GTM-8.5	Gas Tanker Manual GTM IMO Gas Code	Form No.: GTM-8.5 Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD

5. IMO GAS CODE

Three IMO codes are presently applicable to gas carriers:

• The Code for Existing Ships Carrying Liquefied Gases in Bulk (Res A.329ix of 12 Nov. 1975) is applicable to gas carriers built before 31st October 1976. This was accepted by most countries except U.S.C.G.
• The Code for the Construction and Equipment of Ships carrying Liquefied Gases in Bulk (Res A328ix of 12 November 1975) is applicable to gas carriers built between the 31st October 1976 and 1st July 1986.
• The International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC) - (Res MSC 5(48) of 17th June 1983 as amendment to SOLAS) is applicable to gas carriers built after 1st July 1986.

You are advised to ascertain into which category your vessel falls and rely upon the appropriate code thereof. The first two IMO codes are not mandatory although most countries and/or charterers require compliance. The IMO International Code IGC is mandatory for new ships. However, you may also find it possible that a vessel coming into our management might not comply with any Code because it may have been built to operate exclusively in countries which do not implement these IMO codes. Please obtain a copy of the relevant Code applicable to your vessel and advise V Ships of special remarks if any.

The code also lists the type of liquefied gases to which it applies, and the ship type for an application, and any special requirements for certain cargoes.

5.1 IMO SHIP TYPES

• Type 1G: intended for the transport of products which require maximum preventative measures to prevent the escape of such cargo.
• Type 2G: intended for the transport of products which require significant preventative measures to prevent the escape of such cargo.
• Type 2PG: is a vessel of 150M in length or less intended to carry products as indicated in Chapter 19 of the IGC code and which is fitted with Type C tanks designed for a MARVS (maximum allowable relief valve setting of a cargo tank) of at least 7 Bar gauge and a containment system design temperature of -55°C or above.
• Type 3G: is a vessel which requires moderate preventative measures to prevent the escape of cargo.

5.2 SURVIVAL CAPABILITIES

The Master of the ship will be supplied with a loading and stability book. This will contain details of typical service conditions, loading, unloading and ballasting operations, provisions for evaluating other conditions of loading and a summary of the ship’s survival capabilities.

The booklet will contain sufficient information to enable the Master to load and operate the ship in a safe and seaworthy manner.

5.2.1 DAMAGE STABILITY

The vessel must have onboard the approved Stability Information Book (SIB) which is written in the working language of the vessel. The Stability Booklet should include damage stability calculations for different loading conditions in case the vessel suffers side or bottom damage on the voyage.

The stability condition upon departure from a port must be compliant with both intact and damage stability criteria. These departure criteria can be checked against one of the approved cases within the Stability Information Booklet, or by running the loading conditions on the loading program. If the program does not take account of damage stability, the ship must manually evaluate the final loading conditions against the KG limit curves presented in the Damage Stability report. The Master and Chief Officer must make sure they are familiar with this approach.

If the vessel does not have damage stability functionality within the approved software and does not have KG / GM limit curves that incorporate damage stability requirements then the vessel has two options:-

1. The vessel must load according to the approved loading conditions in the stability information booklet (allowance of 2cm of GM and 1% of deadweight would be considered acceptable and not be treated as an alternate condition).
2. If the vessel has to load outside the approved conditions then the vessel should apply (well in advance) to have the planned condition approved by the vessel’s Classification Society and Flag Administration.

5.3 CARGO CONTAINMENT

• Type A tanks: are constructed of plain surfaces and suitable for low pressures only 0.7 Bar gauge i.e. fully refrigerated cargoes.
• Type B tanks: may be constructed of plain surfaces or pressure vessel type but to a much more stringent stress analysis than required for Type A.
• Type C tanks: are normally spherical or cylindrical pressure vessels with design vapour pressure higher than 2 bar gauge. Cylindrical vessels may be vertically or horizontally mounted. This type of containment system is always used in semi-refrigerated and fully pressurised liquid gas carriers; it is also commonly used for fully refrigerated transport provided appropriate low-temperature steels are used in the tank construction. Type C tanks are designed and built to conventional pressure vessel codes and, as a result, can be subjected to accurate stress analysis. Furthermore, design stresses are kept reasonably low so, where this type of system is used, no secondary barrier is required and the hold space can be filled with either inert gas or air.

Furthermore, design stresses are kept reasonably low so, where this type of system is used, no secondary barrier is required and the hold space can be filled with either inert gas or air.

In the case of a typical fully pressurised ship, i.e. where the cargo is carried at ambient temperature, the tanks may be designed for a maximum working pressure of 17 barg or more. For a semi-pressurised/fully refrigerated ship, the cargo tanks and associated equipment are designed for a maximum working pressure of approximately 5-7 barg and up to 50 per cent vacuum. The tank steels are typically capable of withstanding carriage temperatures of -48°C for LPG and -104°C for ethylene and LPG.

5.4 IMO SURVEYS

• An initial survey before the ship is put into service.
• A periodic survey not exceeding 5 years.
• A minimum of one intermediate survey.
• A mandatory annual survey within three months before or after the anniversary date of the IMO Certificate of Fitness.
• An additional survey general or partial whenever any important repairs or renewals are made.

The above surveys are additional to normal ship surveys.

5.5 GENERAL

These General International standards for the construction and equipment of ships carrying dangerous chemicals in bulk are defined in the IBC Code. When carriers have the capability of transporting chemical cargoes then the vessel must meet the requirements of both IGC and IBC codes as designated in Chapter 19 of the IGC Code.