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Sources of ignition on ships carrying LNG/LPG

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LNG fire protection and emergency response during transportation are critical components of ensuring the safe handling of liquefied natural gas (LNG). Given the highly flammable nature of LNG, robust safety measures and well-coordinated emergency response plans are essential.

By implementing the fire protection measures and emergency response plans described below, LNG can be transported safely while minimizing risks to people, property and the environment.

Sources of Ignition

The following are precautions commonly taken on board:

Matches and lighters

  • lighters are particularly dangerous and will therefore be prohibited on board gas carriers;
  • use only safety matches – other types are dangerous;
  • never carry matches or lighters on deck or into the cargo area;
  • the Master will decide when and where smoking is allowed (the shore/terminal authorities may also impose restrictions on smoking areas);
  • obey all instructions about smoking;
  • never smoke outside on the open deck;
  • secret smoking is more dangerous than controlled smoking;
  • never smoke in bed.

Galley

If cargo gases are likely to enter the galley, the cooking equipment should be shut down until the source of the leak has been located, stopped and the cargo gases dispersed.

Engine room

There are many possible sources of ignition in the engine room and gas carriers are designed to reduce the chances of cargo gases entering these spaces. Doors are located away from the cargo area and ventilation fan intakes are positioned at high level. Entrances to the engine room will be kept shut at all times and opened only for access.

Accommodation

Cargo gases must be excluded from the accommodation areas and the potential sources of ignition within them. All external doors and ports will be kept shut, and opened only for access, especially during loading and discharging operations. Engine room ventilation fans are high above the deck to prevent cargo gases entering these spaces and intakes are fitted with closing devices. Some doors may be fitted with airlocks and, if so, it is essential that these are used correctly.

Under IGC Code requirements, if the ship can carry toxic cargoes, ventilation intakes must be able to be secured from inside the accommodation.

Intrinsically safe equipment is described in detail in the article “Liquefied Petroleum Gas Reliquefaction Plant and Boil-Off ControlElectrical Equipment

Torches and portable lighting

Only certified safety torches of an approved type will be available on board.

Handheld radios

An unapproved radio can be a source of ignition. Therefore, only portable radios of an approved type that have a certification plate or stomp will be used.

Mobile telephones

Mobile telephones are known to be capable of igniting flammable vapours. If dropped, they may break open, exposing the battery and electrical circuit.

Fire and Fire-Fighting Management

On ships, the only mandatory requirement for fire detection equipment in the cargo area is the fusible plugs specified in the IGC Code. These have to be fitted in the vicinity of tank domes and at cargo manifolds. The fusible plugs provide for the automatic cargo shutdown in the event of fire. However, many ships have fire detectors installed in motor rooms and compressor rooms.

In terminals, where storage tanks and miscellaneous plant are located, fire detection equipment is extensively provided. Fire detectors are categorised as either heat, smoke, or flame detectors, and are chosen according to the materials that may burn in a particular location. Typical locations are electrical control rooms, boil-of gas (BOG) compressor houses and at cargo pumps.

Cargo related fires may be broadly categorised as follows:

The most important function of a fire-fighting system is the protection of personnel. The loss of equipment and product, the minimisation of environmental impact and protection of brand reputation are usually secondary considerations.

The guiding principle for fire control is that effective attack should normally be carried out as early as possible. In understanding how best to control a liquefied gas fire, the behaviour of gas spills, their characteristics and potential hazards, for example, will need to be understood. In certain circumstances, however, an appropriate strategy might be to stop the spillage and protect the surroundings while allowing the fire to burn itself out.

To help support this guiding principle, the following items, amongst others, will usually be considered:

  • ensuring that monitors are aligned to cover appropriate areas;
  • inspection of fire-fighting equipment on a regular basis. Such inspections will usually include the weighing of cylinders where necessary and the actuation of fire monitors;
  • he immediate reporting (and suitable maintenance) of any faulty fire-fighting equipment in the shortest possible time should be a responsibility of all concerned;
  • the provision for clear plans showing the fire-fighting equipment provided and its proper location;
  • the correct marking of all fire-fighting equipment showing its intended function and proper operation;
  • the provision for adequate emergency plans and procedures covering ship’s crew, terminal personnel and local authorities, as applicable;
  • the training (and refresher training) of all personnel who may be involved in fire-fighting. Training will usually be carried out in an area where realistic fires can be arranged under controlled conditions.

Fire-fighting facilities installed on board ships, in terminals and on jetties depend on factors such as location of the terminal, type and size of storage, sizes of ships and types of products handled. These facilities are more fully described in “Liquefied Gas Fire Hazard Management” (Reference 2.86).

Extinguishing Mediums

Water

Water is widely used to protect exposed plant and storage from fire and heat radiation. It may be used in the form of jets, sprays, fixed deluge systems or water curtain radiation screens. Water is a heat source for refrigerated product spills, promoting rapid evaporation of spilled product, and will, therefore, not be appropriate for use against a burning liquid gas pool. Being abundantly available, however, water is an excellent cooling agent for surfaces exposed to radiation or direct fire impingement.

Pypeline for water
Fig. 1 Deck spray line

Fixed deluge systems, designed to provide a layer of water over exposed surfaces, are customary for storage tanks and plant in potential fire areas. This provides a screen against fire radiation. Sprays from fixed monitors or handheld hoses can provide essential radiation protection for personnel in their approach to shut valves. Such spray shields are also beneficial in an approach to a jet fire, where an attack using dry chemicals to extinguish the flame is envisaged.

Deluge systems on the ship
Fig. 2 Deck and accommodation deluge systems

Fixed water deluge systems are customary for surfaces such as ships’ structures, deck tanks and piping, shore storage tanks, plant and jetties, all of which can be exposed to liquefied gas fires. Provided a water layer of sufficient thickness can be maintained, the surface temperature cannot exceed 100 °C. Application rates vary with the distance of the structure to be protected from the envisaged fire source and range from 2 to 10 + litres of water per square metre of protected surface.

Foam

High expansion foam, adequately applied to the surface of a burning liquid pool (when confined within a bunded area), suppresses the radiation from the flame into the liquid beneath and reduces the vaporisation rate. Consequently, the intensity of the pool fire is limited.

Foam applied to unignited LNG pools can reduce the horizontal extent of gas clouds because the heat input from the foam to the evolving vapour increases the vapour’s buoyancy. The foam, as it breaks down into the liquid beneath, may increase the vaporisation rate. However, if the foam is stable, it can freeze at the interface and thereby reduce vaporisation rates.

However, foam will not extinguish a liquefied gas fire and, while likely effective for the above purposes, needs to be applied to a substantial depth. For liquefied gases, therefore, foam is only appropriate for use in bunded areas (where the foam can be built up to sufficient depth) and, for this reason, it is only found at terminals and not on gas carriers.

Foam is normally applied by fixed monitors, controlled by remote means and suitably located around tank bund areas.

Dry chemical powders

Dry chemical powders such as sodium bicarbonate, potassium bicarbonate and urea potassium bicarbonate, can be very effective in extinguishing small LNG or LPG fires. These powders attack the flame by absorbing the free radicals in the combustion process, but have a negligible cooling effect. Just a few seconds application is required for mixing with the flame before the chemical begins to affect the fire. The time taken to extinguish the fire is a function of the burning rate and the application rate of the chemical.

Dry chemicals ma y be applied from fixed, mobile or portable equipment. A well-trained operator can extinguish a fire of about 100 m2 using a hand line or monitor nozzle discharging at the rate of 23 kg/sec. Larger areas require more operators or a number of fixed systems. Required application rates for successful extinguishment depend on wind speed and direction.

Gas carriers are required by the IGC Code to be fitted with fixed dry powder systems capable of delivering powder to any port of the cargo area by means of fixed monitors and handheld hoses.

Hose for Dry powder
Fig. 3 Dry powder hose and gun

It is also usual for jetty manifold areas to be protected by substantial portable or fixed dry powder systems. Dry chemical powders are effective in dealing with gas fires on deck or in extinguishing jet fires from a holed pipeline.

Dry chemicals should never be used in combination with sprayed water as it is important to keep the dry chemical dry and “fluid“.

The presence of objects such as steel supports may cause problems by shielding parts of the fire from the chemical. Because of their negligible cooling effect, powders can leave hot spots that may re-ignite after the initial extinguishment. Special attention should, therefore, be given to eliminating the source of spillage when using dry chemical powders. Dry chemicals are considered particularly effective in dealing with fires at sumps, tank vents and leaking flanges. Fixed and portable dry chemical extinguisher systems provide an important defence against liquefied gas fires on many jetties.

Carbon dioxide (CO2) systems

Enclosed spaces, on ships and in terminals, containing cargo plant such as compressors, heat exchangers or pumps, will normally be provided with a fixed and remotely activated fire extinguishing system such as CO2. Provided no major disruption to the enclosure has occurred, these systems should be immediately effective.

While many of these areas are protected by inert gas systems, because of the comparatively low rate at which such gas can be delivered, such systems are not used for the rapid inerting of an enclosed space in which a fire has already begun.

It is important to be sure that there actually is a fire in one of these enclosed areas, because CO2 systems are delivered as a “single shot“. While CO2 systems are effective in enclosed spaces they have a major disadvantage in that their fire extinguishing action is achieved by reducing oxygen in the space to a level that will not support combustion or life and it is, therefore, essential that all personnel evacuate the space before injection begins. All spaces protected by CO2 extinguishing systems will therefore have a safety placard to this effect.

A further concern is that the injection of CO2 produces electrostatic charging, which can be an ignition hazard if CO2 is injected inadvertently or as a precautionary measure into a flammable atmosphere.

After CO2 has been injected into an enclosed space, the boundaries of the space will be kept cool, usually with a water spray.

The space will remain sealed until it is established that the fire is extinguished and has sufficiently cooled so that it will not reignite with the introduction of oxygen.

CO2 or nitrogen injected into safety relief valve outlets may be used as an effective means of extinguishing vapour fires at the vent risers. This is particularly valuable once the initial pressure flow has subsided.

CO2 extinguishers may be of reduced value on open jetties, except for the local extinguishment of electrical fires in junction boxes and in similar equipment.

Alarm procedures

Each gas ship and terminal will have fire-fighting plans and muster lists prominently displayed and it will be expected that all personnel will have read and understood them. As a general guide, if a liquid gas fire occurs, the intended procedure will usually be to:

  • raise the alarm;
  • assess the fire’s source and extent and whether personnel are at risk;
  • implement the emergency plan;
  • stop the spread of the fire by isolating the source of fuel;
  • cool surfaces under radiation or flame impingement with water;
  • extinguish the fire with appropriate equipment or, if this is not possible or desirable, control the spread of the fire.

Raising the alarm and initial action

Fundamental to emergency procedures is how to report and how the alarm should be given to all concerned. These procedures will, by their nature, usually be developed independently for the terminal, the ship and the ship/shore system.

It will be interesting: Cargo Related Spaces on Liquefied Natural Gas Carriers

Commonly, procedures will warn that a seemingly minor may incident may quickly escalate to one of a more serious nature and that much may be gamed by immediately reporting any abnormal occurrence.

In the case of incidents on a ship, or on a jetty while a ship is alongside, the manpower and facilities immediately available on the ship will generally make it appropriate that the ship takes first action by initiating cargo transfer ESD (see article “Cargo equipment for gas carriers carrying LNG/LPGEmergency shutdown (ESD) systems“) by the agreed safe means, alerting the terminal to provide assistance as quickly as possible and immediately putting into action the ship’s own emergency procedure.

Training

For effective use of any of these fire-fighting systems a thorough knowledge of the capabilities of each will usually be essential. Speed in correctly tackling a fire is vital if escalation is to be minimised and life, property and environment safeguarded. Training of ship and shore personnel who may have to lead a fire party will usually be given in shore-based fire schools where fire extinguishing techniques can be demonstrated and practiced in a controlled environment. Training will commonly be consolidated by frequent exercises on board ship and in terminals and be realistically staged.

Inspection and maintenance of fire-fighting systems are commonly incorporated into on board and on-site training programmes, which are intended to help to familiarise personnel with the equipment and to provide them with a fuller understanding of their operation.

For further information on fire-fighting training for liquefied gas cargoes, see “Liquefied Gas Fire Hazard Management” (Reference 2.86).

Author
Author photo - Olga Nesvetailova
Freelancer
Literature
  1. The international group of liquefied natural gas importers (GIIGNL). LNG custody transfer handbook / 6th Edition: 2020-2021.
  2. ©Witherby Publishing Group Ltd. LNG Shipping Knowledge / 3rd Edition: 2008-2020.
  3. CBS Publishers & Distributors Pvt Ltd. Design of LPG and LNG Jetties with Navigation and Risk Analysis / 4th Edition.
  4. NATURAL GAS PROCESSING & ITS ENERGY TRANSITION ROLE: LNG, CNG, LPG & NGL Paperback – Large Print, November 14, 2023.
  5. OCIMF, ICS, SIGTTO & CDI. Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases / 1st Edition, 2013.
  6. The Society of International Gas Tanker and Terminal Operators (SIGTTO). Ship/Shore Interface / 1st Edition, 2018.

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Май, 20, 2024 80 0
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