Figure 3.7 Compressor room/electric motor room on a gas carrier
The Gas Codes detail the requirements for mechanical ventilation of these rooms. Positive pressure ventilation must be provided for the electric motor room and negative pressure ventilation for the cargo compressor area. This ensures an appropriate pressure differential between the rooms. An airlock entrance to the electric motor room from the ship's deck, with two gastight doors at least 1.5 metres apart, prevents loss of air pressure on entry. To ensure that both doors are not opened simultaneously they must be self-closing with audible and visual alarms on both sides of the airlock. (However an airlock is required only where access to the motor room is within 2.4 metres of the ship's main deck). In addition, loss of over-pressure in the motor room should trip the electric motors within.
Another safety feature associated with the compressor room area concerns the sealing of the drive-shafts penetrating the gas-tight bulkhead between the compressor and motor room and this is discussed further in Chapter Four.
The cargo containment and handling systems must be completely separate from the accommodation and machinery spaces. A cofferdam, or other means of gas-tight segregation, is required between the cargo area and the engine room and fuel tanks. The Gas Codes also give specific advice for positioning doors leading from accommodation spaces into cargo areas. In addition, air intakes for accommodation and engine spaces must be sited away from cargo vent risers. All air intakes into accommodation and service spaces should be fitted with closing devices.
Gas tankers are fitted with a fixed water spray system for fire protection purposes. This covers areas such as:
· Cargo tank domes
· Cargo tank areas above deck
· Cargo manifold areas
· The front of the accommodation including lifeboat boarding areas, and
· Control room bulkheads facing the cargo-deck
Minimum water flow rates of 10 litre/m2 per minute for horizontal surfaces and 4 litre/m2 per minute for vertical surfaces must be achieved. In addition to the fixed water spray systems, all gas tankers must be fitted with a fixed dry powder installation capable of fighting fires in the cargo area. At least two hand-held hoses and a minimum of one fixed monitor must be provided to cover the deck area. The dry powder installation is activated by nitrogen pressure which is stored in cylinders adjacent to the powder containers.
Finally, cargo tanks cannot be used for ballast purposes and separate ballast tanks are required.
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3.6 SURVIVAL CAPABILITY
The Gas Codes divide gas carriers into four categories; Ship Types 1G, 2G, 2PG and 3G, according to the hazard rating of the cargoes for the carriage of which the ship is certified. This categorisation can be seen in Appendix 2. For example, Type 1G ships (where the cargo tanks are located at the greatest distance from the side shell and which may also be restricted in capacity) must be used for cargoes representing the greatest hazard, such as chlorine. Ship Types 2G/2PG and 3G can carry cargoes which represent progressively decreasing environmental hazards and, therefore, progressively less stringent structural requirements in respect of damage survival capability in the event of collision or grounding.
A fully refrigerated ship, say with Type 'A' tanks, designed for LPG, must comply with the requirements for tank location and survival capability of a category 2G ship whereas a semi-pressurised ship, with Type 'C' tanks carrying LPG can comply with the requirements either of a 2G or a 2PG ship. For the latter case, the Type 'C' pressure vessels must have a design pressure of at least 7 barg, and a design temperature not lower than -55°C. The 2PG category takes into account the fact that the pressure vessel design provides increased survival capability when the ship is damaged by collision or grounding.
The Gas Codes and classification society rules should be referred to for the detailed construction requirements for each category of ship.
3.7 SURVEYS AND CERTIFICATION 3.7.1 Certificate of Fitness
A ship complying with an older IMO Gas Code (Existing Ship or GC Code) should be surveyed and issued with a Certificate of Fitness for the Carriage of Liquefied Gases in Bulk. This certificate remains valid, dependant on the results of intervening surveys, for a period of five years assuming the ship has not changed flag. (Where the ship is designed in accordance with the new IGC Code, then the certificate is called the International Certificate of Fitness for the Carriage of Liquefied Gases in Bulk). Such certificates signify that a minimum standard of constructional safety has been achieved and list the cargo grades which the ship is certified to carry.
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To comply with the relevant Gas Code, throughout the period of validity of the certificate, a gas carrier must be re-surveyed periodically. Such surveys ensure the continuing validity of the Certificate of Fitness.
Surveys required to maintain the validity of the Certificate of Fitness are listed below:—
· An Initial Survey before the ship is put into service
· A Periodical Survey before the end of a period not exceeding five years (a new Certificate of Fitness is issued)
· An Intermediate Survey half way between each Periodical Survey
· An Annual Survey
· An Additional Survey after serious accident or important renewals
3.7.2 Other certification
The surveys as outlined in 3.7.1 are, of course, additional to usual cargo-ship certification as required under various IMO conventions, where the surveys are carried out by the individual flag state administration.
This certification will include:
· International Tonnage Certificate
· International Load Line Certificate
· International Oil Pollution Prevention Certificate
· Cargo Ship Safety Construction Certificate
· Cargo Ship Safety Equipment Certificate
· Cargo Ship Safety Radio Certificate
Chapter 4
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