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LGTCO/LGTF exit exam questions and answers — Set 11 — Liquefied Gas Tanker Cargo Operations

Set 11 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.

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Set 11 — 30 LGTCO/LGTF exit exam questions with answers

1. What is the primary role of the cargo venting system on a liquefied gas tanker?

  • A. Relieving excess cargo tank pressure
  • B. Improving cargo containment
  • C. Heating the cargo
  • D. Increasing cargo vaporisation
Answer: A — Relieving excess cargo tank pressure
The cargo tank venting system with its pressure relief valves and vent masts discharges vapour to the atmosphere if tank pressure exceeds the set pressure. This protects the tanks from overpressure as required by IGC Code Chapter 8.

2. What is the primary purpose of a pressure relief valve on liquefied gas tanker cargo tanks?

  • A. Reducing cargo vaporisation
  • B. Improving cargo containment
  • C. Preventing overpressure in cargo tanks
  • D. Providing fuel for the ship's engines
Answer: C — Preventing overpressure in cargo tanks
Pressure relief valves open at their set pressure (MARVS) and vent vapour to the vent mast, so the tank pressure cannot rise above its design limit. The IGC Code requires at least two such valves on tanks larger than 20 m³.

3. What is the primary function of the emergency shutdown (ESD) system on a liquefied gas tanker?

  • A. Increasing cargo transfer speed
  • B. Preventing cargo vaporisation
  • C. Ensuring a safe and rapid shutdown of cargo transfer in emergencies
  • D. Reducing cargo pressure
Answer: C — Ensuring a safe and rapid shutdown of cargo transfer in emergencies
The ESD system, required by IGC Code Chapter 18, stops cargo pumps and compressors and closes the ESD valves when activated manually or by fire, high level or other trips. It is linked between ship and shore to stop cargo flow safely and quickly.

4. Which liquefied gas tanker cargo operation involves removing inert gas from the cargo tanks with cargo vapour before loading?

  • A. Gassing up (purging with cargo vapour)
  • B. Cargo heating
  • C. Loading
  • D. Discharging
Answer: A — Gassing up (purging with cargo vapour)
Before loading, inert gas is displaced from the tanks with cargo vapour in an operation called gassing up or purging. This prevents non-condensable inert gas from causing high pressures in the reliquefaction plant or contaminating the cargo, and it is followed by cool-down.

5. What is the primary purpose of a cargo tank high-level alarm on a liquefied gas tanker?

  • A. Improving cargo containment
  • B. Preventing cargo crystallisation
  • C. Warning that the liquid level is approaching the maximum filling limit
  • D. Reducing cargo vaporisation
Answer: C — Warning that the liquid level is approaching the maximum filling limit
The IGC Code (Chapter 13) requires a high-level alarm independent of the gauging system that warns when the tank is nearly full. A separate overflow control sensor then shuts the loading valve to prevent the tank becoming liquid full.

6. What is the primary function of the cargo pumps on a liquefied gas tanker?

  • A. Reducing cargo pressure
  • B. Improving cargo containment
  • C. Transferring cargo to shore facilities during discharge
  • D. Providing fuel for the ship's engines
Answer: C — Transferring cargo to shore facilities during discharge
Cargo pumps, usually deepwell or submerged electric pumps, discharge liquid cargo from the tanks through the manifold to the terminal. They are also used for internal transfers and for spraying during cool-down. Loading is done by shore pumps.

7. What is the primary purpose of a cargo tank pressure control system on a liquefied gas tanker?

  • A. Reducing cargo vaporisation
  • B. Keeping cargo tank pressure within safe limits
  • C. Increasing cargo transfer speed
  • D. Preventing cargo crystallisation
Answer: B — Keeping cargo tank pressure within safe limits
The IGC Code (Chapter 7) requires that cargo tank pressure be kept below the relief valve set pressure by means such as reliquefaction, using boil-off as fuel, or a gas combustion unit. This control avoids venting cargo to the atmosphere.

8. In liquefied gas tanker cargo operations, what does a reliquefaction plant do?

  • A. Heats the cargo
  • B. Recovers inert gas
  • C. Measures the cargo vaporisation rate
  • D. Cools and reliquefies the cargo boil-off
Answer: D — Cools and reliquefies the cargo boil-off
A reliquefaction plant compresses the boil-off vapour, cools and condenses it, often in a cascade or nitrogen-refrigerant cycle, and returns the liquid to the tanks. It controls tank pressure without venting or burning the cargo.

9. Under the IGC Code, ship type 2PG is a type 2G ship of 150 m or less in length carrying products in independent Type C tanks designed for a MARVS of at least:

  • A. 7 barg
  • B. 2 barg
  • C. 18 barg
  • D. 0.7 barg
Answer: A — 7 barg
The IGC Code defines type 2PG as a type 2G ship of 150 m length or less in which products are carried in independent Type C tanks designed for a MARVS of at least 7 barg and a containment design temperature of -55°C or above.

10. Under the IGC Code, a type 3G gas carrier is intended to carry products requiring:

  • A. No preventive measures as the products are non-hazardous
  • B. Moderate preventive measures to preclude their escape
  • C. Maximum preventive measures to preclude their escape
  • D. Significant preventive measures to preclude their escape
Answer: B — Moderate preventive measures to preclude their escape
The IGC Code grades ship types by the hazard of the products: type 1G for maximum, type 2G/2PG for significant and type 3G for moderate preventive measures to preclude the escape of cargo.

11. Why are fully pressurised LPG carriers generally limited to relatively small cargo capacities?

  • A. The IGC Code prohibits pressurised tanks above 1,000 m³
  • B. They cannot be fitted with deepwell cargo pumps
  • C. Their heavy, thick-walled pressure-vessel tanks make large capacities uneconomic
  • D. They must always carry a reliquefaction plant
Answer: C — Their heavy, thick-walled pressure-vessel tanks make large capacities uneconomic
Fully pressurised ships carry cargo at ambient temperature in Type C pressure vessels designed for about 18 barg; the tank steel weight and cost rise steeply with size, so these ships are typically small coastal or short-sea carriers.

12. Which type of gas carrier typically uses prismatic independent Type A tanks, a full secondary barrier and a reliquefaction plant to carry LPG near atmospheric pressure?

  • A. Fully pressurised LPG carrier
  • B. Semi-pressurised ethylene carrier
  • C. Moss-type LNG carrier
  • D. Fully refrigerated LPG carrier
Answer: D — Fully refrigerated LPG carrier
Large fully refrigerated LPG carriers carry cargo at about its atmospheric boiling point in prismatic Type A tanks; being low-pressure tanks, they need a complete secondary barrier and a reliquefaction plant to control boil-off.

13. Semi-pressurised (semi-refrigerated) gas carriers control cargo condition by:

  • A. A combination of reduced temperature and moderate pressure
  • B. Inert gas blanketing at atmospheric pressure
  • C. Refrigeration alone at atmospheric pressure
  • D. Pressure alone at ambient temperature
Answer: A — A combination of reduced temperature and moderate pressure
Semi-pressurised ships carry cargo in Type C tanks designed for moderate pressures, with a reliquefaction plant to keep the cargo below ambient temperature, giving them flexibility to load from pressurised or refrigerated terminals.

14. Why are expansion bellows or loops fitted in cargo pipelines of refrigerated gas carriers?

  • A. To reduce static electricity build-up
  • B. To accommodate thermal contraction and expansion of the pipes
  • C. To act as liquid traps for condensate
  • D. To increase the cargo flow rate
Answer: B — To accommodate thermal contraction and expansion of the pipes
Cargo pipes shrink considerably when cooled from ambient to cryogenic temperature; bellows, loops or offsets absorb this contraction and later expansion so that excessive stresses are not transmitted to pipe supports, flanges and tank connections.

15. Cargo pipes and valves intended for LNG service at about -163°C are commonly made of:

  • A. Ordinary carbon steel
  • B. Galvanised mild steel
  • C. Austenitic stainless steel
  • D. Cast iron
Answer: C — Austenitic stainless steel
Ordinary carbon steel and cast iron become brittle at cryogenic temperatures. The IGC Code requires materials with proven low-temperature toughness, so LNG piping and valves are typically of austenitic stainless steel.

16. What is the main purpose of a removable spool piece in gas carrier cargo or inert gas piping?

  • A. To measure cargo flow
  • B. To dampen pressure surges during ESD
  • C. To heat the cargo before discharge
  • D. To provide positive separation between systems when the spool is removed
Answer: D — To provide positive separation between systems when the spool is removed
A removable spool piece allows two systems, such as inert gas and cargo lines, to be positively segregated by physically removing the pipe section and blanking the ends, which valves alone cannot guarantee.

17. Why are cargo pipelines on gas carriers electrically bonded across flanges and to the hull structure?

  • A. To prevent accumulation of static electricity on the pipework
  • B. To allow cathodic protection of the pipes
  • C. To act as an earth for the ship's lighting
  • D. To reduce corrosion of flange bolts
Answer: A — To prevent accumulation of static electricity on the pipework
Flowing liquids and vapours can generate static charge; electrical bonding of pipe sections and earthing to the hull ensures that charge drains away and cannot accumulate to produce an incendive spark.

18. Under the IGC Code, the outlet of cargo tank pressure relief valve vent masts must be located at a height of at least:

  • A. 2 m above the weather deck
  • B. B/3 or 6 m above the weather deck, whichever is greater
  • C. The height of the wheelhouse top
  • D. 1 m above the highest manifold
Answer: B — B/3 or 6 m above the weather deck, whichever is greater
The IGC Code requires vent outlets to be not less than B/3 or 6 m, whichever is greater, above the weather deck and at least 6 m above the working area and fore-and-aft gangway, so that vented vapour disperses safely.

19. Why is the filling limit of a liquefied gas cargo tank set below 100% of the tank volume?

  • A. To keep the cargo pump submerged
  • B. To reduce sloshing in pressure vessels
  • C. To allow for thermal expansion of the liquid so the tank does not become liquid-full
  • D. To allow space for inert gas
Answer: C — To allow for thermal expansion of the liquid so the tank does not become liquid-full
If cargo warms, it expands; the IGC filling limit ensures that even when the cargo reaches the reference temperature the tank does not become liquid-full, which would allow liquid to reach the relief valves.

20. In the IGC Code loading limit formula LL = FL × ρR / ρL, the term ρR represents the:

  • A. Relative density of the cargo at the loading temperature
  • B. Rated capacity of the relief valve
  • C. Reliquefaction plant capacity
  • D. Relative density of the cargo at the reference temperature
Answer: D — Relative density of the cargo at the reference temperature
The loading limit is the filling limit corrected by the ratio of cargo density at the reference temperature (ρR) to its density at the loading temperature (ρL), so that the tank reaches its filling limit only when the cargo has warmed to the reference temperature.

21. For the IGC filling limit calculation, the reference temperature is the temperature corresponding to the:

  • A. Vapour pressure of the cargo at the set pressure of the pressure relief valves
  • B. Atmospheric boiling point of the cargo
  • C. Ambient sea water temperature at the loading port
  • D. Design temperature of the secondary barrier
Answer: A — Vapour pressure of the cargo at the set pressure of the pressure relief valves
The IGC Code defines the reference temperature as the temperature corresponding to the vapour pressure of the cargo at the set pressure of the pressure relief valves; the cargo cannot exceed it without the relief valves lifting.

22. During a ballast voyage, how are the cargo tanks of an LNG carrier normally kept cold before arrival at the loading terminal?

  • A. By injecting nitrogen into the tanks
  • B. By spraying the retained heel through the tank spray system
  • C. By keeping the tanks under vacuum
  • D. By circulating sea water around the tanks
Answer: B — By spraying the retained heel through the tank spray system
The heel retained after discharge is pumped through spray rails into the tanks during the ballast passage to keep the containment and pipes cold, so the ship can begin loading without a lengthy cool-down.

23. Why is vapour supplied to the cargo tanks from shore or from the ship's vaporiser during discharge?

  • A. To inert the tanks for the next cargo
  • B. To cool down the discharge lines
  • C. To maintain tank pressure and prevent a vacuum forming as liquid is pumped out
  • D. To speed up the reliquefaction plant
Answer: C — To maintain tank pressure and prevent a vacuum forming as liquid is pumped out
As liquid is removed, the vapour space increases and tank pressure would fall; returning vapour from shore or producing it in the vaporiser maintains a positive pressure and protects the tanks against vacuum and air ingress.

24. On a steam-turbine LNG carrier during the loaded passage, cargo tank pressure is normally controlled by:

  • A. Raising the tank pressure until the relief valves lift
  • B. Venting boil-off gas to atmosphere
  • C. Injecting liquid nitrogen into the tanks
  • D. Burning the boil-off gas as fuel in the boilers
Answer: D — Burning the boil-off gas as fuel in the boilers
Boil-off generated by heat ingress is sent by low-duty compressors and heaters to the boilers as fuel; this removes the vapour and holds tank pressure within limits, with a GCU or dumping steam arrangements as back-up.

25. During care in transit on a gas carrier, which readings must be monitored and logged at regular intervals?

  • A. Cargo tank pressures, temperatures and liquid levels
  • B. Only the accommodation temperatures
  • C. Only the main engine revolutions
  • D. Only the ballast tank soundings
Answer: A — Cargo tank pressures, temperatures and liquid levels
Care of cargo in transit requires regular monitoring of tank pressures, liquid and vapour temperatures, levels and hold/interbarrier space conditions so that any abnormal boil-off, leak or containment problem is detected early.

26. Which industry publication gives guidance for ship-to-ship transfer of liquefied gases?

  • A. International Grain Code
  • B. Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases (ICS/OCIMF/SIGTTO/CDI)
  • C. International Aeronautical and Maritime SAR Manual
  • D. Code of Safe Practice for Cargo Stowage and Securing
Answer: B — Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases (ICS/OCIMF/SIGTTO/CDI)
The joint ICS, OCIMF, SIGTTO and CDI Ship to Ship Transfer Guide, which replaced the earlier ICS/OCIMF liquefied gas STS guide referenced in the course, covers planning, compatibility, equipment and safety for STS transfers of gas cargoes.

27. Rollover in an LNG cargo tank is caused by:

  • A. Failure of the cargo pump
  • B. Overfilling the tank above 98%
  • C. Stratification into layers of different density which suddenly mix, releasing a large volume of vapour
  • D. Excessive ship motion in heavy weather
Answer: C — Stratification into layers of different density which suddenly mix, releasing a large volume of vapour
When layers of LNG of different density and temperature form, heat ingress gradually equalises their densities until they suddenly mix; the superheated lower layer then releases a large volume of boil-off, which can overpressurise the tank.

28. To minimise the risk of stratification when loading LNG into a tank containing heel, cargo denser than the heel should normally be loaded:

  • A. Through the bottom filling line
  • B. Only after warming up the tank
  • C. Only by the vapour return line
  • D. Through the top filling (spray) line
Answer: D — Through the top filling (spray) line
Industry guidance is to top-fill cargo that is denser than the tank contents and bottom-fill lighter cargo; the denser liquid then sinks and the lighter one rises through the existing liquid, promoting mixing rather than stable layering.

29. Why are membrane-type LNG tanks subject to partial-filling restrictions during a sea passage?

  • A. To avoid sloshing impact loads damaging the containment system
  • B. To keep the tank within its MARVS
  • C. To allow inspection of the secondary barrier
  • D. To reduce boil-off
Answer: A — To avoid sloshing impact loads damaging the containment system
In membrane tanks, liquid sloshing at intermediate filling levels can produce high impact pressures on the insulation and membrane; the designer therefore specifies filling ranges that must be avoided at sea.

30. Ballast operations during cargo loading on a gas carrier are coordinated mainly to:

  • A. Prevent static electricity in the cargo tanks
  • B. Keep the ship within its stress, stability, draft and trim limits
  • C. Supply water to the water spray system
  • D. Cool the cargo tanks
Answer: B — Keep the ship within its stress, stability, draft and trim limits
Deballasting is planned in step with loading so that shear forces, bending moments, stability, draft, trim and air draught stay within limits throughout the operation, as required by the loading plan.

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