Liquefied Gas Carrier

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LNG operating instructions - various important terms

Absolute Vapour Density – The mass of a unit volume of gas under stated conditions of temperature and pressure.

Adiabatic – Without transfer of heat.

Administration – The government of the country in which the ship is registered (Flag Administration).

Approved Equipment – Equipment of a design that has been tested and approved by an appropriate authority such as an Administration or Classification Society.

BOG – Boil off gas, vapour produced above a cargo liquid surface due to evaporation caused by heat ingress.

Asphyxia – When a person is deprived of an adequate supply of oxygen a loss of consciousness will follow.

Asphyxiant – A gas or vapour which has no toxic properties but when present in sufficient concentration excludes oxygen and will cause asphyxia.

Auto-ignition Temperature – The lowest temperature to which a solid liquid or gas requires to be raised to cause self-sustained combustion without initiation by spark or flame. Bar Gauge – Pressure in bar.

Boil Off – Vapour produced above a cargo liquid surface due to evaporation caused by heat ingress.

Boiling Point – The temperature at which the vapour pressure of a liquid equals that of the atmosphere above its surface. This temperature varies with pressure.

Bonding (Electrical) – The connecting together of electrically conducting metal parts to ensure electrical continuity.

Brittle Fracture – Fracture of a material caused by a lack of ductility in the crystal structure resulting from low temperature.

Cargo Area – That part of the ship, which contains the cargo containment system, cargo pumps and compressor rooms.

Cargo Containment System Independent Tanks – Independent tanks are self-supporting. They do not form part of the ship’s hull and are not essential to the hull’s strength.

Type A – Are tanks, which are designed primarily using “Recognized Standards” of classical ship structural analysis procedures.

Type B – Are tanks which are designed using model tests, refined analytical tools and analysis methods to determine stress levels, fatigue life and crack propagation characteristics.

Type C – Referred to as pressure vessels are tanks meeting pressure vessel criteria.

Cargo Operations – Any operation on board a gas carrier involving the handling of cargo liquid or vapour including cargo transfer, reliquefaction, venting etc.

Cavitation – Uneven flow caused by vapour pockets within a liquid.

Certified Gas Free – Means that a tank, compartment or container has been tested using an approved instrument and proved to be sufficiently free at the time of the test of toxic or explosive or inert gas for a specified purpose, such as hotwork by an authorized person. E.g. shore chemist and that a certificate to this effect has been issued.

Chemical Absorption Detector – (e.g. Draeger Tubes) – Used for the detection and determining the concentration of gases or vapours based on the principle of a reaction between the gas and a chemical agent, the gas discolours the agent.

Closed Gauging System – A system whereby the contents of a tank can be measured by means of a device which penetrates the tank but is part of a closed system e.g. float system, electronic probe, magnetic probe.

Combustible Gas Indicator – An instrument for detecting a combustible gas/air mixture.

Critical Temperature – The temperature above which a gas cannot be liquefied by pressure alone.

Explosion Proof Enclosure – One, which will withstand, without injury, any explosion of the prescribed flammable gas that may occur within it under practical conditions of operation within the rating of the apparatus and will prevent the transmission of flame such as will ignite the prescribed flammable gas, which may be present in the surrounding atmosphere.

Filling Limit (or Ratio) – The volume of a tank expressed as a percentage of the total volume, which can be safely filled, having due regard to the possible expansion (and change in density) of the liquid.

Flame Arrestor – A device used in gas vent lines to arrest the passage of flame into enclosed spaces.

Flammable – Capable of being ignited and burning in air. The term “flammable gas” is used to mean a vapour air mixture within the flammable range.

Flammable Range – The flammable (explosive) range, lies between the minimum and maximum concentrations of vapour in air, which form flammable (explosive) mixtures. This is usually abbreviated to LFL (Lower Flammable Limit) and UFL (Upper Flammable Limit). These are synonymous with “Lower Explosive Limit” (LEL) and Upper Explosive Limit” (UEL).

Flash Point – The lowest temperature at which a liquid gives off sufficient vapour to form a flammable mixture with air near the surface of the liquid or within the apparatus used. This is determined by laboratory testing in a prescribed apparatus.

Gas-Dangerous Space or Zone – A space or zone within the cargo area which is designated as likely to contain flammable vapours and which is not equipped with approved arrangements to ensure that its atmosphere is maintained in a safe condition at all times.

Gas Free – Gas free means that a tank, compartment or container has been tested using appropriate gas detection equipment and found to be sufficiently free, at the time of the test, of toxic or explosive or inert gases for a specific purpose.

Gas-Freeing – The introduction of fresh air into a tank, compartment or container to remove toxic, flammable or inert gas to such a level as is required for a specified purpose (e.g. tank entry, hot work).

Gas-Safe Space or Zone – A space or zone not designated as a gas-dangerous space or zone.

Gauze Screen (sometimes called Flame Screen) – A portable or fixed device incorporating one or more corrosion resistant wire meshes used for preventing sparks from entering an open deck hole, or for a SHORT PERIOD OF TIME preventing the passage of flame, yet permitting the passage of gas.

Hot Space – The space enclosed by the ship’s structure in which a cargo containment system is situated (see Cargo Containment System).

Hot Work – Hot work is any work involving welding or burning, and other work including drilling, grinding, electrical work and the use of non-intrinsically safe electrical equipment, which might produce an incendiary spark.

Hot Work Permit – A document issued by an authorized person permitting specific work for specified time to be done in a defined area employing tools and equipment which could cause ignition of flammable gases.

Hydrates – The compounds formed at certain pressures and temperatures by the interaction between water and hydrocarbons.

IMO – International Maritime Organization: this is the United Nations specialized agency dealing with maritime affairs.

IMO Codes – The IMO Codes for the Design, Construction and Equipment of Ships carrying Liquefied Gases in Bulk.

IGC Code – The International Code for the Construction and Equipment of Ships carrying Liquefied Gases in Bulk.

Inert Gas – A gas or vapour, which will not support combustion and will not react with the cargo.

Inerting – This is the introduction of inert gas into a space to reduce and maintain the oxygen content to a level at which combustion cannot be supported.

Inhibited Cargo – Any cargo which contains a inhibitor.

Inhibitor – A substance used to prevent or retard cargo deterioration or a potentially hazardous chemical reaction.

Insulating Flange – An insulating device placed between metallic flanges, bolts and washers, to prevent electrical continuity between pipelines, sections of pipelines, hose strings and loading arms or equipment/apparatus.

Intrinsically Safe – Intrinsically safe equipment, instruments, or wiring are such equipment, instruments or wiring that are capable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmosphere mixture in its most easily ignited concentration.

Liquefied Gas – A liquid, which has an absolute vapour pressure exceeding 2.8kp/cm2 at 37.8° C, and certain other substances specified in the IMO Codes.

LNG – Liquefied natural gas, principal constitute of LNG is Methane.

LPG – Liquefied Petroleum Gases – these are mainly propane and butane, shipped either separately or in mixtures. They may be refinery by product gases or may be produced in conjunction with crude oil or natural gas.

MARVS – The maximum Allowable Relief Valve Setting of a cargo tank.

Peroxide – A compound, formed by the chemical combination of cargo liquid or vapour with atmospheric oxygen, or oxygen from another source. These compounds may, in some cases, be highly reactive or unstable and constitute a potential hazard.

Polymerization – The phenomenon whereby the molecules or a particular compound can be made to link together into a larger unit containing anything from two to thousands of molecules, the new unit being called a polymer. A compound may thereby change from a free flowing liquid to a viscous one or even a solid. A great deal of heat may be evolved when this occurs. Polymerization may occur spontaneously with no outside influence, or it may occur if the compound is heated, or if a catalyst or impurity is added. Polymerization may, under some circumstances be dangerous.

Pressure – Force per unit area.

Purging – The introduction of nitrogen or suitable inert gas or suitable cargo vapour to displace an unacceptable tank atmosphere.

Relative Liquid Density – The mass of a liquid at a given temperature compared with the mass of an equal volume of fresh water at the same temperature or at a different given temperature. Term Specific Gravity being discontinued in general.

Relative Vapour Density – The weight of the vapour compared with the weight of an equal volume of air, both at standard conditions of temperature and pressure. Thus vapour density of 2.9 means that the vapour is 2.9 times heavier then an equal volume of air under the same physical conditions.

Reliquefaction – Converting cargo boil-off vapour back into a liquid by refrigeration.

Responsible Officer – The Master or any officer to whom the Master may relegate responsibility for any operation or duty.

Responsible Terminal Representative – The shore supervisor in change of all operators and operations at the terminal associated with the handling of products, or his responsible delegate.

Restricted Gauging System – A system employing a device, which penetrates the tank and which, when in use, permits a small quantity of cargo vapour or liquid to be exposed to the atmosphere. When not in use the device is completely closed.

RPT – Rapid Phase Transition - When LNG contacts water under specific conditions, it transforms explosively from the liquid to vapour state.

Secondary Barrier – The liquid resisting outer element of a cargo containment system designed to afford temporary containment of any envisaged leakage of liquid cargo through the primary barrier and to prevent the lowering of the temperature of the ship’s structure to an unsafe level.

Self-Reaction – The tendency of a chemical to react with itself, usually resulting in polymerization or decomposition.

Sloshing – Wave formations which may arise at the liquid surface in a cargo tank from the effects of ship motions.

Span Gas – A vapour sample of known composition and concentration and concentration used to calibrate (or “span”) a ship’s gas detection equipment.

Static Electricity – The electrification of dissimilar materials through physical contact and separation.

Threshold Limit Value (TLV) – The TLVs refer to the maximum concentration of gases, vapours, mists or sprays to which it is believed that nearly all persons on board may be repeatedly exposed, day after day, without adverse effect assuming an 8 hours per day, 40 hours per week exposure. Because of the wide variation in individual susceptibility, exposure of an occasional individual at, or even below, the TLV may not prevent discomfort or aggravation of a pre-existing condition.

Vapour Pressures – The pressure exerted by the vapour above the liquid at a given temperature.

Venting – The release of cargo vapour or inert gas from cargo tanks and associated systems.

Void Space – The enclosed space in the cargo area external to a cargo containment system, not being a hold space, ballast space, fuel oil tank, cargo pump or compressor room, or any space in normal use by personnel.

CTS – Custody Transfer System, this is the cargo measurement and level gauging instrumentation

External links :

The Center for Liquefied Natural Gas : ( CLNG ) - A trade association of LNG producers, shippers, terminal operators and developers, energy trade associations and natural gas consumers

Reference publications

Related Information:

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  2. LNG spill risk during marine transportation and hazards associated

  3. How to tackle fire on board LNG ship

  4. Fire fighting plan for LNG cargo

  5. Increased Cargo Capacity for LNG ships & Advantages of the dual fuel diesel electric propulsion

The sea transport of liquefied gases in bulk -Where do the products come from ?

LNG vessel construction - advantages of membrane technology

Advantages of Moss rosenberg cargo containment system

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