Fig:LNG ship underway
Gas carriers are classed in three types based on hazard
i) type 1G, designed to carry the most hazardous
ii) type 2G and 2PG, designed to carry cargoes
having a lesser degree of hazard
iii) type 3G, designed to carry cargoes of the least
Gas carrier types
All gas cargoes are transported in liquid form (ie they
are not carried as a gas in its vapour form) and, because
of their physical and chemical properties, they are
carried either at:
- pressures greater than atmospheric, or at
- temperatures below ambient, or a combination
Therefore, gas carriers are generally grouped as follows:
i) Fully Pressurised
ii) semi-pressurised and refrigerated
iii) fully refrigerated
Note. These grouping names are more prevalently used
when discussing the classes and types of LPG carriers
rather than LNG carriers.
In principle, the design is ‘a box within a box that is separated by a void
space’, similar in effect to the principle of a flask. Gas Carriers can be split
into two distinct groups. One is the liquefied natural gas (LNG) carrier. The
other is the liquefied petroleum gas (LPG) carrier.
LNG is mainly methane and ethane. LNG ships carry their cargo
at -161°C, at a relative density of approximately 0.600 with a volume contraction
ratio of 1 in 600. LNG cargo is carried at ambient pressure.
LPG is mainly propane and butane. LPG ships carry their cargo
at -42°C, at a relative density of approximately 0.500 with a volume contraction
ratio of 1 in 300. LPG cargo may be carried under pressure.
The cargo tank construction of LNG and LPG ships can be of (a) prismatic
design (b) membrane design or (c) spherical design. Materials used
for these cargo tanks can be aluminium, balsa wood, plywood, invar or
nickel steel, stainless steel, with pearlite and polyurethane foam.
Because of the demand for insulation at these extremely low cargo temperatures,
the first cost of these specialised ships are extremely high. A very
high standard of workmanship is required for the building of these types of
Fig:LNG carrier moss tanks
Their capacity ranges from 75000 to 138000m3 of gas, their LBPs up to
280 m and their Br. Mld from 25 to 46 m. When fully loaded, their CB can be
0.660 up to 0.680 with service speed in the range of 16–20.75 kt. They are
fine-form vessels .
Gas carriers must comply with the standards set by the Gas Codes or national rules, and
with all safety and pollution requirements common to other tankers.
The safety features
inherent in the tanker design requirements have helped considerably in the safety of these
tankers. Equipment requirements for gas carriers include temperature and pressure
monitoring, gas detection and cargo tank liquid level indicators, all of which are provided
with alarms and ancillary instrumentation. The variation of equipment as fitted can make
the gas carrier one of the most sophisticated tankers afloat today.
There is much variation in the design, construction and operation of gas carriers due to the variety of cargoes carried and the number of cargo containment systems utilized. Cargo containment systems may be of the independent tanks (pressurized, semi-pressurized or fully refrigerated) or of the membrane type.
Some of the principal features of these design variations are described in our additional pages below:Fully pressurized ships
Semi pressurized ships
Ethylene carriers Fully refrigerated ships
Various design LNG ships
We have extracted gas carrier images from the publication ‘LNG Shipping Knowledge’ by Witherby Seamanship
- Training requirement for working onboard various gas carriers types
- Construction materials used for cargo tanks
- Gas cargo containment systems - primary barrier (the cargo tank),secondary barrier, thermal insulation and more
- Preparatory operations for drydocking
- Various type LPG tanker - Design characteristics and usability
- LPG tanker cargo work equipments & product line system
- LPG tanker cargo pipe line inspection and testing guideline
- Carriage of LPG cargo at sea & safety guideline
- LPG reliquefaction plant safety guideline
- Preparations for LPG cargo discharging, pumping & stripping guideline
- Preparations for loading compatible cargo onboard LPG tanker
- Preparation for changing different grade cargo or drydocking -LPG tanker guideline
- Cargo tank inerting prior to gassing up - LPG tanker procedure
- LPG cargo tank purging & safety guideline
- LPG cargo tank cooling safety procedure
- LPG cargo loading special guideline
- Tackling fire onboard LNG & LPG ships
- Detail guideline for Ballast operation at sea by LPG carrier
- Handling cargo related documents for LPG carrier
- Cargo sampling procedure for liquefied gas cargo
- Cargo measurement and calculation guideline for LPG carriers
- Handling Propylene oxide, Ethylene oxide mixtures
- Special characteristics of Vinyl Chloride Monomer & Butadiene
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