Gas carriers built after June 1986 (the IGC Code)
The Code which applies to new gas carriers (built after 30th June 1986) is the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk. In brief, this Code is known as the IGC Code. The IGC Code, under amendments to Safety of Life at Sea Convention (SOLAS), is mandatory for all new
ships. As proof that a ship complies with the Code, an International Certificate of Fitness for the Carriage of Liquefied Gases in Bulk should be on board.
In 1993, the IGC Code was amended and the new rules came into effect on 1st July 1994. Ships on which construction started on or after 1st October 1994 should apply the amended version of the Code but ships built earlier may comply with previous editions of the IGC Code.
Gas carriers built between 1976 and 1986 (the GC Code)
The regulations covering gas carriers built after 1976 but before July 1986 are included in the Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk. It is known as the Gas Carrier Code or GC Code in short.
Since 1975, IMO has approved four sets of amendments to the GC Code. The latest was adopted in June 1993. It should be noted that all amendments are not necessarily agreed by every government. Although this Code is not mandatory, many countries have implemented it into national law. Accordingly, most charterers will expect such ships to meet with Code standards and, as proof of this, to have on board a Certificate of Fitness for the Carriage of Liquefied Gases in Bulk.
Gas carriers built before 1977 (the Existing Ship Code)
The regulations covering gas carriers built before 1977 are contained in the Code for Existing Ships Carrying Liquefied Gases in Bulk. Its content is similar to the GC Code, though less extensive.
The Existing Ship Code was completed in 1976 after the GC Code had been written. It therefore summarises current shipbuilding practice at that time. It remains as an IMO recommendation for all gas carriers in this older fleet of ships. The Code is not mandatory but is applied by some countries for ship registration and in other countries as a necessary fulfilment prior to port entry. Accordingly, many ships of this age are required by charterers to meet with Code standards and to have on board a Certificate of Fitness for the Carriage of Liquefied Gases in Bulk.
Some of the factors to be taken into consideration which affect the design of gas ships are:—
· Types of cargo to be carried
· Condition of carriage (fully pressurised, semi-pressurised, fully refrigerated)
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· Type of trade and cargo handling flexibility required by the ship
· Terminal facilities available when loading or discharging the ship
Perhaps more than any other single ship type, the gas tanker encompasses many different design philosophies. Nowhere is this more apparent than in considering the different types of cargo containment system which have been adopted.
3.2 CARGO CONTAINMENT SYSTEMS
A cargo containment system is the total arrangement for containing cargo including, where fitted:
· A primary barrier (the cargo tank),
· Secondary barrier (if fitted),
· Associated thermal insulation,
· Any intervening spaces, and
· Adjacent structure, if necessary, for the support of these elements.
For cargoes carried at temperatures between -10°C and -55°C the ship's hull may act as the secondary barrier and in such cases it may be a boundary of the hold space.
The basic cargo tank types utilised on board gas carriers are in accordance with the list below:—
| • Independent Type 'A' • Independent Type 'B' • Independent Type 'C' • Membrane |
Some other types such as:
• Internal insulation Type '1'
• Internal insulation Type '2'
• Integral
have been fully designed and approved but have not been commercially used yet.
3.2.1 Independent tanks
Independent tanks are completely self-supporting and do not form part of the ship's hull structure. Moreover, they do not contribute to the hull strength of a ship. As defined in the IGC Code, and depending mainly on the design pressure, there are three different types of independent tanks for gas carriers: these are known as Types 'A', 'B' and 'C'.
Type 'A' tanks
Type 'A tanks are constructed primarily of flat surfaces. The maximum allowable tank design pressure in the vapour space for this type of system is 0.7 barg; this means cargoes must be carried in a fully refrigerated condition at or near atmospheric pressure (normally below 0.25 barg). Figure 3.1 shows a section through this type of
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