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GASCO exit exam questions and answers — Set 24 — Advanced Training for Liquefied Gas Tanker Cargo Operations

Set 24 of the Advanced Training for Liquefied Gas Tanker Cargo Operations (GASCO) exit exam bank — 20 of 710 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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20 of 20 shown · Set 24

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Set 24 — 20 GASCO exit exam questions with answers

1. Cargo tank pressure relief valves on gas carriers are sized primarily for which design case?

  • A. Fire exposure of the tank, plus vapour from purging or other sources
  • B. Normal boil-off at sea
  • C. Maximum loading rate only
  • D. Thermal expansion of liquid in the lines
Answer: A — Fire exposure of the tank, plus vapour from purging or other sources
The IGC Code bases required relief capacity mainly on the vapour generated when the tank is exposed to fire, using a formula that relates flow to the tank's external surface area, latent heat and insulation. This governs the size of valves and vent risers.

2. Apart from relief valves, what does the IGC Code require for a cargo tank not designed to withstand the maximum external pressure difference?

  • A. Two independent pressure switches to alarm and then stop suction of liquid or vapour from the tank, or vacuum relief valves
  • B. A gas-freeing fan permanently connected
  • C. A second spool piece at the manifold
  • D. A water spray nozzle inside the tank
Answer: A — Two independent pressure switches to alarm and then stop suction of liquid or vapour from the tank, or vacuum relief valves
Where tanks could be damaged by vacuum, IGC Code 8.4 requires vacuum protection, such as two independent pressure switches that first alarm and then stop all suction from the tank, or vacuum relief valves admitting inert gas, cargo vapour or air.

3. What is the main advantage of a pilot-operated relief valve over a simple spring-loaded valve on a low-pressure cargo tank?

  • A. It stays tight close to its set pressure and opens fully with little overpressure, giving accurate settings at low pressures
  • B. It does not need to be tested or maintained
  • C. It can only lift when the vent mast is on fire
  • D. It can be set above MARVS
Answer: A — It stays tight close to its set pressure and opens fully with little overpressure, giving accurate settings at low pressures
In a pilot-operated valve, a small pilot senses tank pressure and controls a large main valve. It stays tight up to near set pressure and opens fully with minimal overpressure, which suits the low MARVS of membrane and type A or B tanks.

4. What is the purpose of the nitrogen purging/snuffing connection at the top of an LNG carrier's vent mast?

  • A. To extinguish a fire at the vent outlet caused by ignited vented vapour
  • B. To raise tank pressure during discharge
  • C. To dry the cargo tanks before loading
  • D. To supply nitrogen to the insulation spaces
Answer: A — To extinguish a fire at the vent outlet caused by ignited vented vapour
If vapour released through the vent riser is ignited, e.g. by lightning, nitrogen injected into the mast smothers the flame at the outlet. Vent masts are also kept free of water and debris so relief valves can discharge freely.

5. Under IGC Code chapter 18, the closing time of ESD valves at the manifold, measured from activation to final closure, must normally not exceed:

  • A. 30 seconds
  • B. 5 seconds
  • C. 2 minutes
  • D. 5 minutes
Answer: A — 30 seconds
IGC Code 18.10 requires manifold ESD valves to close fully within 30 seconds of activation. The closing time must be known and compatible with the shore system so that loading rates can be set to avoid unacceptable surge pressures in the hoses or arms.

6. To avoid surge pressure, how does the IGC Code relate the maximum loading rate to the ESD valve closing time?

  • A. Loading rate in m³/h should not exceed 3600 U/t, where U is the ullage volume at the operating signal level and t the time to stop flow
  • B. Loading rate should equal the shore pump capacity
  • C. Loading rate should be reduced only when topping off the last tank
  • D. There is no relationship; closing time is fixed
Answer: A — Loading rate in m³/h should not exceed 3600 U/t, where U is the ullage volume at the operating signal level and t the time to stop flow
IGC Code 18.10 gives the maximum loading rate as LR = 3600 U/t m³/h. U is the ullage volume at the level that triggers the ESD signal, and t is the time in seconds from that signal to full valve closure, so the tank cannot overfill while the valves close.

7. In SIGTTO terminology, what distinguishes ESD-2 from ESD-1 on a loading arm system?

  • A. ESD-2 activates the powered emergency release coupling to separate the arms, after ESD-1 has stopped the transfer
  • B. ESD-2 only sounds an alarm
  • C. ESD-2 restarts the cargo pumps
  • D. ESD-2 opens the manifold valves
Answer: A — ESD-2 activates the powered emergency release coupling to separate the arms, after ESD-1 has stopped the transfer
ESD-1 stops cargo pumps and closes ESD valves to halt transfer. ESD-2, triggered for example by the ship moving beyond the arm envelope, operates the emergency release system: double valves close and the coupling releases, letting the ship separate with minimal spillage.

8. What is the key safety feature of an emergency release coupling (ERC) on a liquefied gas loading arm?

  • A. Two valves close either side of the coupling before it separates, so only a small amount of liquid is released
  • B. It cuts the arm with a hydraulic blade
  • C. It allows the arm to stretch to follow the ship
  • D. It vents the arm contents to atmosphere before release
Answer: A — Two valves close either side of the coupling before it separates, so only a small amount of liquid is released
An ERC (or PERC) is a dry-break device: interlocked ball or butterfly valves on each side of the coupling close first, then the coupling separates. Only the small volume trapped between the valves is released, protecting people and structure from cryogenic spills.

9. When should the ship-shore ESD link be tested during a cargo port call?

  • A. After connection and before cargo transfer starts, as agreed at the pre-transfer meeting
  • B. Only during dry-dock
  • C. Only after discharge is complete
  • D. Only if a leak has occurred
Answer: A — After connection and before cargo transfer starts, as agreed at the pre-transfer meeting
Ship/shore safety checklists and SIGTTO guidance require the ship-to-shore and shore-to-ship ESD functions to be tested once the link is connected and before transfer begins. This confirms both sides will stop cargo flow when either activates its ESD.

10. Why is a fibre-optic ship-shore link often preferred to an electrical link at LNG terminals?

  • A. It can also carry telephone and data signals, is immune to electrical interference and carries no spark hazard
  • B. It does not need a connector at the manifold
  • C. It works without any shore equipment
  • D. It replaces the need for ESD valves
Answer: A — It can also carry telephone and data signals, is immune to electrical interference and carries no spark hazard
Fibre-optic links carry ESD signals plus hotline telephone, mooring-load and data channels over one cable. As light rather than current carries the signal, they do not suffer electromagnetic interference and pose no ignition risk in the hazardous manifold area.

11. What happens to the ship's ESD system if electrical or control-air power to it is lost?

  • A. It is designed to fail safe, closing the ESD valves and stopping cargo transfer
  • B. The valves stay in their last position until power returns
  • C. Only an alarm is given and transfer continues
  • D. The cargo pumps speed up to finish quickly
Answer: A — It is designed to fail safe, closing the ESD valves and stopping cargo transfer
ESD systems are fail-safe: loss of control power or instrument air closes the ESD valves and trips cargo pumps and compressors. A component failure therefore stops transfer rather than leaving the transfer running without emergency protection.

12. During a planned stop, why should cargo flow be reduced and shore pumps stopped before ESD valves are closed?

  • A. To prevent surge pressure from rapid valve closure against a full-rate flow
  • B. To allow the ESD link to be disconnected faster
  • C. To increase the boil-off rate
  • D. To warm up the manifold
Answer: A — To prevent surge pressure from rapid valve closure against a full-rate flow
Closing a valve rapidly against full-rate liquid flow creates a pressure surge (liquid hammer) that can burst hoses, arms or piping. ESD valves are an emergency device; normal stops should be by reducing rate and stopping pumps first.

13. When gassing-up a cargo tank with methane vapour after inerting, how is the vapour introduced and the inert gas removed?

  • A. Warm methane vapour is introduced at the tank top and the heavier inert gas is pushed out through the bottom filling line
  • B. Methane is introduced at the bottom and inert gas vented from the top
  • C. Liquid LNG is sprayed in at full rate with all vents closed
  • D. Inert gas and methane are mixed by the tank fans
Answer: A — Warm methane vapour is introduced at the tank top and the heavier inert gas is pushed out through the bottom filling line
Methane is lighter than inert gas, so it is introduced at the top and the heavier inert gas is displaced from the bottom by a piston effect, mixing as little as possible. For heavier vapours like propane the process is reversed.

14. Why must inert gas from a combustion-type generator be replaced by cargo vapour before LNG cool-down?

  • A. Its carbon dioxide would freeze at LNG temperatures, forming solids that can damage pumps and block valves and spray nozzles
  • B. Inert gas reacts violently with LNG
  • C. Inert gas makes the cargo off-spec only for odour
  • D. Inert gas raises the boiling point of LNG
Answer: A — Its carbon dioxide would freeze at LNG temperatures, forming solids that can damage pumps and block valves and spray nozzles
Combustion inert gas contains about 14% CO2, which solidifies at about -78°C. At LNG temperatures it would form dry ice that blocks lines, sprays and pump inlets, so tanks are gassed up with LNG vapour before cooling down.

15. To what dew point should cargo tanks be dried before loading a refrigerated cargo?

  • A. Below the lowest temperature the tank and cargo system will reach in service
  • B. Equal to ambient seawater temperature
  • C. 0°C in all cases
  • D. Above the cargo boiling point
Answer: A — Below the lowest temperature the tank and cargo system will reach in service
If moisture remains when the tank is cooled below its dew point, water condenses and freezes. The resulting ice and hydrates can block pumps, valves and instrument lines, so tanks are dried with dry air or nitrogen until the dew point is below the lowest expected system temperature.

16. During the warming-up of LNG cargo tanks before gas freeing, how is the heat normally supplied?

  • A. Cargo vapour is heated in the cargo heater and circulated to the tanks by the compressor
  • B. Steam is injected directly into the tanks
  • C. Seawater is pumped into the hold spaces
  • D. The tanks are left open to atmosphere
Answer: A — Cargo vapour is heated in the cargo heater and circulated to the tanks by the compressor
Warm-up uses a closed cycle: cold vapour is drawn from the tanks by the high-duty compressor, heated in the cargo heater and returned to the tank bottoms. This evaporates residual liquid and raises the temperature before inerting and aeration.

17. Why are cargo tanks inerted after warming up and before aeration with dry air?

  • A. To lower the hydrocarbon content so that the tank atmosphere does not pass through the flammable range when air is introduced
  • B. To cool the tanks again
  • C. To increase the oxygen content
  • D. To prevent the cargo heater from overheating
Answer: A — To lower the hydrocarbon content so that the tank atmosphere does not pass through the flammable range when air is introduced
Admitting air directly to a tank full of hydrocarbon vapour would create a flammable mixture. Inert gas or nitrogen first displaces the vapour until the hydrocarbon content is low enough that adding air will not bring the mixture into the flammable range.

18. Why is the loading rate reduced during topping-off of cargo tanks?

  • A. To control the final level accurately and avoid exceeding the permitted filling limit or triggering high-level ESD
  • B. To increase the boil-off rate
  • C. To allow the tank to warm up
  • D. To save shore pump power
Answer: A — To control the final level accurately and avoid exceeding the permitted filling limit or triggering high-level ESD
As tanks approach their filling limits, rates are reduced in agreement with the terminal so levels can be monitored and valves closed in time. This avoids overfilling past the IGC Code filling limit and unplanned high-level shutdowns causing surge.

19. On a laden LNG voyage, which parameters must be continuously monitored as part of care of cargo in transit?

  • A. Tank pressures, cargo temperatures, levels and the operation of the BOG handling system
  • B. Only the ballast tank soundings
  • C. Only the manifold pressure
  • D. Only the cargo pump motor insulation
Answer: A — Tank pressures, cargo temperatures, levels and the operation of the BOG handling system
In transit, heat ingress produces boil-off that must be consumed as fuel, reliquefied or burnt in the GCU to keep tank pressure within limits. Watchkeepers monitor tank pressures, temperatures, levels, cofferdam and insulation space conditions and gas detection.

20. During bulk loading of LNG at a terminal, how is cargo tank pressure normally controlled?

  • A. Displaced vapour and flash gas are returned to shore through the vapour return line, using the high-duty compressor if needed
  • B. Vapour is vented continuously through the vent mast
  • C. The loading rate is kept at maximum and pressure ignored
  • D. Tank pressure is controlled by opening the insulation space relief valves
Answer: A — Displaced vapour and flash gas are returned to shore through the vapour return line, using the high-duty compressor if needed
Incoming liquid displaces vapour and flashes as it enters the warmer tanks. The vapour is sent back to the terminal through the vapour return line, boosted by the HD compressor when tank pressure rises, keeping pressure well below relief settings without venting.

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