Liquefied Gas Carrier

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Principle type and design characteristics of modern liquefied gas carriers

Gas carriers range in capacity from the small pressurised tankers of between 500 and 6,000 m3 for shipment of propane, butane and the chemical gases at ambient temperature up to the fully insulated or refrigerated seagoing tankers of over 100,000 m3 capacity for the transport of LNG and LPG. Between those two distinct types is a third tanker type – semipressurised gas carrier.

These very flexible tankers are able to carry many cargoes in a fully refrigerated condition at atmospheric pressure or at temperatures corresponding to carriage pressure of between five and nine bar. The movement of liquefied gases by waterways is now a mature industry, served by a fleet of many tankers, a network of export and import terminals and a wealth of knowledge and experience on the part of various people involved.

Gas carriers have certain features common with other tankers used for the carriage of bulk liquids such as oil and chemical tankers.

A feature almost unique to the gas carrier is that the cargo is kept under positive pressure to prevent air entering the cargo system. This means that only cargo liquid and cargo vapour are present in the cargo tank and flammable atmospheres cannot develop.

Furthermore all gas carriers utilise closed cargo systems when loading or discharging, with no venting of vapour being allowed to the atmosphere.

In the LNG trade, provision is always made for the use of a vapour return line between tanker and shore to pass vapour displaced by the cargo transfer. In the LPG trade this is not always the case as, under normal circumstances during loading, reliquefaction is used to retain vapour on board. By these means cargo release to the atmosphere is virtually eliminated and the risk of vapour ignition is minimised.

Gas carriers are divided into two main groups.

Liquefied Petroleum Gas (LPG) Carriers, which are designed to carry mainly butane, propane, butadiene, propylene, vinyl chloride monomer (VCM) and are able to carry anhydrous ammonia.

Liquefied Natural Gas (LNG) Carriers, which are designed to carry liquefied natural gas (which is mostly methane).

LNG ship underway
Fig:LNG ship underway

Gas carriers are classed in three types based on hazard potential:

i) type 1G, designed to carry the most hazardous cargoes

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 hazardous nature.

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 of both.

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 vessel.

LNG carrier moss tanks
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.

Cargo handling equipment on a VLGC

A Very Large Gas Carrier has normally the capacity to load about 80,000m3 of LPG. In order for the gas carrier to safely load, unload and carry liquefied gases a set of systems and equipment installations are required: a VLGC is equipped with 4 IMO type A tanks of prismatic shape located below deck. The tanks are self-supporting and structurally independent of the ship’s hull.

Each tank is enclosed in a cargo hold and is bounded by the transverse bulkheads, the double bottom, the ship’s sides and the main deck. The tanks are resting on the double bottom on specially constructed supports. They are stiffened against rolling and pitching on the bottom and on the top, against rolling and floating. Each tank is equipped with a dome protruding the deck level. All connections for piping and equipment to the tanks are arranged through this dome. The equipment associated with cargo handling operations are cargo pumps, cargo booster pumps, cargo heater and vaporizer and cargo reliquefaction unit.

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

Related Information:

  1. Training requirement for working onboard various gas carriers types

  2. Construction materials used for cargo tanks

  3. Gas cargo containment systems - primary barrier (the cargo tank),secondary barrier, thermal insulation and more

  4. Preparatory operations for drydocking

We have extracted gas carrier images from the publication ‘LNG Shipping Knowledge’ by Witherby Seamanship

  1. Various type LPG tanker - Design characteristics and usability

  2. LPG tanker cargo work equipments & product line system

  3. LPG tanker cargo pipe line inspection and testing guideline

  4. Carriage of LPG cargo at sea & safety guideline

  5. LPG reliquefaction plant safety guideline

  6. Preparations for LPG cargo discharging, pumping & stripping guideline

  7. Preparations for loading compatible cargo onboard LPG tanker

  8. Preparation for changing different grade cargo or drydocking -LPG tanker guideline

  9. Cargo tank inerting prior to gassing up - LPG tanker procedure

  10. LPG cargo tank purging & safety guideline

  11. LPG cargo tank cooling safety procedure

  12. LPG cargo loading special guideline

  13. Tackling fire onboard LNG & LPG ships

  14. Detail guideline for Ballast operation at sea by LPG carrier

  15. Handling cargo related documents for LPG carrier

  16. Cargo sampling procedure for liquefied gas cargo

  17. Cargo measurement and calculation guideline for LPG carriers

  18. Handling Propylene oxide, Ethylene oxide mixtures

  19. Special characteristics of Vinyl Chloride Monomer & Butadiene

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