Tanker Definitions

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The International safety guide for oil Tankers and Terminals (ISGOTT) provides definitions of tanker terminology in common usage. Some important definitions are given below. These are for the information of non–tanker students. Students on tankers should use the latest edition of the ISGOTT.

Students sailing on ships other than tankers need to know these definitions, as they are common to all hazardous cargoes. As a cargo watch keeper we must be a couple of steps ahead of hazards. Take for example, when you are bunkering, the ship needs to follow the same precautions as on a tanker.

Anti-static additive:

A substance added to a petroleum product to raise its electrical conductivity above 100 Pico Siemens/metre (pS/m) to prevent accumulation of static electricity.

Approved equipment

Is equipment tested and approved by an appropriate authority such as the Administration or a Classification Society. The approval indicates that the equipment is safe for use in a specified hazardous atmosphere.


Is the ignition of a combustible material without initiation by a spark or flame, when the material has been raised to a temperature at which self-sustaining combustion occurs.

Boiling point

The temperature at which a liquid changes from liquid to vapour state, at atmospheric pressure.


Is the connecting together of metal parts to ensure electrical continuity.

Cathodic protection

Is the process of the prevention of corrosion by electrochemical techniques. On tankers it may be applied either externally to the hull or internally to the surfaces of tanks. At terminals, it is frequently applied to steel piles and fender panels.


Is the oil remaining I clinging on the walls of a pipe or on the internal surfaces of tanks after the bulk of the oil has been removed.

Cold work

Means any repair or maintenance work, which cannot create a source of ignition.

Combination carrier

Means a ship designed to carry either petroleum cargoes or dry bulk cargoes.

Combustible (also referred to as ‘Flammable’)

Means a substance capable of being ignited and of burning. For the purposes of this guide the terms ‘combustible’ and ‘flammable’ are synonymous. Combustible gas indicator means an instrument for measuring the composition of hydrocarbon gas/air mixtures, usually giving the result as a percentage of the lower flammable limit (LFL)


It is the continuous combination of an inflammable substance with oxygen, generating heat, light or both. Dangerous area means an area on a tanker, which for the purposes of the installation and use of electrical equipment is regarded as dangerous.

Dry chemical powder

Means a flame inhibiting powder, used in fire fighting Earthing (also referred to as ‘Grounding’) means the electrical connection of any equipment to the main body of the earth to ensure that it is at earth potential On board ship, the connection is made to the main metallic structure of the ship, which is at earth potential because of the conductivity of the sea.

Entry permit

Is a document issued by a responsible person permitting entry to a space or compartment for a specific time interval


See ‘Combustible gas indicator’ Explosion proof (flame -proof) equipment is an equipment which does not provide ignition source in a specified hydrocarbon gas / air mixture and which is capable of withstanding the explosion of a hydrocarbon gas/air mixture or other specified flammable gas mixture. The equipment must operate at such an external temperature that a surrounding flammable atmosphere will not be ignited.
Examples: 1) Pump Room Lights 2) Pump Room switches.

Flame arrester

Means a permeable matrix of metal, ceramic or other heat resisting materials, which can cool a deflagration and any following combustion products, below the temperature required for the ignition of the flammable gas on the other side of the arrester.

Flame screen

Means a portable or fitted device incorporating one or more corrosion resistant wire woven fabrics of very small mesh used for preventing sparks from entering a tank or vent opening. Such device may prevent the passage of flame for a short period. (Not to be confused with flame arrester).

Flammable (also referred to as ‘Combustible’)

Means capable of being ignited and of burning. For the purposes of this guide, the terms ‘flammable’ and ‘combustible’ are synonymous.

Flammable range (also referred to as ‘Explosive range’)

The range of hydrocarbon gas concentrations in air between the lower and upper flammable (explosive) limits. Mixtures within this range are capable of being ignited and of burning.

Flash Point

It is the lowest temperature at which a liquid gives off sufficient vapours so as to sustain combustion when an external source of ignition is applied. If the concentration of hydrocarbon in the mixture with air is reduced below the LEL, the mixture will not be able to support combustion. In this case i.e. the mixture will be considered to be too lean and so also if the concentration of hydrocarbon is increased above the UEL, the mixture will be unable to support combustion, – i.e. the mixture will be considered to be too rich.

Closed cup and open cup method

May determine the flash point. In the open cup method the surface of the liquid is continuously open to the atmosphere as the liquid is heated. The result is the open cup flash point and it is applied for kerosene and heavier fuels whose flash point is above 80°F. In the closed cup method the space above the liquid is kept closed except for the brief intervals when the source is being introduced. This type of flash point is applied to gasoline and most crude oils, which have low flash point. Because of loss of vapour in the open cup method the open cup flash point is always higher than the closed cup flash point of the same material by about 5°C.

Flashlight (also referred to as ‘Torch’)

A battery operated hand lamp. An approved flashlight is one, which is approved by a competent authority for use in a flammable atmosphere.

Foam also referred to as Froth’)

Means an aerated solution which is used for fire prevention and fire fighting. Foam solution means the mixture produced by diluting foam concentrate with water before proceeding to make foam. Free-fall means the unrestricted fall of liquid into a tank Froth see ‘Foam’

Gas free certificate

Means a certificate issued by an authorised person confirming that, at the time of testing the tank, compartment or container was gas free for a specific purpose. Gas free. A tank, compartment or container is gas free where sufficient fresh air has been introduced into it to lower the level of any flammable, toxic, or inert gas to that required for a specific purpose, e.g. hot work, entry, etc.


Means halogenated hydrocarbon used in fire fighting which inhibits flame propagation. (This means of fire fighting is being taken off, as it is a threat to environment.)

Hazardous area

Means an area on shore, which for the purposes of the installation and use of electrical equipment is regarded as dangerous. Such hazardous areas are graded into hazardous zones depending upon the probability of the presence of a flammable gas mixture.

Hot work permit

Means a document issued by an authorised person permitting specific hot work to be done during a specific time interval in a defined area.

Hydrocarbon gas

Means a gas composed entirely of hydrocarbons If the concentration of hydrocarbon in the mixture with air is reduced below the LEL, the mixture will not be able to support combustion. In this case i.e. the mixture will be considered to be too lean and so also if the concentration of hydrocarbon is increased above the UEL, the mixture will be unable to support combustion, – i.e. the mixture will be considered to be too rich.

Ignition point

It is the temperature at which an inflammable material will spontaneously ignite without the application of an external source of ignition. It is also called the spontaneous ignition temperature.

Inert condition

Is a condition in which the oxygen content throughout the atmosphere of a tank has been reduced to 8 per cent or less by volume by the addition of inert gas.

Inert gas distribution system

Includes all piping, valves and associated fittings to distribute inert gas from the inert gas plant to the cargo tanks, and the arrangements to vent gases to atmosphere to protect tanks against excessive pressure or vacuum.

Inert gas

Means a gas or a mixture of gases, such as flue gas, containing insufficient oxygen to support the combustion of hydrocarbons. Inert gas plant includes all equipment fitted to supply, cool, clean, pressurise, monitor and control the delivery of inert gas to the cargo tank systems.

Inert gas system (IGS)

Means the inert gas plant and gas distribution system together with means for preventing backflow of cargo gases to the machinery spaces, fixed and portable measuring instruments and control devices.


Means the introduction of inert gas into a tank with the object of attaining the inert condition.

Insulating flange

Means a flanged joint incorporating an insulating gasket, sleeves and washers to prevent electrical continuity between pipelines, hose strings of loading arms.

Interface detector

Means an electrical instrument for detecting the boundary between oil and water.

Intrinsically safe

Means an electrical circuit or part of a circuit incapable, under prescribed test conditions, of igniting a prescribed gas mixture, even when any spark or thermal effect is produced normally (i.e., by breaking or closing the circuit) or accidentally (e.g. by short circuit or earth fault)

Loading overall

Is the loading of cargo or ballast ‘over the top’ through an open ended pipe or by means of an open ended hose entering a tank through a hatch or other deck opening, resulting in the free fall of liquid.

Lower Explosive limit (LEL) / Lower Flammable limit(LFL):

Means the concentration of a hydrocarbon gas in air below, which there is insufficient hydrocarbon to support and propagate combustion. Sometimes referred to as lower explosive limit (LEL)

Mooring winch brake design capacity

Is the percentage of the minimum breaking load (MBL) of a new mooring rope or wire it carries at which the winch brake is designed to render. Winch brakes will normally be designed to hold 80% of the line’s MBL and will be set in service to hold 60% of the mooring line’s MBL. line’s MBL. Brake holding capacity may be expressed either in tonnes or as a Percentage of a line’s MBL.

Mooring winch design heaving capacity

Means the power of a mooring winch to heave in or put a load on its mooring rope or wire. Usually expressed in tonnes.

Naked lights

Include open flames or fires, lighted cigarettes, cigars, pipes or similar smoking materials, any other unconfined sources of ignition, electrical and other equipment liable to cause sparking while in use and unprotected light bulbs.

Non-volatile petroleum

Means petroleum having a flashpoint of 60°C or above as determined by the closed cup method of test.

Oxygen analyser/ meter

Means an instrument for determining the percentage of oxygen in a sample of the atmosphere drawn from a tank, pipe or compartment.

Packaged cargo

Means petroleum or other cargo carried in drums, containers or other packages

Permissible Exposure Limits PEL

Means the maximum exposure to a toxic substance that is allowed by appropriate regulatory standards, including those of flag States, PEL is usually expressed as:

a) Time weighted Average (TWA) – the airborne concentrations of a toxic substance averaged over an 8-hour period, expressed in parts per million. (ppm)

b) Short Term Exposure (STE) – the airborne concentration of a toxic substance averaged over any 15-minute period, usually expressed in parts per million (ppm).

Petroleum gas

Means a gas evolved from petroleum. The main constituents of petroleum gases are hydrocarbons, but they may also contain other substances, such as hydrogen sulphide or lead alkalis, as minor constituents.


Means crude oil and liquid hydrocarbon products derived from it.

Pour point

Means the lowest temperature at which petroleum oil will remain fluid.

Pressure surge

Means a sudden increase in the pressure of the liquid in a pipeline brought about by an abrupt change in flow velocity.

Pressure/vacuum relief valve (p/v valve)

Means a device, which provides for the flow of the small volumes of vapour, air or inert gas mixtures caused by thermal variations in a cargo tank.


Means the introduction of inert gas into a tank already in the inert condition with the object of

a) further reducing the existing oxygen content, and/or

b) Reducing the existing hydrocarbon gas content to a level below which combustion cannot be supported if air is subsequently introduced into the tank.

Pyrophoric iron sulphide

Means iron sulphide capable of a rapid exothermic oxidation causing incandescence when exposed to air and potential ignition of flammable hydrocarbon gas/air mixtures.

Reid vapour pressure (RVP)

Means the vapour pressure of a liquid determined in a standard manner in the Reid apparatus at a temperature of 37.80 C and with a ratio of gas to liquid volume of 4:1. Used for comparison purposes only. See ‘True Vapour Pressure’

Responsible officer (or person)

Means a person appointed by the employer or the master of the ship and empowered to take all decisions relating to a specific task, having the necessary knowledge and experience for that purpose.


Means equipment to assist or restore the breathing of personnel overcomes by gas or lack of oxygen. Self-stowing mooring winch means a mooring winch fitted with a drum on which a wire or rope is made fast and automatically stowed

Sour crude oil

Means crude oil containing appreciable amounts of hydrogen sulphide and/or mercaptans.

Specific Gravity

It is the ratio of the weight of a body to the weight of an equal volume of a standard substance.

Specific heat

It is the number of calories required to raise the temperature of a unit mass of a substance through one-degree centigrade of temperature. It is also defined as the capacity for heat of a unit mass of a substance.

Spontaneous combustion

Is said to occur when any material, which undergoes exothermic chemical reaction, catches fire due to rise in temperature and evolution of flammable gas without any external source of fire, like spark or flame, being brought near it.

Static accumulator oil

Means oil with an electrical conductivity less than 50 pico Siemens/metre (pS/M), so that it is capable of retaining a significant electrostatic charge.

Static electricity

Means the electricity produced by dissimilar materials through physical contact and separation.

Static non-accumulator oil

Means an oil with an electrical conductivity greater than 50 Pico Siemens / metre (pS/m), which renders it incapable of retaining a significant electrostatic charge.

Tank cleaning

Means the process of removing hydrocarbon vapours, liquid or residue. Usually carried out so that the tanks can be entered for inspection or hot work.


Means a ship designed to carry liquid petroleum cargo in bulk, including a combination carrier when being used for this purpose.


Means a place where tankers are berthed or moored for the purpose of loading or discharging petroleum cargo.

Terminal representative

Means a person designated by the terminal to take responsibility for an operation or duty.

Thermal conductivity:

The volume of heat, which in unit time passes through a plate, the temperature of whose surfaces is different by one degree and which is of unit area and thickness.

Threshold limit value (TLV)

Means the time weighted average concentration of a substance to which workers may be repeatedly exposed, for a normal 8-hour workday or 40-hour workweek, day after day, without adverse effect. (See also Permissible Exposure Limits)

Topping off

Means the operation of completing the loading of a tank to a required ullage.

Topping up

Means the introduction of the inert gas into a tank, which is already in an inert condition with the object of raising the tank pressure to prevent any ingress of air.


Means poisonous to human life

True vapour pressure (TVP)

Means the absolute pressure exerted by the gas produced by evaporation from a liquid when gas and liquid are in equilibrium at the prevailing temperature and the gas/liquid ration is effectively zero.


Means the depth of the space above the liquid in a tank

Upper flammable limit (UFL)

Means the concentration of a hydrocarbon gas in air above, which there is insufficient oxygen to support and propagate combustion. Sometimes referred to as upper explosive limit (UEL)

Vapour emission control system

Means an arrangement of piping and equipment used to control vapour emissions during tanker operations, including ship and shore vapour collection systems, monitoring and control devices and vapour processing arrangements

Vapour lock system

Means equipment fitted to a tank to enable the measuring and sampling of cargoes without release of vapour/inert gas pressure.


Means gas below its critical temperature


It is the resistance to flow exhibited by a liquid. It is defined as the force in Newton that must be exerted between two parallel layers ‘1 sq. m in area and 1 m apart in order to maintain a velocity of 1 m per second of one layer past the other. The unit is ‘Pascals / sec. The effect of temperature on the viscosity of a liquid is quite pronounced. As the temperature of the liquid is raised the viscosity decreases. The relation can be best explained in the form of an exponential equation.
The viscosity of petroleum was indicated as ‘Redwood Sec.’, ‘Saybolt Sec.’ Etc. but now it is indicated in MKS units.

Vapour pressure

Is the pressure exerted by the vapour of a substance on the surface of the substance such that no more vapour is formed. It is the state of equilibrium between the vapour and the liquid. The true vapour pressure gives a good indication of the ability of a liquid to give rise to gas but it is a parameter, which is difficult to determine. When a volatile liquid is released in a gas free compartment, it starts to vaporise and the space on top of the liquid is occupied by the vapour. This vapour will exert a pressure in the surface of the liquid. As this pressure increases the rate of evaporation will decrease and will stop at a certain pressure.

Volatile liquids

Are liquids, which have a flash point of less than 600 C (as determined by the closed cup method). Some of these liquids, at normal ambient temperatures, produce gas whose concentration lies within the flammable range of that gas.

Water spray

Means a suspension in the atmosphere of water divided into coarse drops by delivery through a special nozzle for use in fire fighting. Work permit means a document issued by a responsible person permitting specific work to be done during a specific period in a defined area.

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