Important definitions for oil and chemical tankers


  1. Adiabatic: Describes an ideal process undergone by a gas in which no gain or loss of heat occurs.
  2. Administration: This means the government of the state whose flag the ship is entitled to fly.
  3. ALARP: As low as reasonably practicable.
  4. Antistatic additive: A substance added to a petroleum product to raise its electrical conductivity to a safe level above 50 picoSiemens/metre (pS/m) to prevent accumulation of static electricity.
  5. Approved equipment: Equipment of a design that has been tested and approved by an appropriate authority, such as a government department or classification society. The authority should have certified the equipment as safe for use in a specified hazardous or dangerous area.
  6. Auto-ignition: 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.
  7. Boil-off: Boil-off is the vapour produced above the surface of a boiling cargo due to evaporation. It is caused by heat ingress or a drop in pressure.
  8. Boiling Point: The temperature at which the vapour pressure of a liquid is equal to the pressure on its surface (the boiling point varies with pressure).
  9. Bonding: The connecting together of metal parts to ensure electrical continuity.
  10. Booster Pump: A pump used to increase the discharge pressure from another pump (such as a cargo pump).
  11. Bulk Cargo: Cargo carried as a liquid in cargo tanks and not shipped in drums, containers or packages.
  12. Carbamates: A white powdery substance produced by the reaction of ammonia with carbon dioxide.
  13. Carcinogen: A substance capable of causing cancer.
  14. Cargo Area: That part of the ship which contains the cargo containment system, cargo pumps and compressor rooms, and includes the deck area above the cargo containment system. Where fitted, cofferdams, ballast tanks and void spaces at the after end of the aftermost hold  space or the forward end of the forwardmost hold space are excluded from the cargo area. (Refer to the Gas Codes for a more detailed definition).
  15. Cargo Containment Systems: The arrangement for containment of cargo including, where fitted, primary and secondary barriers, associated insulations, interbarrier spaces and the structure required for the  support of these elements. (Refer to the Gas Codes for a more detailed definition.)
  16. Cascade Reliquefaction Cycle: A process in which vapour boil-off from cargo tanks is  condensed in a cargo condenser in which the coolant is a refrigerant gas such as R22 or equivalent. The refrigerant gas is then compressed and passed through a conventional sea water-cooled condenser.
  17. Cathodic protection: 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.
  18. Cavitation: A process occurring within the impeller of a centrifugal pump when pressure at the inlet to the impeller falls below that of the vapour pressure of the liquid being pumped. The bubbles of vapour which are formed collapse with impulsive force in the higher pressure regions of the impeller. This effect can cause significant damage to the impeller surfaces and, furthermore, pumps may loose suction.
  19. Certificate of Fitness: A certificate issued by a flag administration confirming that the structure, equipment,  fittings, arrangements and materials used in the construction of a gas carrier are in compliance with the relevant Gas Code or applicable legal requirements. Such certification may be issued on behalf of the administration by an approved classification society.
  20. Certified Gas Free: A tank or compartment is certified to be gas-free when its atmosphere has been tested with an approved instrument and found in a suitable condition by an independent chemist. This means it is not deficient in oxygen and sufficiently free of toxic or flammable gas for a specified purpose.
  21. Clingage: Oil remaining on the walls of a pipe or on the internal surfaces of tanks after the bulk of the oil has been removed.
  22. Closed operations: Ballasting, loading or discharging operations carried out without recourse to opening ullage and sighting ports. During closed operations, ships will require the means to enable closed monitoring of tank contents, either by a fixed gauging system or by using portable equipment passed through a vapour lock.
  23. CMR Substance: A substance that is carcinogenic, mutagenic or reprotoxic.
  24. Cold Work: Work that cannot create a source of ignition.
  25. Combustible (also referred to as ‘Flammable’): Capable of being ignited and of burning. For the purposes of this Guide, the terms ‘combustible’ and ‘flammable’ are synonymous.
  26. Compression Ratio: The ratio of the absolute pressure at the discharge from a compressor divided by the absolute pressure at the suction.
  27. Condensate: Reliquefied gases which collect in the condenser and which are then returned to the cargo tanks.
  28. Competent person: A person who has been adequately trained to undertake the tasks they are required to perform within their job description. For personnel in the shipping industry, they should be able to demonstrate this competence by the production of certificates recognised by the ship’s administration.
  29. Critical Pressure: The pressure at which a substance exists in the liquid state at its critical temperature. (In other words it is the saturation pressure at the critical temperature).
  30. Critical Temperature: The temperature above which a gas cannot be liquefied by pressure alone.
  31. Cryogenics: The study of the behaviour of matter at very low temperatures.
  32. Density: The mass per unit volume of a substance at specified conditions of temperature and pressure.
  33. Dew point: The temperature at which condensation will take place within a gas if further cooling occurs.
  34. Earthing (also referred to as ‘Grounding’): The electrical connection of 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.
  35. Enclosed space: A space that has limited openings for entry and exit, unfavourable natural ventilation, and that is not designed for continuous worker occupancy. This includes cargo spaces, double bottoms, fuel tanks, ballast tanks, pump rooms, cofferdams, void spaces, duct keels, inter-barrier spaces, engine crankcases and sewage tanks.
  36. Endothermic: A process which is accompanied by the absorption of heat.
  37. Enthalpy: Enthalpy is a thermodynamic measure of the total heat content of a liquid or vapour at a given temperature and is expressed in energy per unit mass (kJoules per 1 kg) from absolute zero. Therefore, for a liquid/vapour mixture, it will be seen that it is the sum of the enthalpy of the liquid plus the latent heat of vaporization.
  38. Entropy: Entropy of a liquid/gas system remains constant if no heat enters or leaves while it alters its volume or does work but increases or decreases should a small amount of heat enter or leave. Its value is determined by dividing the intrinsic energy of the material by its absolute temperature. The intrinsic energy is the product of specific heat at constant volume  multiplied by a change in temperature. Entropy is expressed in heat content per mass per unit of temperature. In the Sl system its units are therefore Joule/kg/K. It should be noted that in a reversible process in which there is no heat rejection or absorption, the change of entropy is zero.
  39. Entry permit: A document issued by a Responsible Person allowing entry into a space or compartment during a specific time interval.
  40. Explosion-proof (also referred to as ‘Flame-proof ’): Electrical equipment is defined and certified as explosion-proof when it is enclosed in a case that is capable of withstanding the explosion within it of a hydrocarbon gas/air mixture or other specified flammable gas mixture. It must also prevent the ignition of such a mixture outside the case either by spark or flame from the internal explosion or as a result of the temperature rise of the case following the internal explosion. The equipment must operate at such an external temperature that a surrounding flammable atmosphere will not be ignited.
  41. Flame arrester: A permeable matrix of metal, ceramic or other heat-resisting materials which can cool even an intense flame, and any following combustion products, below the temperature required for the ignition of the flammable gas on the other side of the arrester.
  42. Flame screen: A portable or fitted device incorporating one or more corrosion resistant wire-woven fabrics of very small mesh, which is used for preventing sparks from entering a tank or vent opening or, for a short time, preventing the passage of flame. (Not to be confused with ‘Flame arrester’.)
  43. Flammable (also referred to as ‘Combustible’): Capable of being ignited and of burning. For the purposes of this Guide, the terms ‘flammable’ and ‘combustible’ are synonymous.
  44. Flammable gas monitors (also referred to as ‘Explosimeter’): An instrument for measuring the composition of hydrocarbon gas/air mixtures, usually giving the result as a percentage of the Lower Explosive Limit (LEL).
  45. 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.
  46. Flashpoint: The lowest temperature at which a liquid gives off sufficient gas to form a flammable gas mixture near the surface of the liquid. It is measured in a laboratory in standard apparatus using a prescribed procedure.
  47. Flow rate: The linear velocity of flow of liquid in a pipeline, usually measured in metres per second (m/s). The determination of the flow rates at locations within cargo pipeline systems is essential when handling static accumulator cargoes.
  48. Gas free: A tank, compartment or container is gas free when 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.
  49. Gas free certificate: A certificate issued by an authorised Responsible Person confirming that, at the time of testing, a tank, compartment or container was gas free for a specific purpose.
  50. Gas-Freeing: The removal of toxic, and/or flammable gas from a tank or enclosed space with inert gas followed by the introduction of fresh air.
  51. Gassing-up: Gassing-up means replacing an inert atmosphere in a tank with the vapour from the next cargo to a suitable level to allow cooling down and loading.
  52. Halon: A halogenated hydrocarbon used in fire-fighting that inhibits flame propagation. Hazardous area 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. (For ships, see ‘Dangerous area’.)
  53. Hot Work: Work involving sources of ignition or temperatures sufficiently high to cause the ignition of a flammable gas mixture. This includes any work requiring the use of welding, burning or soldering equipment, blow torches, some power driven tools, portable electrical equipment which is not intrinsically safe or contained within an approved explosion-proof housing, and internal combustion engines.
  54. Hot Work Permit: A document issued by a Responsible Person permitting specific Hot Work to be done during a particular time interval in a defined area.
  55. Hydrocarbon gas: A gas composed entirely of hydrocarbons.
  56. Inert condition: 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.
  57. Inert gas: A gas or a mixture of gases, such as flue gas, containing insufficient oxygen to support the combustion of hydrocarbons.
  58. Inert gas plant: All equipment fitted to supply, cool, clean, pressurise, monitor and control the delivery of inert gas to the cargo tank systems.
  59. Inert Gas System (IGS): An inert gas plant and inert gas distribution system together with means for preventing backflow of cargo gases to the machinery spaces, fixed and portable measuring instruments and control devices.
  60. Inerting: The introduction of inert gas into a tank with the object of attaining the inert condition. Insulating flange A flanged joint incorporating an insulating gasket, sleeves and washers to prevent electrical continuity between ship and shore.
  61. International Safety Management (ISM) Code: An international standard for the safe management and operation of ships and for pollution prevention. The Code establishes safety management objectives and requires a Safety Management System (SMS) to be established by the Company and audited and approved by the flag administration.
  62. Intrinsically safe: An electrical circuit, or part of a circuit, is intrinsically safe if any spark or thermal effect produced normally (i.e. by breaking or closing the circuit) or accidentally (e.g. by short circuit or earth fault) is incapable, under prescribed test conditions, of igniting a prescribed gas mixture.
  63. Isothermal: Descriptive of a process undergone by an ideal gas when it passes through pressure or volume variations without a change of temperature.
  64. Latent Heat: The heat required to cause a change in state of a substance from solid to liquid (latent heat of fusion) or from liquid to vapour (latent heat of vaporisation). These phase changes occur without change of temperature at the melting point and boiling point, respectively.
  65. Latent Heat of Vaporisation: Quantity of heat to change the state of a substance from liquid to vapour (or vice versa) without change of temperature.
  66. Loading over the top (also referred to as ‘Loading overall’): The loading of cargo or ballast through an open-ended pipe or by means of an open-ended hose entering a tank through a deck opening, resulting in the free fall of liquid.
  67. Lower Explosive Limit (LEL): The concentration of a hydrocarbon gas in air below which there is insufficient hydrocarbon to support and propagate combustion. Sometimes referred to as Lower Flammable Limit (LFL).
  68. MARVS: This is the abbreviation for the Maximum Allowable Relief Valve Setting on a ship’s cargo tank – as stated on the ship’s Certificate of Fitness.
  69. Material Safety Data Sheet (MSDS): A document identifying a substance and all its constituents. It provides the recipient with all necessary information to manage the substance safely. The format and content of an MSDS for MARPOL Annex I oil cargoes and oil fuel are prescribed in IMO Resolution MSC.286(86). See SDS.
  70. Mercaptans: A group of naturally occurring organic chemicals containing sulphur. They are present in some crude oils and in pentane plus cargoes. They have a strong odour.
  71. Non-volatile petroleum: Petroleum having a flashpoint of  or above, as determined by the closed cup method of test.
  72. Odor threshold: The lowest concentration of vapour in air that can be detected by smell.
  73. Oxygen analyser or oxygen meter: An instrument for determining the percentage of oxygen in a sample of the atmosphere drawn from a tank, pipe or compartment.
  74. Packaged cargo: Petroleum or other cargo in drums, packages or other containers.
  75. Pellistor: An electrical sensor unit fitted in a flammable gas detector for measuring hydrocarbon vapors and air mixtures to determine whether the mixture is within the flammable range.
  76. Phases of oil: Oil is considered to have three phases in which it can exist depending on the grade of oil and its temperature. The three phases are the solid phase, the liquid phase and the vapour phase. The phases do not exist in isolation and operators must manage the carriage of oil with an understanding of the combinations of the phases of oil in the cargo being carried.
  77. PolymerizationThe chemical union of two or more molecules of the same compound to form a larger molecule of a new compound called a polymer. By this mechanism, the reaction can become self-propagating causing liquids to become more viscous and the end result may even be a solid.
  78. Pour point: The lowest temperature at which a petroleum oil will remain fluid.
  79. Pressure surge: A sudden increase in the pressure of the liquid in a pipeline brought about by an abrupt change in flow rate.
  80. Pressure/vacuum relief valve (P/V valve): A device that provides for the flow of the small volumes of vapour, air or inert gas mixtures caused by thermal variations in a cargo tank.
  81. Purging: The introduction of inert gas into a tank already in the inert condition with the object of further reducing the existing oxygen content and/or reducing the existing hydrocarbon gas content to a level below which combustion cannot be supported if air is subsequently introduced into the tank.
  82. Pyrophoric iron sulphide: Iron sulphide capable of a rapid exothermic oxidation causing incandescence when exposed to air and potential ignition of flammable hydrocarbon gas/air mixtures.
  83. Reid Vapour Pressure (RVP): The vapour pressure of a liquid determined in a standard manner in the Reid apparatus at a temperature of  and with a ratio of gas to liquid volume of 4:1. Used for comparison purposes only. 
  84. 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.
  85. Resuscitator: Equipment to assist or restore the breathing of personnel overcome by gas or lack of oxygen.
  86. Rollover: The phenomenon where the stability of two stratified layers of liquid of differing relative density is disturbed resulting in a spontaneous rapid mixing of the layers accompanied in the case of liquefied gases, by violent vapour evolution.
  87. Safety Data Sheet (SDS): A document identifying a substance and all its constituents. It provides the recipient with all necessary information to manage the substance safely. Guidance on the format and content of an SDS are given in the European Globally Harmonized System of Classification and Labelling of Chemicals (GHS). See MSDS.
  88. Secondary Barrier: The liquid-resisting outer element of a cargo containment system designed to provide temporary containment of a 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.
  89. Self-stowing mooring winch: A mooring winch fitted with a drum on which a mooring wire or rope is made fast and automatically stowed.
  90. Settling time: The time it takes for tank contents to stop moving once filling has stopped, and therefore the cessation of further static electricity generation. Typically, this time is 30 minutes. Not to be confused with ‘Relaxation time’ – see definition.
  91. Slops: A mixture of cargo residues and washing water, rust or sludge which is either suitable or not suitable for pumping.
  92. Sour crude oil or products: A term used to describe crude oil or products containing appreciable amounts of hydrogen sulphide and/or mercaptans.
  93. Spiked crude oil: A crude oil blended with a liquefied gas or condensate.
  94. Spontaneous combustion: The ignition of material brought about by a heat producing (exothermic) chemical reaction within the material itself without exposure to an external source of ignition.
  95. Spread loading: The practice of loading a number of tanks simultaneously to avoid static electricity generation when loading static accumulator cargoes.
  96. Static accumulator oil: An oil with an electrical conductivity of less than 50 picoSiemens/metre (pS/m), so that it is capable of retaining a significant electrostatic charge.
  97. Static electricity: The electricity produced by movement between dissimilar materials through physical contact and separation.
  98. Static non-accumulator oil: An oil with an electrical conductivity greater than 50 picoSiemens/metre (pS/m), so that it is incapable of retaining a significant electrostatic charge.
  99. Stripping: The final operation in draining liquid from a tank or pipeline.
  100. Surge Pressure: A phenomenon generated in a pipeline system when there is a change in the rate of flow of liquid in the line. Surge pressures can be dangerously high if the change of flow rate is too rapid and the resultant shock waves can damage pumping equipment and cause rupture of pipelines and associated equipment.
  101. Tank cleaning: The process of removing hydrocarbon vapours, liquid or residue from tanks. Usually carried out so that tanks can be entered for inspection or Hot Work or to avoid contamination between grades.
  102. Threshold Limit Value (TLV): Airborne concentrations of substances under which it is believed that nearly all workers may be exposed day after day with no adverse effect. TLVs are advisory exposure guidelines, not legal standards, and are based on industrial experience and studies. There are three different types of TLVs:
    • Time Weighted Average (TLV-TWA): The airborne concentration of a toxic substance averaged over an 8-hour period, usually expressed in parts per million (ppm).
    • Short-Term Exposure Limit (TLV-STEL): The airborne concentration of a toxic substance averaged over any 15 minute period, usually expressed in parts per million (ppm).
    • Ceiling (TLV-C):The concentration that should not be exceeded during any part of the working exposure.
  103. Topping-off: The operation of completing the loading of a tank to a required ullage.
  104. Topping-up: The introduction of inert gas into a tank that is already in the inert condition with the object of raising the tank pressure to prevent any ingress of air.
  105. Toxicity: The degree to which a substance or mixture of substances can harm humans or animals. ‘Acute toxicity’ involves harmful effects to an organism through a single short term exposure. ‘Chronic toxicity’ is the ability of a substance or mixture of substances to cause harmful effects over an extended period, usually upon repeated or continuous exposure, sometimes lasting for the entire life of the exposed organism.
  106. True Vapour Pressure (TVP): 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 ratio is effectively zero. 
  107. Ullage: The space above the liquid in a tank, conventionally measured as the distance from the calibration point to the liquid surface.
  108. Upper Explosive Limit (UEL): The concentration of a hydrocarbon gas in air above which there is insufficient oxygen to support and propagate combustion. Sometimes referred to as Upper Flammable Limit (UFL).
  109. Vapour Emission Control System (VECS): An arrangement of piping and equipment used to control vapour emissions during tanker operations, including ship and shore vapor collection systems, monitoring and control devices and vapour processing arrangements.
  110. Vapour lock system: Equipment fitted to a tank to enable the measuring and sampling of cargoes without release of vapour or inert gas pressure.
  111. Void Space: An enclosed space in the cargo area external to a cargo containment system, other than a hold space, ballast space, fuel oil tank, cargo pump or compressor room or any space in normal use by personnel.
  112. Volatile petroleum: Petroleum having a flashpoint below  as determined by the closed cup method of test.
  113. Water fog: A suspension in the atmosphere of very fine droplets of water usually delivered at high pressure through a fog nozzle for use in fire-fighting.

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