Liquefied Gas Carrier

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Cargo piping system for LNG carriers

LNG vessel cargo piping systems comprise of the following:
Liquid lines (or Headers)

Liquid lines are comprised of butt welded, cryogenic stainless steel pipeline connecting each of the cargo tanks to the cargo manifolds by means of a common line. At each tank, there is a manifold which connects to the loading and discharge lines from the tank to allow for the loading and discharge of cargo. This manifold connects to the cargo pump discharge lines, the loading line and the spray line. All sections of the liquid line outside the cargo tanks are insulated and covered with a moulded cover to act as a tough water and vapour tight barrier.
LNG carrier
Fig:LNG carrier underway

Vapour lines

The vapour lines are comprised of cryogenic stainless steel pipeline connecting each of the cargo tanks by means of a common line to the vapour manifold, the compressor room and the forward vent mast. The line to the compressor room allows for the vapour to be used in the following procedures:
The line to the forward vent mast acts as a safety valve to all tanks and is used to control the tank pressure during normal operations. All sections of the vapour line outside the cargo tanks are insulated and covered with a moulded cover to act as a tough water and vapour tight barrier.

Spray lines

The spray lines are comprised of cryogenic stainless steel pipeline connecting the spray pump in each tank to the stripping/spray header and serves the following functions by supplying LNG to:
  1. The spray rails in each tank, used for cooldown and gas generation.
  2. The main liquid line, used for cooling down lines prior to cargo operations.
  3. Priming of discharge lines in the cargo tanks to prevent line surge when starting cargo pumps.
  4. Supply of LNG to the vapourisers for gas generation to the compressors and heaters. All sections of the spray line outside the cargo tanks are insulated and covered with a moulded cover to act as a tough water and vapour tight barrier.

Fuel gas line

During transportation of LNG at sea, gas vapour is produced due to the transfer of heat from the outside sea and air through the tank insulation. In addition, energy is absorbed from the cargo motion due to the vesselís movement. Under normal power conditions, the boil-off is used as fuel in the shipís boilers.

The gas vapour is taken from the vapour header and passed on into the LD compressor. It then passes through the LD heater before going to the boilers.

The main gas isolating valve is located immediately forward of the accommodation block and a nitrogen purging connection will also be located at this point. From the point of entry into the machinery space, the pipe runs through a ventilated duct which is served by vent fans situated on the open deck to draw the surrounding air to the atmosphere. The vent duct is fitted with gas detection.

Vent line

During normal operations, the pressure in the tanks is controlled by the use of the boil-off gas in the boilers as fuel, or controlled via the forward vent mast and the common vapour line.

Each cargo tank is also fitted with an independent means of venting, comprising of two lines exiting the tank top into their own pilot operated relief valve. From here the gas passes through a line into a vent mast where it is vented to atmosphere.

All vent masts are protected by the N2 purge and fire smothering system. At certain points along the vent line, sample points are fitted to facilitate inerting and aeration of the system during refit. Sections of the vent line outside the cargo tanks are insulated with a rigid polyurethane foam covered with a molded GRP cover to act as a tough water and vapour tight barrier.

Inerting / Aeration line

The system comprises of a flanged line which supplies inert gas or dry air to the cargo tanks and pipelines for inerting and drying during refit periods.

The line is connected to the gas header and the liquid header by means of a spool piece. By selective use of the spool pieces and flexible hoses, it is possible to inert or aerate all tanks or a single cargo tank.

Below is our additional guideline for handling LNG cargo:

Connection and disconnection of cargo hoses and hard arms

Preparation for loading LNG cargo

Drying of Cargo Tanks and preparation for loading LNG cargo

Inerting of Cargo Tanks prior loading LNG cargo

Gassing-up requirement for cargo tanks

Initial Cool Down of cargo tanks

LNG spill risk during marine transportation and hazards associated

Toxicity and associated health hazards in liquefied Gas Carrier

Safety check items prior loading LNG cargo

Related Information:

  1. Liquefied gas carrier required cargo handling equipment

  2. Increased Cargo Capacity for LNG ships & Advantages of the dual fuel diesel electric propulsion

External links :

  1. LNG jornal, news, forums and latest industry news

Liquefied Gas Carriers !
Transporting bulk liquefied gases in trans-ocean services

Carrying and handling LNG cargo onboard poses significant potential hazards including risk of injury or death,threats to environment and each person working on a gas carrier and terminal ashore needs to understand the risks involved, obtain the necessary training and take all the needed precautions.

Defining various gas carrier types

Fuel flexibility of LNG ships

LNG shipment

Initial Cool Down of cargo tanks

Leaks on the Cargo System, Continuous Flow - how to prevent

LNG tank leaks and immediate action by gas carriers

Leaks from a Loading Arm due to Tidal or Current Effects

Minor or major leaks from LNG tanks

Procedures for LNG cargo discharging

Cargo conditioning, reliquefaction and boil-off control requirement for a liquefied gas carrier

Cargo Containment Systems in Liquefied Gas Carriers

cargo emergency shutdown requirement

damage stability guideline for liquefied gas carriers

Various Cargo handling equipments onboard

Cargo hoses connection guideline

Documents accompanying a liquid gas cargo

How LNG transferred from shore to ships cargo tanks ?

Cargo operation guideline onboard a liquefied gas carrier

Cargo piping Systems in Liquefied Gas Carriers

cargo planning requirement

cargo and pumproom safety precautions

cargo stripping guideline

Emergency response for cargo system leaks

Emergency response for cargo tank rupture

Risk of overfilling of cargo tank during loading onboard a liquefied gas carrier

Preparation for cargo transfer

cargo transfer between vessels- safety guideline

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