Particular attention should be paid to pneumatic valves and control systems which can freeze in cold weather if the control air supply is damp, and to relief valves and cooling water systems. If fitted, heating systems should be used. Any water collected on the discharge side of relief valves should be drained off. Cooling water systems should either be dosed with anti-freeze or drained.
If a system is drained, the fact should be logged and the system refilled before subsequent use.
Water in fire main or spray systems should be circulated continuously or drained where there is a risk of freezing. Attention should be paid to emergency showers or eye-wash stations to ensure availability of facilities.Cold weather can also cause cargo vapour trapped in rotating equipment (e.g. in a cargo compressor) to condense, enter the crankcase and dilute the lubricating oil, and cause damage.
Crankcase heaters should be used if fitted, and started in ample time before running up cargo compressors (see paragraph 1.8.4).
In addition, when the liquefied gases being handled present a health hazard, further notices in appropriate languages should be prominently displayed stating:
Local regulations may require additional notices and such requirements should be observed.
Cargo vapour, whether toxic or flammable, should be vented to atmosphere with extreme caution, taking account of regulations and weather conditions (see Section 2.8).
If the temperature of the vented vapour is below atmospheric dewpoint, clouds of condensed water vapour will form. Condensed water vapour (fog) is heavier than air whereas the cargo vapour may or may not be heavier than air, depending on temperature. The cargo vapour cloud is likely to be oxygen deficient, and should only be entered by personnel wearing breathing apparatus. Furthermore, it should never be assumed that the cargo vapour is contained entirely within the boundaries of the visible water vapour cloud.
If the cargo vapour is heavier than air it may accumulate on deck and enter accommodation spaces. The precautions in Section 2.10 should therefore be observed. In some cases it may be possible to heat vapour before venting to reduce its density and assist dispersion. If such facilities are provided they should be used.
Regulations require that superstructures are designed with certain portholes fixed shut and openings positioned to minimise the possibility of vapour entry. These design features should nc be modified in any way.
All doors, portholes and other openings to gas-safe spaces should be kept closed during cargo operations. Doors should be clearly marked if they have to be kept permanently closed in port, but in no circumstances should they be locked.
Mechanical ventilation should be stopped and air conditioning units operated on closed cycle or stopped if there is any possibility of vapour being drawn into the accommodation.
2.11.1 Combustion Equipment
Boiler tubes, uptakes, exhaust manifolds and combustion equipment should be maintained in good condition as a precaution against funnel fires and sparks. In the event of a funnel fire, or if sparks are emitted from the funnel, cargo operations should be stopped and, at sea, the course should be altered as soon as possible to prevent sparks falling onto the tank deck.
2.11.2 Blowing Boiler Tubes
Funnel uptakes and boiler tubes should not be blown in port.
At sea they should only be blown in conditions where soot will be blown clear of the tank deck.
2.11.3 Cargo Vapour
Care should be taken to ensure that cargo vapour does not enter the engine or boiler room from any source. Special attention should be paid to engine room equipment connected to the cargo plant e.g. the inert gas plant, with its cooling water system. Particular care is necessary if LNG cargo vapour is used as fuel (see paragraph 4.9.3).
If malfunction of equipment, explosion, collision or grounding damage should give rise to a situation where cargo vapour is likely to enter the machinery space, immediate consideration should be given to its possible effect on the operation of equipment. Any necessary action should be taken; e.g. isolating the source, closing access doors, hatches and skylights, shutting down mechanical ventilation system, auxiliary and main machinery, or evacuation.
Apart from the obvious hazards, diesel engines are liable to overspeed and destroy themselves if flammable vapour is present in the air supply, even at concentrations well below the lower
Cargo vapour may be present in cargo pump or compressor rooms, and gas detection systems are installed to warn of its presence. In ships carrying cargoes whose vapours are lighter than air (e.g. ammonia) and heavier than air (e.g. LPG) gas detector points are fitted at high and low levels and the relevant detector points should be used for the cargo carried.
Ventilation systems are provided to disperse any vapour that may collect in the pump or compressor room. The space should be ventilated for at least ten minutes before cargo operations begin and throughout their duration, and also if liquid or vapour leakage is suspected.
Ventilation systems should be maintained carefully; if the fans fitted are of non-sparking design their design features should not be modified in any way.
The precautions given in Section 6.3 should be observed before personnel enter cargo machinery rooms.
Lighting systems in cargo machinery rooms must be certified flame proof. It is essential to ensure that such systems are properly maintained. Additional lighting, if required, should be of a suitably safe type (see paragraph 3.5.2).
Gas-tight bulkhead gland seals and air lock doors to cargo machinery electric motor rooms should be carefully checked and maintained to ensure that cargo vapour does not enter.
At all times during discharge, loading and ballasting operations the ship should have adequate stability and suitable trim to allow for departure at short notice in the event of an emergency.
While berthed at a terminal the ship's boilers, main engines, steering machinery, mooring equipment and other essential equipment should be kept ready to permit the ship to move from the berth at short notice, and in accordance with the terminal regulations.
Repairs and other work which may immobilise the ship should not be undertaken at a berth without the prior written agreement of the terminal. It may also be necessary to obtain permission from the local Port Authority before carrying out such work.
The normal high standards of navigation should be maintained and any navigational restrictions (routeing, reporting requirements etc) should be observed. If the ship is permitted to burn LNG vapour in the main machinery at sea, it may be necessary to change over to oil fuel when manoeuvring or when entering restricted or territorial waters.
It is the responsibility of the master or those in charge of transfer operations involving cargo or bunkers to know the applicable pollution prevention regulations and to ensure that they are not violated. Exercises should be held to train personnel in accordance with the Shipboard Oil Pollution Emergency Response Plan, and recorded.
There is a danger of violating pollution prevention regulations if ballast taken on in polluted waters is discharged in another port. If ballast has to be taken on in polluted areas, it may be necessary to exchange it for clean ballast when in deep water on passage. Some terminals have specific requirements in this respect, and the master should ensure that they are observed.
flammable limit (LFL). It is recommended that diesel engines are fitted with a valve on the air intake to stop the engine in these circumstances.
Fire-fighting appliances should always be kept in good order, tested regularly, and available for immediate use at all times (see Section 3.8).
Gas carriers are recommended not to undertake routine helicopter operations unless a purpose- built helicopter platform is provided. Whenever helicopter services are used the safety measures recommended in the ICS 'Guide to Helicopter/Ship Operations' should be taken into account.
This chapter addresses the hazards presented by flammable liquefied gases and vapour emissions, and recommends practices to prevent the risk of fire. Information is also provided on precautions against the dangers of inhaling vapour and of fire hazards from sources other than the cargo.
The avoidance of cargo fires depends upon preventing flammable cargo vapour, oxygen and sources of ignition coming together.
Cargo vapours in flammable concentrations are likely to be present in areas such as cargo tanks, cargo machinery spaces and at times on deck. It is essential that all possible sources of ignition are eliminated from these areas, both by design and operation.
Sources of ignition are inevitably present in spaces such as the accommodation, galleys and engine rooms, and it is essential to prevent cargo vapour entering these spaces.
Personnel should be continuously on their guard, not only against the more obvious dangers, but also against unforeseen circumstances which could lead to flammable vapours and sources of ignition coming together.
It is the vapour given off by a liquid and not the liquid itself which burns. A mixture of vapour and air cannot be ignited unless the proportions of vapour and air lie between two
concentrations known as the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL). The limits vary according to the cargo (see data sheets). Concentrations below the lower limit (too lean) or above the upper limit (too rich) cannot burn. However, it is important to remember that concentrations above the upper limit can be made to burn by diluting them with air until the mixture is within the flammable range, and that pockets of air may exist in any system, leading to the creation of a flammable mixture.
A liquid has to be at a temperature above its flash point before it evolves sufficient vapour to form a flammable mixture. Many liquefied gas cargoes are flammable, and since they are shipped at temperatures above their flash points flammable mixtures can be formed.
The source of flammable material may be vapour from the cargo, or from anything else that will burn. Oxygen normally derives from the atmosphere, which contains approximately 21 % oxygen by volume. Ignition can be caused by anything capable of providing the necessary energy, such as a naked flame, an electrical or electrostatic spark, or a hot metal surface.
Fire is prevented by ensuring that at least one of these three elements is excluded.
In the presence of a flammable substance, sources of ignition or oxygen should be excluded.
Oxygen can be restricted to a safe level within the cargo system by keeping the pressure above atmospheric pressure with cargo vapour or inert gas. Many sources of ignition are eliminated during the design stage and care should be taken to ensure that design features are not impaired in any way. Other sources of ignition need to be excluded by correct operational practices.
Where sources of ignition and oxygen are likely to be present, such as in accommodation, engine and boiler rooms, galley, motor rooms etc., it is vital to exclude flammable vapour. Particular care is necessary if there is a direct connection between the engine room and the cargo system (e.g. when cargo vapour is burnt as fuel, see paragraph 4.9.3), or if the inert gas plant is located in the engine room.
3
Liquefied gas cargoes are usually carried either fully refrigerated or pressurised in order to avoid loss of cargo. Cargo vapour is evolved and is normally treated in the following ways:
• During loading, vapour is displaced by cargo liquid; this vapour is either reliquefied and returned to the tanks as a boiling liquid or returned to shore through a vapour return line.
• During carriage, the cargo will boil off because of heat transfer through the insulation. In this case the vapour is either reliquefied or (in the case of LNG only) burnt in the main engines. If the cargo system is fully pressurised any vapour will be retained within the cargo tank.
• During gas-freeing at sea, the vapour is normally a mixture of cargo vapour and inert gas or inert gas and air. It cannot be used as fuel or reliquefied, and is vented to atmosphere. During gas-freeing in port, the vapour is returned through a shoreline.
Whatever methods are provided for handling vapour, it is essential to ensure that they function properly and are operated correctly. Failure to do so may create a hazard to the ship, the ship's crew or the environment.
3.4.1 General
When carrying a flammable cargo the cargo system contains liquid and vapour. The atmosphere around the cargo tanks is normally inerted to prevent the formation of flammable mixtures. The IMO Codes use the term 'environmental control' to describe this process. The precautions necessary to ensure safety are dealt with in the following paragraphs.
3.4.2 Hold and Interbarrier Spaces
These spaces may have to be filled with inert gas if the cargo is flammable. Different cargo containment systems require different procedures, as follows:
Containment System Hold or Interbarrier Space Atmosphere Full secondary Dry inert gas or nitrogen;
barrier maintained with make-up gas provided by the shipboard inert gas generation system, or by shipboard storage which should be sufficient for at least 30 days at normal rates of
consumption
Partial secondary Dry inert gas or nitrogen;
barrier maintained with make-up gas provided by the shipboard inert gas generation system, or by shipboard storage which should be sufficient for at least 30 days at normal rates of
consumption. Alternatively, subject to certain conditions, the space may be filled with dry air (see Regulation 9.2.2.2 of the IGC Code).
No secondary Dry air or dry inert gas depending on the cargo;
barrier maintained either with dry air provided by suitable air drying equipment, or with make-up inert gas provided by the shipboard inert gas generation system or shipboard storage.
3.4.3 Cargo Tanks and Piping Systems
The formation of a flammable vapour mixture in the cargo system should be prevented by replacing the air in the system with inert gas before loading, and by removing cargo vapour by inert gas after discharge, prior to changing cargoes or gas-freeing. Suitable pipe connections should be provided for this purpose. Inerting should be continued until the concentration of oxygen or cargo vapour in the space is reduced to the required level. The tank atmosphere
should be monitored at different levels to ensure there are no pockets of excessive concentrations of oxygen or cargo vapour, particularly in tanks with complex internal structures or bulkheads.
Some cargoes require the oxygen content in the vapour space to be kept extremely low (in some cases less than 0.2%) to prevent a chemical reaction occurring. For instance, ethylene
oxide/propylene oxide mixtures can decompose spontaneously unless special precautions are taken to control the atmosphere; and butadiene can react with oxygen to form unstable peroxide compounds. The oxygen content in the tanks must be reduced as necessary before loading begins. While such cargoes remain on board, oxygen is excluded either by keeping the ullage space full of inert gas at a positive pressure or, in the case of butadiene, by keeping the cargo vapour above atmospheric pressure. In every case, shippers' requirements should be strictly observed.
3.4.4 Inert Gas Quality
Inert gas used for atmosphere control should be suitable for the intended purpose, regardless of source. In particular it should:
• be chemically compatible with the cargo and the materials of construction throughout the full range of operating temperatures and pressures;
• have a sufficiently low dewpoint to prevent condensation, freezing, corrosion, damage to insulation etc. at the minimum operating temperature;
• have an oxygen concentration not exceeding 5%, but as low as 0.2% if the cargo can react to form peroxides;
• have a low concentration of CO2 to prevent it freezing out at the anticipated service temperature (see paragraph 4.6.1);
• have minimal capacity for accumulating a static electrical charge.
3.4.5 Inert Gas Hazards and Precautions
The main hazard associated with inert gas is asphyxiation of personnel due to lack of oxygen.
Asphyxiation can occur in those parts of the cargo system which have been inerted, or in other enclosed spaces into which inert gas has leaked. Nobody should enter spaces which are not in common use until it has been established that the atmosphere will support life (see Chapter 6).
As the inert gas plant is often situated in the engine room, great care should be taken to ensure that cargo vapour does not flow back along inert gas supply lines; non-return valves should be tested for effectiveness, at regular intervals. Any temporary connection between the inert gas plant and the cargo systems should be disconnected and tightly blanked after use.
If a liquid nitrogen system is used, care should be taken to avoid contact with skin and eyes, or severe cold burns will be caused. Any metal structure or component likely to come into contact with liquid nitrogen could suffer brittle fracture unless it has been designed for a service temperature of -196°C. Great care should be taken to ensure that vaporisers are used correctly.
3.5.1 Smoking
Company policy and local regulations should be strictly observed.
Smoking can only be permitted under controlled conditions. The designated smoking places on a gas carrier must be known to the crew, and when in port should be agreed in writing between the master and the terminal representative before cargo operations start. The master is
responsible for ensuring that all persons on board the tanker are informed of the places in which smoking is permitted, and for posting suitable notices.
The agreed smoking places should be confined to locations abaft all cargo tanks, and should not have doors or portholes which open directly to open decks.
The use of matches and cigarette lighters outside designated smoking spaces should be
prohibited. The risks involved in carrying matches and, more particularly, cigarette lighters should be impressed on all personnel. The use of lighters should be discouraged. Matches used on board should be of the safety type.