Session 17: Gas Carrier

Question 1

What does boil off and boiling point mean?

Answer 1

Boil off: vapour produced above the surface of a boiling liquid due to evaporation

Boiling point: the temperature at which the product exhibits a vapour pressure equal to atmospheric pressure

Question 2

What does cofferdam and critical temp mean?

Answer 2

Cofferdam: isolating space between two adjacent steel bulkheads or decks

Critical temperature: temp above which a gas cannot be liquified by a pressure alone

Question 3

What’s the definition of liquified gas in general and the IMO definition?

Answer 3

Liquified gas: liquid form of a substance, at ambient temperature and at atmospheric pressure, would be a gas

IMO definition: defines a liquified gas as a liquid having a vapor pressure in excess of 2.8 bar absolute at 37.8 Celsius

Question 4

What publications do you need for gas carriers?

Answer 4

IGC Code (International gas carrier code)

SIGTTO: Society International gas tankers and terminal operators

Question 5

What certificate do you need for gas carrier?

Answer 5

International certificate of fitness for the carriage of liquified gases in bulk

Question 6

What are the 4 types of cargos that gas carrier can carry?

Answer 6

Liquified natural gas - methane

Liquified petroleum gas - Propane, butane

Ethylene

Ammonia

Question 7

What are some gas carrier types?

Answer 7

Fully refrigerated ships (cryogenic cargo/LNG

Fully pressurized ships (LPG)

Semi-pressurized ships (LPG)

Ethylene ships

Question 8

What are the two types of tanks and their features?

Answer 8

Independent tank - self supporting, not part of the ship’s construction(Hull)

Membrane tank - not self supporting, consist of a thin layer(membrane) supported through insulation which supported by ship’s hull, secondary barrier required to ensure integrity, for LNG only

Question 9

Describe me the type A tank?

Answer 9

-Prismatic shape
-Found on fully refrigerated LPG Carrier
-Self supporting tank which requires conventional internal stiffening
-tank is surrounded by a skin of foam insulation
-Type A capable of carrying cargoes below -10C

Question 10

Describe type B tank?

Answer 10

-Spherical shape
-LNG carrier
-tank requires only a partial secondary barrier in the form of a drip tray
-type B spherical tank is almost exclusively applied to LNG ships

Question 11

Describe type C tank?

Answer 11

-spherical or cylindrical pressure vessels
-type of containment system is always used for semi-pressurized and fully pressurized gas carriers
-tank steels for the semi-pressurized ships are capable of withstanding carriage temperatures of -48C LPG or -104C for ethylene

Question 12

Give me 3 features of membrane tanks

Answer 12

-membrane containment system is based on a very thin primary barrier (membrane 0.5 to 1.2 mm thick) which is supported through insulation

-Such tanks are not self-supporting like the independent tanks

-these systems must always be provided with a secondary barrier to ensure the integrity for the total system in the event of primary barrier leakage

Question 13

What is the main purpose of a linked ESD(Emergency, Shutdown, System)?

Answer 13

Main is to have safe ship and shore control over the entire ESD system. In order to ensure a safe shut-down in line with appropriate valve closure times.

The ESD system should allow for control from both the ship and a shore control center. This dual control ensures that critical decisions can be made and actions taken from both locations in the event of an emergency.

Question 14

Why is bunkering not allowed during cargo operations on gas carriers?

Answer 14

Bunkering during cargo operations alongside terminal from a bunker barge is not allowed, because bunker barge is a possible source of ignition.

Question 15

During cargo handling operations, why is cooling down necessary?

Answer 15

Cooling down is necessary to avoid excessive tank pressure (due to flash evaporation) during bulk loading

Question 16

What does the cooling down process consist of?

Answer 16

it consists of spraying cargo liquid into a tank at a slow rate. The lower the cargo carriage temperature, the more important the cool down temperature.

Question 17

Before loading a refrigerated cargo, what does one must do?

Answer 17

A ship’s tank must be cool down slowly in order to minimize thermal stresses. The rate at which the cargo tank can be cooled, without creating high thermal stress, depends on the design of containment system and is typically 10C per hour

Question 18

what are 5 general steps before loading the cargo?

Answer 18

-drying the tank using dry air
-inerting or nitrogen
-gassing up
-cooling down the tank with cargo vapor
-slowly load the cargo