue, not a trip; the ladder then runs 97% alarm, 98% normal maximum, 98.5% the filling valve shuts, and only 99% initiates an ESD. Loss of air, a fusible element and blackout (options 1–3) are all genuine fail-safe ESD initiators.

16. After discharge a tank is still full of cargo vapour and must be made safe for entry or dry-dock. Which sequence takes it correctly back towards breathable air?

  • A. Aerate, then inert, then warm up, then liquid-free
  • B. Liquid-free, then warm up, then inert, then aerate
  • C. Warm up, then aerate, then inert, then liquid-free
  • D. Inert, then aerate, then warm up, then liquid-free
Answer: B — Liquid-free, then warm up, then inert, then aerate
The return trip is the loading path in reverse: strip out any liquid, warm the steel to ambient, inert the tank to purge the hydrocarbon down to about 1–2%, then aerate. Aerating first (option 0) would walk the atmosphere straight through its flammable range on the way from cargo vapour to air.

17. A tank is empty of liquid but still full of cargo vapour. A junior officer suggests starting the air blowers straight away because it is quicker. What actually happens inside that tank?

  • A. Nothing — the vapour is far too rich to burn, so it is perfectly safe
  • B. The vapour condenses on the cold steel and drains to the sump
  • C. The atmosphere passes down through its flammable range as air dilutes the vapour
  • D. The oxygen analyser alarms and stops the blowers automatically
Answer: C — The atmosphere passes down through its flammable range as air dilutes the vapour
A rich hydrocarbon atmosphere cannot reach fresh air without crossing its flammable range as it dilutes, so the tank is inerted first and only then aerated — inert gas, not air, makes that journey. Assuming it stays safely rich (option 0) ignores that the mixture must pass through the burnable band on the way down.

18. A tank has been aerated for entry and four sets of gas readings come back. Which ONE set permits entry with no further action?

  • A. O₂ 20.9%, hydrocarbon 3% LFL, toxics nil
  • B. O₂ 19%, hydrocarbon 0% LFL — so enter wearing a BA set
  • C. O₂ 23.5%, hydrocarbon 0% LFL, toxics nil
  • D. O₂ 20.9%, hydrocarbon 0% LFL, and toxic gases within their limits
Answer: D — O₂ 20.9%, hydrocarbon 0% LFL, and toxic gases within their limits
The entry criteria are absolute and must all pass together: oxygen at the normal 20.9%, hydrocarbon at 0% LFL, and any toxic gases below their limits. A reading of 19% oxygen (option 1) fails — a BA set is rescue equipment, never a licence to enter a tank that failed its gas test — and 23.5% (option 2) is oxygen-enriched.

19. A ship must change from an ammonia cargo to an LPG grade. Why must the inert gas plant NOT be used to purge the tanks after ammonia?

  • A. Its CO₂ reacts with ammonia to form solid carbamates that block lines and valves — ventilate with warm dry air instead
  • B. Ammonia is too flammable in a tank to be inerted safely
  • C. Inert gas dissolves into the residual ammonia and contaminates it
  • D. The nitrogen in the inert gas would condense at ammonia temperature
Answer: A — Its CO₂ reacts with ammonia to form solid carbamates that block lines and valves — ventilate with warm dry air instead
NH₃ reacts with the CO₂ in ship-generated inert gas to form solid ammonium carbamates that block lines, valves and instruments, so ammonia tanks are ventilated with warm dry air and may skip inerting altogether. Ammonia is not readily ignitable in a tank atmosphere (option 1), so flammability is not the reason.

20. Where are the responsibilities of the officers and crew in an emergency formally defined?

  • A. In the charter party agreed with the cargo owner
  • B. In the Safety Management System under the ISM Code, with individual emergency duties set out on the Muster List
  • C. In the cargo manifest carried for the voyage
  • D. In the classification society's construction rules
Answer: B — In the Safety Management System under the ISM Code, with individual emergency duties set out on the Muster List
The ISM Code requires a Safety Management System, and each person's emergency station and duties appear on the Muster List — know your station before the alarm sounds. The charter party and manifest (options 0 and 2) are commercial documents and carry no emergency duties.

21. While burning cargo as fuel the boil-off is drawn off faster than the tank generates it. Cargo tank pressure must never be allowed to fall below atmospheric. Why is that the danger?

  • A. The tank could over-pressurise and lift a relief valve
  • B. The cargo would warm and stratify into layers
  • C. Air is drawn in through any imperfect seal, forming a flammable mixture inside the cargo tank
  • D. The reliquefaction plant would overfill with condensate
Answer: C — Air is drawn in through any imperfect seal, forming a flammable mixture inside the cargo tank
Cargo tank pressure is always kept above atmospheric because a sub-atmospheric tank sucks air in through any imperfect seal, and air plus cargo vapour is a flammable mixture inside your own tank. Over-pressure and a lifted relief (option 0) is the opposite failure — the concern here is too little pressure, not too much.

22. Two-thirds of the way through discharge the cargo pumps begin to run erratically and the rate falls away, though the pumps themselves are healthy. What is happening, and what fixes it?

  • A. The tanks are nearly empty; nothing can be done but stop the pumps
  • B. The shore line is blocked; reduce the number of pumps running
  • C. The cargo has warmed; run the reliquefaction plant to cool it
  • D. The pumps have lost their NPSH margin — raise the vapour-space pressure with shore vapour or the ship's spray pump and vaporiser
Answer: D — The pumps have lost their NPSH margin — raise the vapour-space pressure with shore vapour or the ship's spray pump and vaporiser
As the level falls, the head above the suction falls too and the liquid flashes at the impeller — cavitation — so the cure is to raise the vapour-space pressure and restore the net positive suction head. Running more or fewer pumps for a blockage (option 1) does not address the loss of suction head; beyond a point extra pumps even reduce the rate.

23. Two LNG parcels of slightly different density were loaded into the same tank. Ten days later, with no operation in progress, the tank pressure suddenly surges and the relief valves lift. What happened?

  • A. The cargo stratified into layers that overturned — a rollover — flashing vapour faster than the reliefs can pass
  • B. A cargo pump was left running and warmed the cargo through
  • C. Insulation damage let heat in and boiled the cargo faster
  • D. The reliquefaction plant tripped and boil-off simply accumulated
Answer: A — The cargo stratified into layers that overturned — a rollover — flashing vapour faster than the reliefs can pass
Different densities settle into stable layers; the trapped lower layer cannot boil off, superheats until the densities equalise, then the layers overturn and it flashes in a burst that overwhelms the reliefs. A tripped plant (option 3) would raise pressure slowly and predictably, not as a sudden spike.

24. A large spill of cold propane produces a vapour cloud that hugs the deck and drifts downwind. What are the main hazards of such a cloud?

  • A. Only that it is cold enough to cause frostbite on contact
  • B. Only that it looks alarming but is otherwise harmless
  • C. It can drift to an ignition source, it may be toxic, and it can displace oxygen — all three
  • D. It rises immediately and disperses harmlessly into the atmosphere
Answer: C — It can drift to an ignition source, it may be toxic, and it can displace oxygen — all three
A cargo vapour cloud carries three hazards at once — it can reach an ignition source and flash back, it may be toxic, and it displaces oxygen — and the flammable cloud extends beyond the visible cold-condensation fog. Propane vapour is heavier than air (option 3), so it stays low and travels with the wind rather than rising away.

25. A small carrier can load ammonia either fully refrigerated or fully pressurised at ambient. Which figures explain why ammonia can be carried fully pressurised?

  • A. It boils at −104 °C and cannot be liquefied by pressure at all
  • B. It boils at −162 °C and its critical temperature is −33 °C
  • C. It boils at −42 °C and its critical temperature is +36 °C
  • D. It boils at −33 °C at atmospheric pressure, and its critical temperature is +132 °C — well above ambient
Answer: D — It boils at −33 °C at atmospheric pressure, and its critical temperature is +132 °C — well above ambient
Ammonia boils at −33 °C at atmospheric pressure, but its critical temperature is +132 °C, far above ambient, so pressure alone can liquefy it and it can be carried fully pressurised. The −33 °C figure is often mistaken for the critical temperature (option 1) — it is actually the atmospheric boiling point.

26. A leak of ethane vapour occurs on deck. Given its vapour relative density (air = 1), where does the vapour tend to go?

  • A. It has a relative density of about 0.55 and rises rapidly away from the deck
  • B. It has a relative density of about 0.60 and disperses upward
  • C. It has a relative density of about 2.01, so it behaves like a liquid on the deck
  • D. It has a relative density of about 1.05, so being slightly heavier than air it settles into low spaces
Answer: D — It has a relative density of about 1.05, so being slightly heavier than air it settles into low spaces
Ethane vapour has a relative density of about 1.05, slightly heavier than air, so a leak sinks and collects in low, enclosed spaces where it can build a flammable pocket. It is not lighter than air (options 0 and 1), so it does not rise clear of the deck.

27. A fixed dry chemical powder installation on a gas carrier uses nitrogen or carbon dioxide within it. What is the role of that gas?

  • A. It is the propellant that expels the powder — the powder itself breaks the fire's chain reaction
  • B. It cools the burning steel to prevent re-ignition
  • C. It is the chemical agent that actually inhibits the flame
  • D. It wets the fuel surface to smother the fire
Answer: A — It is the propellant that expels the powder — the powder itself breaks the fire's chain reaction
The N₂ or CO₂ is simply the propellant that pushes the powder out; the powder is what knocks the flame down chemically. Because powder gives no cooling and leaves no lasting blanket (so option 1 is wrong), hot steel can re-ignite the gas — cool first, extinguish, then keep cooling.

28. During a fire lecture a trainee insists that the pool of refrigerated liquid cargo on deck is itself what is burning. Why is that wrong, and what is actually alight?

  • A. The liquid burns directly, so removing the vapour above it would not change the fire
  • B. The liquid burns only after it has frozen the surrounding deck steel solid
  • C. The liquid does not burn — it boils, and the vapour mixed with air burns between the flammable limits
  • D. Both the liquid and the vapour burn equally, so cooling the pool has no useful effect
Answer: C — The liquid does not burn — it boils, and the vapour mixed with air burns between the flammable limits
On a gas carrier the liquid does not burn; it evaporates and the vapour, mixed with air, burns only between the lower and upper flammable limits. That is why cooling the pool and suppressing vapour matter far more than attacking the visible flame — treating the liquid as the fuel would lead to the wrong tactics.

29. Two small leaks of equal size develop — one of methane, one of propane. Which reaches an ignitable mixture at the lower vapour concentration in air?

  • A. Propane, because it forms an ignitable mixture at a very low vapour concentration (around 2%)
  • B. Methane, because its flammable range only begins at about 15% by volume
  • C. Neither, because no hydrocarbon can ignite below 10% by volume in air
  • D. Both, but only once the mixture rises above its upper flammable limit
Answer: A — Propane, because it forms an ignitable mixture at a very low vapour concentration (around 2%)
Propane and butane reach their lower flammable limit at only about 2% vapour in air, so even a small leak can make an ignitable mixture; methane burns between roughly 5% and 15%. A mixture above the upper flammable limit is too rich to burn, so distractor D is backwards.

30. Why does flash point tell you almost nothing useful about LNG, while auto-ignition temperature matters after a fire has heated the steelwork?

  • A. Methane has no flash point at all, so neither property is relevant to it
  • B. Flash point and auto-ignition are the same figure for methane, about 580 °C
  • C. Auto-ignition needs a spark, whereas flash point needs only a hot surface
  • D. Methane is always far above its very low flash point in service, so it warns of nothing; auto-ignition (~580 °C) is what a hot surface can deliver with no spark
Answer: D — Methane is always far above its very low flash point in service, so it warns of nothing; auto-ignition (~580 °C) is what a hot surface can deliver with no spark
Methane's flash point is around −188 °C, so a cryogenic cargo is effectively always above it — the concept gives no useful warning. Auto-ignition, about 580 °C for methane, is the real hazard: hot steel after a fire can ignite vapour with no flame or spark, which is why hot metal is cooled before the flame is extinguished.

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