Skip to content
DG Exit Exam
Home GASCO sets
Answer Key

GASCO exit exam questions and answers — Set 23 — Advanced Training for Liquefied Gas Tanker Cargo Operations

Set 23 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.

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.

Bank last rebuilt .

Practise these GASCO questions GASCO mock exit exam, 30 marks GASCO exit exam sets and syllabus

30 of 30 shown · Set 23

Nothing in this set matches that. Try a shorter word, or search the whole key.

Set 23 — 30 GASCO exit exam questions with answers

1. Normal butane and isobutane have the same chemical formula, C4H10, but different molecular structures. They are called:

  • A. Isomers
  • B. Isotopes
  • C. Polymers
  • D. Allotropes
Answer: A — Isomers
Isomers are compounds with the same molecular formula but different structural arrangements. Normal butane has a straight carbon chain while isobutane is branched, which gives them different boiling points and vapour pressures.

2. What is the chemical formula of propylene?

  • A. C3H6
  • B. C3H8
  • C. C2H4
  • D. C4H6
Answer: A — C3H6
Propylene (propene) is an unsaturated hydrocarbon with one carbon-carbon double bond and the formula C3H6. Propane is C3H8, ethylene C2H4 and butadiene C4H6.

3. Which of the following liquefied gas cargoes is a saturated hydrocarbon?

  • A. Propane
  • B. Ethylene
  • C. Propylene
  • D. Butadiene
Answer: A — Propane
Saturated hydrocarbons (alkanes) contain only single carbon-carbon bonds; propane is an alkane. Ethylene, propylene and butadiene contain double bonds, making them unsaturated and more chemically reactive.

4. Why must the temperature of an inhibited cargo such as butadiene or vinyl chloride monomer be kept as low as practicable during the voyage?

  • A. Polymerisation is accelerated at higher temperature and the inhibitor is depleted faster
  • B. Higher temperature makes the cargo freeze
  • C. Higher temperature converts the cargo to hydrates
  • D. Higher temperature makes the cargo non-flammable
Answer: A — Polymerisation is accelerated at higher temperature and the inhibitor is depleted faster
Polymerisation is exothermic and its rate rises with temperature, while the inhibitor is consumed more quickly. Keeping the cargo cool extends the inhibitor's effective life, which is why the inhibitor certificate may state temperature limits.

5. Why is a water spray or water curtain effective in reducing an ammonia vapour cloud?

  • A. Ammonia is highly soluble in water
  • B. Ammonia reacts with water to form an inert gas
  • C. Water raises ammonia's flash point
  • D. Water freezes ammonia into solid hydrates
Answer: A — Ammonia is highly soluble in water
Ammonia is extremely soluble in water, so a water spray absorbs a large amount of vapour and helps knock down a toxic cloud. Large quantities of water must be used, as the dissolution releases heat and the run-off is alkaline.

6. Under IGC Code chapter 19, which ship type is required for the carriage of chlorine?

  • A. Type 1G
  • B. Type 2G
  • C. Type 2PG
  • D. Type 3G
Answer: A — Type 1G
Chlorine is highly toxic and is assigned to ship type 1G in the IGC Code summary of minimum requirements, with independent Type C tanks and additional special requirements, because it presents the most severe hazards.

7. Before loading a cargo, compatibility must be checked between the cargo and which of the following?

  • A. Materials of construction, previous cargo residues, other cargoes carried and the refrigerant of the reliquefaction plant
  • B. Only the paint on the deck
  • C. Only the ship's fuel oil
  • D. Only the lifeboat equipment
Answer: A — Materials of construction, previous cargo residues, other cargoes carried and the refrigerant of the reliquefaction plant
Liquefied gases may react with tank and seal materials, previous cargo residues, adjacent cargoes or the refrigerant if a condenser leaks. Chemical data sheets, compatibility charts and the IGC Code requirements are used to confirm compatibility before loading.

8. Mixing which two cargoes, if carried on the same ship, would require completely separate cargo handling and reliquefaction systems because they react dangerously?

  • A. Ammonia and chlorine
  • B. Propane and butane
  • C. Methane and ethane
  • D. Propane and propylene
Answer: A — Ammonia and chlorine
Ammonia and chlorine react violently, so segregation requires separate piping, vent and reliquefaction systems. Propane, butane, methane, ethane and propylene are compatible hydrocarbons that may share common systems with appropriate grade separation.

9. Which industry publication provides guidance for ship-to-ship transfer of liquefied gases together with petroleum and chemicals?

  • A. Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases (ICS, OCIMF, SIGTTO, CDI)
  • B. International Code of Signals
  • C. IMDG Code
  • D. Bridge Procedures Guide
Answer: A — 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 for Petroleum, Chemicals and Liquefied Gases covers planning, risk assessment, equipment, mooring, communications and emergency procedures for STS operations, including liquefied gas cargoes.

10. In an STS transfer at sea, which ship normally maintains a steady course and speed while the other manoeuvres alongside?

  • A. The constant heading ship, usually the larger vessel
  • B. The manoeuvring ship, usually the larger vessel
  • C. Neither ship; both must stop engines
  • D. The ship with the most cargo remaining
Answer: A — The constant heading ship, usually the larger vessel
In an underway STS approach the constant heading ship keeps a steady heading, usually at slow speed, while the manoeuvring ship, normally the smaller vessel, approaches and berths alongside. Roles are agreed in advance in the STS plan.

11. What type of primary fenders is most commonly used between two ships during an STS operation?

  • A. Large floating pneumatic (Yokohama-type) fenders
  • B. Wooden fender boards
  • C. Old tyres tied to the rails
  • D. Rope fenders
Answer: A — Large floating pneumatic (Yokohama-type) fenders
Floating pneumatic fenders are used as primary fenders in STS operations because they absorb large berthing energies with low reaction force. Their number and size are selected according to the ships' displacements and approach conditions.

12. In an STS operation, what is the role of the 'Person in Overall Advisory Control' (POAC)?

  • A. To advise and coordinate the whole STS operation, without relieving the masters of their responsibility
  • B. To take command of both ships
  • C. To act as cargo surveyor for custody transfer
  • D. To represent the port state control authority
Answer: A — To advise and coordinate the whole STS operation, without relieving the masters of their responsibility
The STS guide defines the POAC as the person who provides advice and coordination for the entire STS operation. Each master nevertheless retains full responsibility for the safety of his ship, crew, cargo and the environment.

13. In the Ship/Shore Safety Checklist, what does the code 'R' against an item indicate?

  • A. The item must be re-checked at the intervals agreed and stated in the declaration
  • B. The item has been rejected
  • C. The item applies only to refrigerated cargo
  • D. The item is the responsibility of the shore only
Answer: A — The item must be re-checked at the intervals agreed and stated in the declaration
In the ISGOTT-based Ship/Shore Safety Checklist, 'R' marks items that are to be re-checked at agreed intervals during the operation. 'A' indicates an agreement or procedure to be identified, and 'P' indicates that permission is to be granted.

14. What is the purpose of an insulating flange fitted in the terminal loading arm or hose string?

  • A. To prevent stray electrical currents flowing between ship and shore and causing arcing at the manifold
  • B. To insulate the cargo from heat ingress
  • C. To act as an emergency release coupling
  • D. To allow nitrogen purging of the arm
Answer: A — To prevent stray electrical currents flowing between ship and shore and causing arcing at the manifold
An insulating flange interrupts electrical continuity between ship and shore, preventing stray currents, such as from cathodic protection, from causing incendive sparks when arms or hoses are connected or disconnected. A ship-shore bonding cable is not a substitute.

15. Which items must be agreed between ship and terminal at the pre-transfer meeting before liquefied gas cargo operations begin?

  • A. Transfer rates, ESD procedures and signals, communications, emergency procedures and vapour return arrangements
  • B. Only the date of departure
  • C. Only the agent's fees
  • D. Only the crew list
Answer: A — Transfer rates, ESD procedures and signals, communications, emergency procedures and vapour return arrangements
The pre-transfer conference covers cargo quantities, sequence and maximum rates, vapour handling, ESD and emergency release procedures, communications and language, emergency signals and the checklist items. Agreement is recorded in the Ship/Shore Safety Checklist.

16. Why must a gas carrier be checked for compatibility with a terminal before first arrival?

  • A. To confirm that mooring, manifold, loading arm envelope, ESD link and gangway arrangements match the berth
  • B. To confirm the crew's nationality
  • C. To agree the freight rate
  • D. To check the ship's paint scheme
Answer: A — To confirm that mooring, manifold, loading arm envelope, ESD link and gangway arrangements match the berth
A ship/shore compatibility study verifies that the ship's size, mooring arrangement, manifold position and flange sizes, loading arm operating envelope, ESD and communication links and access arrangements are suitable for the terminal before the first call.

17. During STS transfer of liquefied gas, what is required for cargo hoses between the two ships?

  • A. Hoses must be suitable for the cargo and temperature, tested and certified, and supported so that their minimum bend radius is not exceeded
  • B. Any hose of the correct diameter may be used
  • C. Hoses need not be tested if new
  • D. Hoses should be left unsupported to allow free movement
Answer: A — Hoses must be suitable for the cargo and temperature, tested and certified, and supported so that their minimum bend radius is not exceeded
STS guidance requires hoses designed for the cargo and its temperature and pressure, with valid test certificates, and proper support so that the minimum bend radius is not exceeded as the ships move relative to each other.

18. Which condition would normally require an STS transfer of liquefied gas to be stopped and hoses disconnected?

  • A. Weather or sea conditions exceeding the limits agreed in the STS plan
  • B. Arrival of a provisions boat at the other ship
  • C. Change of watch on either ship
  • D. Completion of one tank in the sequence
Answer: A — Weather or sea conditions exceeding the limits agreed in the STS plan
STS plans set limits for wind, sea state and relative ship motion. If these are exceeded, or if an emergency, loss of communication or a cargo leak occurs, transfer must be stopped, lines drained and hoses disconnected so that the ships can separate safely.

19. Under the IGC Code, a type 2PG ship is one of 150 m in length or less that carries products in which type of cargo tank?

  • A. Independent type C tanks with MARVS of at least 0.7 MPa gauge and design temperature of -55°C or above
  • B. Membrane tanks with MARVS not exceeding 0.025 MPa gauge
  • C. Independent type A tanks designed for -163°C
  • D. Integral tanks forming part of the hull structure
Answer: A — Independent type C tanks with MARVS of at least 0.7 MPa gauge and design temperature of -55°C or above
The IGC Code defines a type 2PG ship as a gas carrier of 150 m or less carrying ship type 2G products in independent type C tanks designed for a MARVS of at least 0.7 MPa gauge and a cargo containment design temperature of -55°C or above.

20. Which cargo is required by chapter 19 of the IGC Code to be carried only in a type 1G ship?

  • A. Chlorine
  • B. Propane
  • C. Butane
  • D. Methane (LNG)
Answer: A — Chlorine
Chlorine is highly toxic and corrosive, so the IGC Code chapter 19 summary of minimum requirements assigns it to ship type 1G in independent type C tanks, with maximum preventive measures against escape of cargo. LPG and LNG are carried in type 2G ships.

21. A fully refrigerated LPG carrier typically carries its cargo in which condition?

  • A. At about atmospheric pressure and at or near its boiling point, down to about -50°C
  • B. At ambient temperature under pressures up to about 18 bar
  • C. At -163°C in membrane tanks
  • D. At about 5 to 8 bar and -10°C in spherical tanks
Answer: A — At about atmospheric pressure and at or near its boiling point, down to about -50°C
Fully refrigerated LPG ships carry propane, butane, ammonia and similar cargoes at near atmospheric pressure, cooled to their atmospheric boiling points (propane about -42°C), usually in independent type A prismatic tanks with a reliquefaction plant controlling tank pressure.

22. Why are ethylene carriers normally of the semi-pressurised/fully refrigerated type with a cascade reliquefaction plant?

  • A. Ethylene boils at about -104°C at atmospheric pressure and has a low critical temperature, so it cannot be liquefied by pressure alone at ambient temperature
  • B. Ethylene is heavier than LPG and needs stronger tanks
  • C. Ethylene must be carried at -163°C like LNG
  • D. Ethylene polymerises if carried under refrigeration
Answer: A — Ethylene boils at about -104°C at atmospheric pressure and has a low critical temperature, so it cannot be liquefied by pressure alone at ambient temperature
Ethylene has an atmospheric boiling point of about -104°C and a critical temperature of about +9.5°C, so it must be refrigerated. A cascade cycle using a secondary refrigerant such as R-1270 or R-404A is needed to condense ethylene vapour at such low temperatures.

23. What is the principal function of a Floating Storage and Regasification Unit (FSRU)?

  • A. To receive and store LNG and convert it back to gas on board for send-out to a shore pipeline
  • B. To liquefy natural gas from offshore wells
  • C. To act as a bunker barge supplying LPG to ships
  • D. To carry compressed natural gas in cylinders
Answer: A — To receive and store LNG and convert it back to gas on board for send-out to a shore pipeline
An FSRU stores LNG delivered by carriers and regasifies it using on-board vaporisers heated by seawater or another medium, sending high-pressure natural gas ashore through a pipeline. It provides a quicker, cheaper import terminal than a land-based regasification plant.

24. According to the IGC Code, entrances and openings to accommodation spaces on the side boundaries of a deckhouse must be located how far from the end facing the cargo area?

  • A. At least L/25, but not less than 3 m, which need not exceed 5 m
  • B. At least 10 m in all cases
  • C. At least B/3 or 6 m, whichever is greater
  • D. At least 1.5 m
Answer: A — At least L/25, but not less than 3 m, which need not exceed 5 m
IGC Code chapter 3 requires entrances, air inlets and openings to accommodation, service and machinery spaces and control stations not to face the cargo area; on the side boundaries they must be at least L/25 (minimum 3 m, need not exceed 5 m) from the end facing the cargo area.

25. What is the main advantage of a semi-pressurised/semi-refrigerated gas carrier over a fully pressurised ship?

  • A. Reduced tank wall thickness and weight, allowing larger cargo capacity, while still able to load from pressurised or refrigerated terminals
  • B. It needs no reliquefaction plant
  • C. It can carry LNG at -163°C
  • D. It does not require any secondary barrier or insulation for any cargo
Answer: A — Reduced tank wall thickness and weight, allowing larger cargo capacity, while still able to load from pressurised or refrigerated terminals
By partly refrigerating the cargo, semi-pressurised ships use type C tanks designed for lower pressures (typically 5 to 8 bar), so tanks are lighter and larger. Their reliquefaction plant and insulation give flexibility to load and discharge at both pressurised and refrigerated terminals.

26. Traditionally, why were steam turbines used on LNG carriers?

  • A. Boilers could burn the boil-off gas together with fuel oil in any proportion, disposing of BOG safely
  • B. Steam turbines were cheaper to run than diesel engines
  • C. Diesel engines were prohibited on gas carriers
  • D. Steam was needed to keep the cargo warm
Answer: A — Boilers could burn the boil-off gas together with fuel oil in any proportion, disposing of BOG safely
Steam plant boilers can fire natural boil-off gas, fuel oil or both, so BOG from the cargo tanks could be used as fuel without a reliquefaction plant. Their poor thermal efficiency later led to DFDE, slow-speed dual-fuel engines and reliquefaction systems.

27. Under the IGC Code, how many pressure relief valves must be fitted to a cargo tank with a volume of more than 20 m³?

  • A. At least two of approximately equal capacity
  • B. At least one
  • C. At least three, one of which is spring-loaded
  • D. At least four, two for pressure and two for vacuum
Answer: A — At least two of approximately equal capacity
IGC Code chapter 8 requires each cargo tank above 20 m³ to have at least two pressure relief valves of approximately equal capacity, so that one can be taken out of service while the other still protects the tank. Tanks of 20 m³ or less may have one.

28. What is the minimum required height of a cargo tank vent outlet above the weather deck according to the IGC Code?

  • A. B/3 or 6 m, whichever is greater
  • B. 2.4 m
  • C. B/15 or 2 m, whichever is less
  • D. 10 m in all cases
Answer: A — B/3 or 6 m, whichever is greater
Vent outlets from pressure relief valves must be at least B/3 or 6 m above the weather deck, whichever is greater, and 6 m above the working area and fore-and-aft gangway, so that released vapour disperses clear of personnel.

29. Cargo vent outlets must be arranged at a distance from the nearest air intake or opening to accommodation, service and machinery spaces of at least:

  • A. B or 25 m, whichever is less
  • B. 5 m
  • C. L/25
  • D. 50 m, whichever is greater
Answer: A — B or 25 m, whichever is less
The IGC Code requires pressure relief vent outlets to be at least B or 25 m, whichever is less, from the nearest air intake, outlet or opening to accommodation, service and machinery spaces and other gas-safe spaces, to stop vapour entering them.

30. When alternative (harbour) pressure relief valve settings are used, which requirement applies?

  • A. Changes are made under the Master's supervision per approved procedures, recorded in the log, and the set pressure is displayed at the cargo control station and at each relief valve
  • B. The chief officer may change settings by adjusting springs without recording
  • C. Settings may be raised above MARVS while in port
  • D. No requirement, as harbour settings are optional
Answer: A — Changes are made under the Master's supervision per approved procedures, recorded in the log, and the set pressure is displayed at the cargo control station and at each relief valve
IGC Code chapter 8 allows more than one relief valve setting only under the Master's supervision, following procedures in the cargo operations manual. The change must be recorded in the ship's log and a sign showing the set pressure posted at the cargo control station and at each valve.

Printing this? Use your browser's Print (Ctrl/⌘ + P) — the buttons, the search box and the site chrome drop out and the questions run on as plain text, so a set fits a few pages. Filter to one set first if you only want that set on paper.

Practise these GASCO questions as a timed paper All 710 GASCO questions and answers GASCO exit exam sets and syllabus All 24 STCW exit exam courses

Send this GASCO answer key to your batch

Send on WhatsApp

More free tools for seafarers

Mariners DigestBrief for seafarers, Tue–SatSIRE PRO: Ship InspectionTanker inspection prepRightShip ProRISQ 3.2 dry-cargo inspection prepCOLREGS ReaderRules of the road, read aloud