Before commencement of the discharge, the cargo pump emergency stop trips are to be tested.

In vessels fitted with an inert gas system, normally only inert gas must be permitted to enter the space displaced by the discharged cargo (this is dependent on the cargo in question and port regulations).

The pressure/vacuum valves must be set to allow air to enter the tank in the event of an inert gas plant failure to avoid damage to the tank structure whilst pumps are being stopped.

One person must be delegated to keep watch within sight of the manifold area at times throughout discharge. If an incident at the manifold occurs, such as a burst pipe or failure of the manifold connection, the cargo pumps must be “tripped” first and the Emergency Alarms sounded. The gangway watchman may perform this duty.

If the person delegated to watch the manifold area is a rating, he must be instructed in the course of action to be taken in the event of an emergency before he commences his watch.

Tank hatches must not be opened, or ullage plugs left open during discharge.

The main consideration for restricting the maximum rate at which a vessel may load is to prevent the excessive build up of pressure within the cargo system. This is governed by a number of factors including the diameter of the smallest section of pipe in the system; the material from which the pipe is constructed, the angle frequency of bends, the capacity of the venting system, etc.

The closing loading system, whereby vapours are exhausted through either vent risers or high velocity vents, must always be used where possible.

The rate of loading must be adjusted to take into consideration the number of tanks or holds open at any one time.
It is important that the maximum loading rate is not exceeded to avoid over pressurisation and undue stress on the pipeline system.

Consideration of the rate of loading must also be taken into account in view of electrostatic hazards when handling static accumulator oils where an electric charge can build up by means of flow or turbulence.  Generally speaking, the method of controlling electrostatic generation is to restrict the flow rate in the initial stages of loading until all splashing and turbulence in the tank has ceased. Full details of such precautions are contained in ISGOTT.

In addition it is also essential to allow gas from the ullage space to be vented and to dissipate to below the L.E.L. at safe distance from the outlet point.

It is also important that the cargo being loaded is correctly distributed. It is not permissible to load at the maximum loading rate into one single tank or into tanks served by only one section of the gas line so that all the gas being given off by the cargo is expelled through a single outlet.

One person must be delegated to keep watch within sight of the manifold area at all times throughout loading. If an incident at the manifold occurs, such as a burst pipe or failure of the manifold connection, when the agreed shut down procedure must be put into action first and the Emergency Alarms sounded. The gangway watchman may perform this duty.

If the person delegated to watch the manifold area is a rating, he must be instructed in the course of action to be taken in the event of an emergency before he commences his watch.

Tank hatches must not be opened, or ullage plugs left open during loading.

The atmospheric conditions at the time of loading are the main factors in the successful dispersal of the gas at a safe distance from the gas line outlet. Hydrocarbon gases are, on average 1.5 times heavier than air and in still air or near calm conditions will fall to deck level and may accumulate there; in these conditions the Master may consider stopping loading operations, if gas concentrations are approaching hazardous levels, until conditions improve.

Similarly, if the existing atmospheric conditions cause the gas to flow towards the accommodation, and the gas is detected inside the accommodation, loading must be stopped immediately and the Emergency Alarms sounded. The Master must ensure all precautions are taken to prevent vapours entering the accommodation. This must include having the air conditioning on recirculation.

The Emergency Organisation is then to take the appropriate action to isolate the electrical supply of the accommodation from the main switchboard, and clear the accommodation of gas.

It is essential that the accommodation is kept under positive pressure to prevent the entry of Hydrocarbon Vapours – the operation of sanitary and galley extraction fans will cause a vacuum and therefore the air conditioning system intakes must not be kept fully closed. The accommodation air conditioning system should be maintained on partial re-circulation during cargo operation.

Before any loading or discharging operations are commenced, the following fire-fighting equipment is to be placed on deck in a position approximately 10 metres forward or aft, but always upwind of the manifold:

• One portable foam-making branch pipe with eductor, connected to a fire hose and hydrant.
• Not less than 75 litres of foam compound.
• An additional hose with jet/spray nozzle attached, and connected to a hydrant.
• An International shore connection and fire plan/wallet is to be available, situated in the vicinity or point of access to the vessel.
• A semi-portable Foam or Dry Powder extinguisher is to be positioned close to the manifold or if semi-portable extinguishers are not available then two 9 litre foam extinguishers are to be used.
• The oil spill materials are to be kept available for use in the event of a spillage, but are not to be placed adjacent to the fire-fighting equipment.
• The above mentioned equipment is to be provided from the nearest Supplementary Equipment Station and Oil Spill Response Centre.
• 75 litres of foam compound will be consumed by any one branch pipe in 5 minutes. If the Master considers that the Emergency Organisation will require more than 5 minutes to supply additional foam compound to the manifold area, then additional 25 litre canisters are to be positioned in readiness.
• The fire main must be pressurised throughout cargo operations.

In ships fitted with fixed foam systems the Chief Officer will satisfy himself that the system is fully operational prior to commencing cargo, C.O.W. or tank cleaning operations.

• Foam tanks must be checked to see that they are full.
• Isolating valves must be tested to see that they are free.
• Foam monitors must be uncovered and made ready.
• At least one portable foam applicator must be connected to a hose and foam hydrant. This is to be placed about 10 metres forward or aft but always upwind of the manifold area.
• A fire hose and jet/spray nozzle must be placed alongside, connected to the fire main.

In addition to the foregoing precautions the Master is to satisfy himself that the ship/shore checklist has been completed before cargo operations commence.

Toxicity – is the ability of a substance to interrupt the correct function of bodily organs. Toxic cargoes are harmful if they are inhaled or swallowed or absorbed into the body through the skin. Most of the chemical cargoes as well as some crude oils and petroleum products are toxic or contain toxic components.

All people on board a vessel must be aware of the toxic characteristics of cargoes on board. When handling a cargo which may be toxic the precautions contained in MSDS, IBC/BCH Code and/or ISGOTT to be followed. In case of any accident involving dangerous, hazardous or harmful substances the IMO Medical First Aid Guide (MFAG) and IMO Emergency Procedure for Ships Carrying Dangerous Goods (EmS) are to be followed.

Information regarding the health hazards of the cargo can be obtained from Product Data Sheets or Material Safety Data Sheets (MSDS). The most common term in use within the industry for determination of product toxicity is Threshold Limit Value (TLV).

1. Threshold Limit Value (TLV)

TLV refers to the maximum concentration of gases, vapours, mist or sprays to which it is believed that nearly all persons may be repeatedly exposed day after day without adverse effects. TLV is stated as Time  Weighted Average (TLV-TWA), Short Term Exposure Limit (TLV-STEL) and Ceiling (TLV-C): usually expressed in parts per million (ppm). Refer to ISGOTT for more details.

2. Odour Threshold

The Odour Threshold is the smallest concentration, expressed in parts per million (ppm) by volume in air, that can be detected by smell in most people. However, it will vary between individuals and will vary from day to day for any person. The odour of a potentially dangerous vapour may be hidden by another odour. In addition, certain vapours are likely to produce a deadening of the sense of smell. For these reasons, the sense of smell alone is not a reliable indicator of the presence or absence of a dangerous vapour.

3. Hydrogen Sulphide gas (H2S)

Many crude oils contain hydrogen sulphide gas in solution and suitable precautions are to be taken to prevent personnel exposure to H2S gas.

4. Hydrocarbon Gases

In general TLV of petroleum gases is 300 ppm which approximately corresponds to 2% of Low Explosive Limit (LEL) however; this TLV must not be applied on gas mixtures containing benzene, toluene or hydrogen sulphide.

5. Aromatic Hydrocarbons

The aromatic hydrocarbons include benzene, toluene and xylene. They are components in varying amounts in many typical petroleum and chemical cargoes such as gasoline’s, gasoline blending components, napthas and special boiling point solvents.
The TLV of an aromatic hydrocarbon is generally less than that of other hydrocarbons and in this respect repeated exposure of benzene may produce serious chronic effects which can lead to disorders of the blood and bone marrow. Personnel engaged in operations involving products listed above, especially benzene, should therefore follow the precautions described in the ISGOTT in order to avoid harmful exposure during cargo handling operations.
For example the TLV-TWA of benzene is only 1 ppm (approx 0.07% LEL) therefore prior to entry into a tank which has contained petroleum products, the tank must be ventilated to a reading of not more than 1% LEL on an Explosimeter and then checked with appropriate Draeger Multigas Detector tube to ensure that the concentration of benzene vapours is at a safe level.

The liquid cargoes capable of being carried by tanker vessels can be subdivided (as a rule based on material chemical property) into a few major groups. These groups and the basic chemistry related to liquid cargoes are discussed briefly in the following sub-sections:

• Acid – Any chemical compound containing hydrogen capable of being replaced by positive elements or radicals to form salts. Usually corrosive.
• Alcohol – Organic compound containing one or more hydroxyl (OH) element. Alcohol may be mono-, di-, tri-, etc. according to the number of hydroxyl elements they contain and primary, secondary or tertiary according to the position of the elements.
• Aldehyde – A group of organic compounds holding a position between alcohol’s and acids. They are produced by the oxidation of hydrocarbons and can be used for the production of alcohols, e.g. acetaldehyde (CH CHO).
• Aliphatic – Organic compounds in which the carbon atoms are not arranged in a ring structure.
• Alkali – A strong base; a substance whose water solution yields a great volume of hydroxyl ions. Usually corrosive.
• Amine – Substance derived from ammonia where hydrogen atoms are replaced by one, two or three alkyl groups.
• Animal Oil – An oily material obtained from animal substances.
• Aromatics – Aromatic hydrocarbon (benzene series) =, with a carbon ring as the nucleus, e.g. aniline, naphthalene, benzene, toluene, xylene.
• Carbohydrates – Any one of a group of compounds composed of carbon, hydrogen and oxygen. They are neutral compounds comprising of sugars, starches cellulose’s, pentosans, galactans, etc.
• Chlorinated Hydrocarbons – A type of halogenated hydrocarbons, which have undergone a substitution process whereby single atoms of hydrogen are replaced by single atoms of chlorine.
• Crude Oil – Rough or unfinished or unrefined oil.
• Esters – Organic compounds (salts) formed from an alcohol (base) and an organic acid. Many esters occur in nature, as fats for instance. Many as important as solvents.
• Ether – An organic oxide containing one or more kinds of hydrocarbon elements, e.g. diethyl ether.
• Fatty acids – Organic monobase aliphatic acids. Palmitic, stearic and oleic acids are the three acids occurring most frequently in fats as glycerol esters. Fatty acids are common in animal and vegetable tissue.
• Glycols – Aliphatic secondary alcohol compounds containing 2 OH groups e.g. ethylene glycol, a colourless liquid produced from ethylene oxide.
• Halogenated hydrocarbons – Hydrocarbons that are the products of a substitution process whereby single atoms of hydrogen are replaced by single atoms of one of the halogens, i.e. fluorine, chlorine, bromine, iodine.
• Heavy chemicals – This that are manufactured industrially on a very large scale (e.g. sulphuric acid, caustic soda), but not including petrochemicals.
• Hydrogenation – The process of introducing hydrogen into the molecules of a substance, usually with the aid of a catalyst. The conversion of carbon monoxide to methanol is an example as is the hydrogenation of vegetable oils to produce fats.
• Hygroscopic – A hydroscopic compound is one, which absorbs moisture when exposed to air, but does not become wet.
• Ketones – A class of organic compounds produced by oxidation of secondary alcohol’s and characterised by the carbonyl group (CO), e.g. acetone, methyl ethyl ketone.
• Mineral acids – The inorganic acids, the commonest of which are hydrochloric, nitric and sulphuric.
• Orimulsion – Natural Bitumen in a water emulsion stabilised with a surfacient package.
• Petroleum Product – A liquid hydrocarbon product derived from crude oil.
• Solvent – The name given to products in the chemical industry which are used for dissolving other substances, e.g. acetone, alcohol’s, glycol’s, benzene, naphtha, carbon tetrachloride, carbon disulphide, turpentine, cyclohexanol, toluene, xylene.
• Vegetable oil – Any oil of plant origin. Usually the colatile (essential) oils are not included in this classification which is primarily intended to designate the fixed oils (i.e. glycerol esters of various fatty acids).

Flash Point
The flash point of a liquid is the lowest temperature at which the liquid will give off vapour to form a flammable gas mixture with air, near the surface of the liquid.
Auto Ignition Temperature
The auto ignition temperature of a solid, liquid or gas is the lowest temperature at which it requires to be raised to support self initiated combustion.
Flammable / Explosive Limits
The flammable (explosive limits) are the minimum and maximum concentrations of flammable gas or vapour in air between which ignition can occur.
• The Minimum vapour concentration is known as:
The Lower Flammable Limit LFL
The Lower Explosive Limit LEL
• The maximum vapour concentration is known as:
The Upper Flammable Limit UFL
The Upper Explosive Limit UEL
Vapour Pressure
Vapour Pressure is an indication of the tendency of a liquid to vaporise. If a liquid is put in a closed container at a constant temperature, a pressure will be exerted which will reach an equilibrium value, known as the Vapour Pressure of the liquid at that temperature.
Boiling Point
The boiling point (BP) of a liquid is the temperature at which its vapour pressure equals the surrounding atmospheric pressure at which point the liquid vaporises.
Petroleum and Chemical Cargos must therefore not, be loaded at or near their boiling points, unless the tank has a closed or restricted ullaging system and the venting system has sufficient capacity to cope with the vapour displaced during loading.
A cargo must not be loaded if the highest temperature which may be expected during loading, or during the voyage will result in the vapour pressure exceeding the safe working pressure as controlled by the pressure relief system.
Vapour Density
Density is the mass of a substance in unit volume. The densities of hydrocarbon vapours are normally greater than air or inert gas thus the possibility of “layering” of gases has to be taken into account during cargo operations.
Water Solubility
The solubility of a substance in water, at a specified temperature is the maximum weight of the substance which will dissolve in a given weight of water.
In the case of a liquid dissolving in, another liquid the term “musclebility” is used instead of “solubility”.
Solubility information is useful in determining cleanup matters for spills, and the fire extinguishing methods for liquid cargoes e.g.
• A cargo which is soluble in water is likely to destroy normal fire fighting foam.
• A cargo which is insoluble in water will form a separate layer above or below the water layer dependant on its specific gravity.
When the specific gravity of the cargo is high a layer of cargo should always be suspended beneath the water which may result in hazardous vapours being released when the layers are disturbed.
Melting Point/Freezing Point
The temperature at which the liquid state of a material is in equilibrium with a solid state, i.e. at a high temperature the solid will melt and at a lower temperature the liquid will solidify. Freezing point and melting point may not always be coincidence, but they are sufficiently close to enable the difference between them to be ignored for the purpose of cargo handling.
Pour Point
The pour point of a liquid is the lowest temperature at which the liquid will flow. It should be noted that oil with thixotropic properties (the properties of showing a temporary reduction in viscosity when shaken or stirred) can be pumped at temperatures well below its pour point, but at very restricted rates.
Viscosity
Viscosity is a measure of a liquid’s ability to flow and is usually determined by measuring the time required for a fixed volume to flow under gravity through a thin tube at a fixed temperature. As the temperature of the liquid increases its viscosity decreases and therefore it flows more readily. It can also be described as a measure of the internal friction of a liquid.
The distinction between viscosity and pour point should be made clear. Oil ceases to flow below its pour point temperature when the wax content solidifies. A viscosity measurement of a liquid depends upon the internal resistance of the liquid to flow. For a simple liquid this internal resistance varies with the temperature in a predictable and regular way. However, when oil approaches its pour point the rate at which viscosity increases as temperature falls accelerates until sufficient wax has precipitated to solidify the product.
Viscosity is important as regards the pumpability of a product. Centrifugal and deepwell pumps are acceptable for the majority of cargoes but high-viscosity products such as bitumen or molasses are more suited for pumping with positive displacement pumps.
Cloud Point
The Cloud Point of crude oils is the temperature at which wax crystals begin to come out of solution, as the temperature is lowered. It is therefore an important factor when the heating requirements for crude oils in transit and during discharge are considered.
Colour
Colour is the comparison between a sample of product and standard colours measured under closely controlled conditions. The colour of clean products is one of the more common causes of cargo rejection or downgrading.This is generally caused by loading a light-coloured product without adequate preparation into a tank that last carried a darker product. Most of the lube oils and white-water products show quitereadily the traces of prior darker lube oils or residual products, and because of this trait, it is most important that the tank cleaning instructions are closely followed.