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GASCO exit exam questions and answers — Set 14 — Advanced Training for Liquefied Gas Tanker Cargo Operations

Set 14 of the Advanced Training for Liquefied Gas Tanker Cargo Operations (GASCO) exit exam bank — 30 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.

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Set 14 — 30 GASCO exit exam questions with answers

1. On board an LNG tanker during gas burning, what option is available if the volume of boil-off is too great for the boilers to handle?

  • A. Vent excess boil-off from the cargo tanks through the common vent riser
  • B. Increase the rate of gas burning beyond the boiler capacity
  • C. Divert excess boil-off to a separate storage tank
  • D. Reduce the temperature of the cargo to decrease boil-off volume
Answer: A — Vent excess boil-off from the cargo tanks through the common vent riser
Conventional steam LNG carriers have no reliquefaction or storage for excess boil-off. Pressure is controlled by burning gas, dumping steam, and, as a last resort, venting surplus vapour through the common mast riser after heating it.

2. What instrument is used to measure the dew point of the cargo tank atmosphere during cargo operations?

  • A. Barometer
  • B. Anemometer
  • C. Thermometer
  • D. Hygrometer
Answer: D — Hygrometer
A dew point meter is a type of hygrometer that measures the moisture content of a gas. Dew point is checked during drying, inerting and aerating to prevent ice and hydrate formation in valves, pumps and gauges.

3. When treating chemical burns, for how long should the affected area be kept under cool running water?

  • A. 5 minutes
  • B. 15 minutes
  • C. 20 minutes
  • D. 30 minutes
Answer: C — 20 minutes
Current first-aid guidance is to flush chemical burns with cool running water for at least 20 minutes to dilute and remove the chemical and cool the tissue, removing contaminated clothing while flushing, and to seek medical advice.

4. On board an LNG carrier during loading, what will happen when the cargo level in a tank reaches the high-high level setting (about 98.6% by volume)?

  • A. The loading operation will continue uninterrupted
  • B. The tank will automatically release excess pressure
  • C. The tank will start venting excess cargo to maintain safe levels
  • D. The high-level alarm will sound and the filling valve will close automatically
Answer: D — The high-level alarm will sound and the filling valve will close automatically
IGC Code Chapter 13 requires an overflow control system independent of the gauging system. At the high-high level it gives an alarm and automatically closes the tank filling valve to prevent overfilling the tank.

5. How would you determine whether a cargo pump is working correctly, i.e. pumping at the correct rate?

  • A. Measure the voltage output of the pump
  • B. Check the motor amperage load on the switchboard
  • C. Monitor the temperature of the pump casing
  • D. Observe the colour of the pump discharge
Answer: B — Check the motor amperage load on the switchboard
For submerged electric cargo pumps the motor current, together with discharge pressure, indicates the pump's operating point on its performance curve. A correct amperage for the given discharge pressure shows the pump is delivering its expected flow rate.

6. What is meant when a liquefied gas cargo is described as flammable?

  • A. The cargo will not ignite under any circumstances
  • B. The cargo is safe to handle without any risk of combustion
  • C. The vapour from the cargo will burn when mixed with air and ignited
  • D. The cargo does not produce any flammable vapour
Answer: C — The vapour from the cargo will burn when mixed with air and ignited
A flammable cargo gives off vapour that, when mixed with air within its flammable range and exposed to an ignition source, will burn. It is the vapour, not the liquid itself, that burns.

7. What is a typical set point for the insulation space pressure relief valves?

  • A. 50 mbar below atmospheric pressure
  • B. 10 mbar above atmospheric pressure
  • C. 100 mbar above atmospheric pressure
  • D. 500 mbar below atmospheric pressure
Answer: B — 10 mbar above atmospheric pressure
Insulation and interbarrier spaces are kept at a small positive nitrogen pressure, a few mbar above atmosphere and below the cargo tank pressure. Their relief valves are therefore set only slightly above atmospheric, of the order of 10 mbar, to protect the membrane.

8. What is the Joule-Thomson effect?

  • A. The change in temperature that accompanies expansion of a gas without production of work or transfer of heat
  • B. The change in pressure that accompanies compression of a gas without production of work or transfer of heat
  • C. The change in volume that accompanies contraction of a gas without production of work or transfer of heat
  • D. The change in entropy that accompanies compression of a gas without production of work or transfer of heat
Answer: A — The change in temperature that accompanies expansion of a gas without production of work or transfer of heat
The Joule-Thomson effect is the temperature change of a real gas expanding through a throttle without doing work or exchanging heat. At ordinary conditions most gases, except hydrogen and helium, cool on expansion, which is used in gas liquefaction.

9. What is Dalton's law of partial pressures?

  • A. The pressure of a gas mixture is inversely proportional to the total number of gas particles in the mixture
  • B. The pressure exerted by a mixture of gases is equal to the sum of the separate pressures which each gas would exert if it alone occupied the whole volume
  • C. The pressure of a gas is directly proportional to the volume it occupies
  • D. The pressure of a gas is independent of the presence of other gases in the mixture
Answer: B — The pressure exerted by a mixture of gases is equal to the sum of the separate pressures which each gas would exert if it alone occupied the whole volume
Dalton's law states that total pressure equals the sum of the partial pressures of the component gases. It explains, for example, how the vapour pressure of a cargo and the pressure of inert gas combine in a tank atmosphere.

10. What is a submerged centrifugal pump?

  • A. A type of positive displacement pump
  • B. A pump installed above the cargo tank
  • C. A centrifugal cargo pump installed at the bottom of the cargo tank with the motor, impeller and bearings totally submerged in the cargo
  • D. A pump used exclusively for ballast operations
Answer: C — A centrifugal cargo pump installed at the bottom of the cargo tank with the motor, impeller and bearings totally submerged in the cargo
Submerged motor pumps, common on gas carriers and terminals, sit in the tank bottom with motor, impeller and bearings immersed. The liquefied gas itself cools and lubricates the bearings and motor, and no shaft seal through the tank top is needed.

11. On board a membrane LNG tanker during the ballast voyage, what is the danger if the cargo tank levels are kept above 10% of tank length (in the filling restriction band)?

  • A. Heavy sloshing may lead to tank damage
  • B. Improved stability of the vessel
  • C. Reduced risk of sloshing
  • D. Increased cargo capacity
Answer: A — Heavy sloshing may lead to tank damage
Membrane tanks have filling restrictions, typically below 10% of tank length or above 70% of tank height. Intermediate levels allow liquid to build up resonant sloshing, whose impact loads can damage the membrane and insulation.

12. Warm LNG vapour is lighter than the inert gas. During gassing-up by the displacement method, what is the best way to supply the warm LNG vapour to the tank?

  • A. Through the vapour line at the top of the tank, exhausting inert gas through the loading line at the bottom
  • B. Through the loading line at the tank bottom, exhausting through the top
  • C. Through the manifold directly to the vent mast
  • D. By releasing it directly to the atmosphere
Answer: A — Through the vapour line at the top of the tank, exhausting inert gas through the loading line at the bottom
Because warm methane vapour is lighter than inert gas, it is introduced at the tank top and forms a layer that pushes the heavier inert gas out from the bottom through the filling line. Feeding it at the bottom would cause mixing and waste gas.

13. The glass bottle under the cap of a soda-acid type water extinguisher contains ____.

  • A. sodium bicarbonate solution
  • B. calcium hydroxide
  • C. dilute sulphuric acid
  • D. hydrogen peroxide
Answer: C — dilute sulphuric acid
In a soda-acid extinguisher the small bottle under the cap holds dilute sulphuric acid. Striking the plunger breaks it, the acid mixes with the sodium bicarbonate solution and the carbon dioxide produced expels the water.

14. In the event of various alarms being activated in relation to the cargo systems and the pumps themselves, what is the worst that can happen to the main cargo pumps?

  • A. Increased speed
  • B. Manual override
  • C. They will be automatically stopped
  • D. Continuous operation
Answer: C — They will be automatically stopped
Main cargo pumps are interlocked with trips such as low tank level, low discharge pressure, overload, low motor current and emergency shutdown. When these alarms or ESD signals are activated, the pumps are stopped automatically to protect the pump and the system.

15. When operating a stripping/spray pump, what should be monitored and adjusted to ensure its most efficient operation as indicated on the performance graph?

  • A. Temperature of the cargo
  • B. The pump discharge pressure and pump motor amperes
  • C. Tank pressure only
  • D. Colour of the sprayed liquid
Answer: B — The pump discharge pressure and pump motor amperes
The pump's performance curve relates flow, head and motor current. By monitoring discharge pressure and motor amperes and adjusting the discharge valve, the operator keeps the pump at its best efficiency point and avoids cavitation or overload.

16. What is an LNG tanker provided with to manage cargo vapour, especially to ensure the desired flow of boil-off gas from the cargo tanks to the engine room (via the boil-off gas heater) for gas burning?

  • A. High duty gas compressor
  • B. A vacuum system
  • C. A system for direct venting to atmosphere
  • D. Low duty gas compressor
Answer: D — Low duty gas compressor
The low duty compressor handles the natural boil-off and sends it through the boil-off heater to the engine room for burning in boilers or engines. The high duty compressor is used for returning vapour to shore during loading.

17. Why is the boil-off gas heated prior to venting to atmosphere via the common mast riser?

  • A. To increase the visibility of the vented gas
  • B. To avoid venting cold gas, which can create a large white plume and stay at deck level
  • C. To increase the quantity of gas vented
  • D. To comply only with noise regulations
Answer: B — To avoid venting cold gas, which can create a large white plume and stay at deck level
Cold methane vapour below about -110°C is heavier than air and condenses atmospheric moisture into a dense white cloud that can fall to deck level. Heating the gas first makes it lighter than air so it disperses safely upward.

18. Why is it necessary to purge all sections of the pipelines and all cargo machinery at the end of the inerting operation (prior to aerating)?

  • A. To avoid potentially hazardous methane pockets
  • B. To increase the concentration of methane in the system
  • C. To create favourable conditions for combustion
  • D. To save time by skipping the purging process
Answer: A — To avoid potentially hazardous methane pockets
Dead ends in lines, compressors, heaters and vaporisers can trap methane. If not purged with inert gas before air is introduced, these pockets could form flammable mixtures during aeration, so every section must be inerted and tested.

19. When droplets of liquid are carried over into the suction of a compressor and drawn into the compressor, this is called ____.

  • A. wet suction
  • B. dry suction
  • C. moisture-free suction
  • D. vaporised suction
Answer: A — wet suction
Liquid carry-over into a compressor suction is known as wet suction. Liquid is incompressible and can cause serious mechanical damage, so liquid separators and suction superheat monitoring are used to prevent it.

20. What is the effect of boil-off on the composition of the LNG delivered at the end of the voyage?

  • A. Higher percentage of nitrogen and methane than in the LNG loaded
  • B. Unchanged composition compared to the LNG loaded
  • C. Higher percentage of nitrogen than in the LNG loaded, with less ethane
  • D. Lower percentage of nitrogen and methane than in the LNG loaded
Answer: D — Lower percentage of nitrogen and methane than in the LNG loaded
Nitrogen and methane are the most volatile components, so they boil off preferentially. This ageing or weathering leaves the delivered LNG richer in ethane, propane and butane, with a slightly higher density and heating value.

21. Failure of one or both main cargo pumps in a tank requires the use of the emergency pump. Which gas is used to purge the pump well before the emergency pump is lowered into it?

  • A. Oxygen
  • B. Nitrogen
  • C. Argon
  • D. Helium
Answer: B — Nitrogen
Before the emergency pump is lowered, the pump column is purged with nitrogen to remove methane and air, so that no flammable mixture forms when the column is opened. Nitrogen is inert, dry and readily available from the on-board generator.

22. On board an LNG tanker, which pump is normally used to supply LNG to the forcing vaporiser during forced boil-off operation?

  • A. Spray pump
  • B. Main cargo pump
  • C. Diaphragm pump
  • D. Peristaltic pump
Answer: A — Spray pump
The small stripping/spray pump in the tank supplies LNG to the forcing vaporiser to generate additional fuel gas when natural boil-off is insufficient. It is also used for spraying during cool-down and to keep tanks cold during the ballast voyage.

23. The Heimlich manoeuvre is used for ____.

  • A. fractures
  • B. choking
  • C. burns
  • D. hypothermia
Answer: B — choking
The Heimlich manoeuvre consists of abdominal thrusts that force air from the lungs to dislodge an object blocking the airway of a conscious choking casualty. It is not used for fractures, burns or hypothermia.

24. What is the minimum requirement for discharge of a fixed carbon dioxide system to fight a fire in the engine room?

  • A. 50% of the carbon dioxide to be discharged within 2 minutes
  • B. 100% of the carbon dioxide to be discharged within 10 minutes
  • C. 75% of the carbon dioxide to be discharged within 5 minutes
  • D. 85% of the carbon dioxide to be discharged within 2 minutes
Answer: D — 85% of the carbon dioxide to be discharged within 2 minutes
FSS Code Chapter 5 requires the fixed CO2 system for machinery spaces to be able to discharge 85% of the required gas into the space within 2 minutes, giving a rapid inerting effect before the fire can develop.

25. Which statement on heat detectors is correct?

  • A. Heat detectors are only used in cargo holds
  • B. Fixed heat detectors are not affected by temperature changes
  • C. Fixed heat detectors are used in the accommodation and machinery spaces depending on the temperature settings
  • D. Heat detectors are only used in open deck areas
Answer: C — Fixed heat detectors are used in the accommodation and machinery spaces depending on the temperature settings
Heat detectors operate at a set temperature or rate of rise and are fitted where smoke detectors would give false alarms, such as galleys, laundries and parts of machinery spaces. The FSS Code requires fixed-temperature heat detectors to operate between 54°C and 78°C.

26. Besides carbon dioxide, which other gas can be used as the propellant gas in a dry powder system?

  • A. Carbon monoxide
  • B. Oxygen
  • C. Helium
  • D. Nitrogen
Answer: D — Nitrogen
Dry powder systems and large extinguishers on gas carriers use inert gas, normally nitrogen, to fluidise and expel the powder. Nitrogen is dry and inert, so it does not cake the powder or support combustion.

27. Which gas can build up in the battery room when batteries are being charged?

  • A. Nitrogen
  • B. Oxygen
  • C. Hydrogen
  • D. Methane
Answer: C — Hydrogen
Lead-acid batteries release hydrogen and oxygen during charging, especially near full charge. Hydrogen is very light and highly flammable, so battery rooms require good ventilation, explosion-proof fittings and no naked lights.

28. A wide variety of mechanical motions and actions may present hazards to the worker. Which of the following is NOT one of the basic types of hazardous mechanical motion?

  • A. Static motion
  • B. Reciprocating motion
  • C. Oscillating motion
  • D. Rotating motion
Answer: A — Static motion
Hazardous mechanical motions are rotating, reciprocating, transverse and oscillating movements of machine parts, which can catch, crush or strike a worker. A static part is not a motion at all.

29. When a new vessel is inducted into operations, what will require close monitoring?

  • A. Navigation equipment
  • B. Crew accommodation
  • C. Weather conditions
  • D. Ship/shore compatibility of loading and unloading procedures
Answer: D — Ship/shore compatibility of loading and unloading procedures
A newly inducted gas carrier must be checked for compatibility with terminals, including manifold arrangement, mooring, emergency shutdown links and cargo transfer procedures. These are monitored closely during the first loading and discharging operations.

30. A radar tank gauge is stamped Ex ia IIC T4. What is the maximum surface temperature allowed?

  • A. 100°C
  • B. 150°C
  • C. 135°C
  • D. 200°C
Answer: C — 135°C
The temperature class indicates the maximum surface temperature of the equipment: T1 450°C, T2 300°C, T3 200°C, T4 135°C, T5 100°C and T6 85°C. T4 equipment therefore must not exceed 135°C.

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