Water washing may be inadequate or inappropriate after the carriage of certain products, because tanks can only be adequately cleaned by special cleaning methods or cleaning agents. Where it is decided to use a special cleaning method, and well documented experience indicates that it is safe to do so, thorough company guidance should be provided that describes the procedures for the ship to follow.

Where a special cleaning method is to be used in port, local authorities may impose additional safety or environmental requirements.

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Some cargoes may react with certain cleaning agents and produce large amounts of toxic or flammable vapours, or render equipment such as pumps inoperable. The choice of a tank cleaning agent should be made with full knowledge of the cargo characteristics.

If a special method involving cleaning agents is to be used, it may create an additional hazard for the crew. Shipboard procedures should ensure that personnel are familiar with, and protected from, the health hazards associated with such a method. The cleaning agents may be added to the wash water or used alone. The cleaning procedures adopted should not entail the need for personnel to enter the tank.

If, however, the only practical means of cleaning involves personnel entering the tank then the precautions should be strictly followed. No one should enter any cargo tank unless express permission to do so has been received from the responsible officer, and all appropriate precautions taken. The tank atmosphere should be safe for entry and an entry permit issued. Chemical absorption detectors should be used for detecting the presence of specific gases and vapours at TLV levels.

In exceptional circumstances the requirement might arise for wiping down product residues from the tank walls by using a chemical solvent in a localised area. The amount used should be small, and the personnel involved should be aware that its use may modify the atmosphere. The introduction of the solvent into the tank might also generate additional risks such as toxicity or flammability. Such risks should be carefully evaluated before starting the operation, which should not be undertaken unless the personnel involved can be effectively protected from those risks. Data sheets for the chemical solvent used should be available on board.

In addition, manufacturer's instructions or recommendations for the use of commercial products should be observed, and the resulting slops disposed of in accordance with the ship's P&A Manual.


Precautions during loading, discharging, sweeping, tank cleaning of various oils

Oxidise – Means, absorption of oxygen from the air, and so the air remaining in the tank containing vegetable or animal oil, or coated with residual quantities of these oils, may not have enough oxygen in to support life. This is most likely to happen when a tank has been closed for a long time after the oils or fats have been discharged. The residual oil or fat on the structures in the tank starts to decompose (rot). When it does this, it not only absorbs oxygen, but also produces various toxic and asphyxiating gases such as methane, carbon dioxide and hydrogen sulphide. These gases may also be present in sumps or drains, where oils and fats of this nature remain.

Sometimes it is essential to sweep the tank during / after discharging of the vegetable, animal oils and fats, as many residues can remain on the bottom of a tank. In this case the physical tank entry is required. Therefore, it is dangerous to enter a tank, which has been used for the vegetable or animal oils or fats unless it is being continuously ventilated.


Tank cleaning best practices (Vegetable Oils - drying or semi-drying)

Animal and Vegetable Oils easily saponify when combined with Caustic Soda. To saponify means converting the oil into soap, or, in practice bringing into suspension. However, alkaline cleaners can only be used if the tank is made from stainless steel or coated with epoxy based coating.

Drying and semi-drying vegetable and animal oils react with oxygen to form a varnish-like polymeric film. This is very difficult to remove from the bulkheads. Heat increases the reaction speed. Therefore the initial washing of these products must be done with water at ambient temperature without any delay after unloading of cargo.

If full tank cleaning in port is not possible after discharge, a quick wash should be given to the tanks to ensure the atmosphere in the tank remains moist. As an alternative, a low pressure steam or limited hand-hosing may be applied.

Heated cargoes in adjacent tanks should be avoided. In case of doubt as to whether oil is non-drying or drying, always use ambient water first. Initial washing must be done using ambient fresh or salt water. Basic cleaning should be done for 3 hours followed by 3 hours hot washing (80C).








Related info:

1. Fixed and portable tank cleaning equipments
   The installation of fixed tank washing machines within a cargo tank allows an inert atmosphere to be maintained during the washing operation, and thus permits cleaning in a closed mode in compliance with port regulations prohibiting release of noxious vapours. Their installation and use also reduces crew exposure to cargo vapours and inert gas.
   


2. Tank cleaning and risk with cargo contact
   Different chemicals affect the human body in many different ways. A general information and some practical advice are available in Appendix,7, of "Medical first aid guide for use in accidents involving dangerous goods" published by IMO, WHO and ILO ref (36).


3. Practical example of solving tank cleaning problems
   Cleaning of tanks is usually the responsibilty of the ship. Tank cleaning and the cleanliness involved have different standards depending upon the previous cargo and the cargo to be loaded. But the matter can be still more complicated, as cleanliness for one and the same product may vary, depending on who the receiver is and for what purpose the cargo is finally intended.


4. Tank cleaning fatality- case study & lessons learned
   In the trade practice onboard chemical tankers, it is not uncommon that the use of methanol or other chemical/detergents is undertaken to achieve the required standard of cleanliness inside the cargo tank, pipelines, cargo hoses, manifold adaptors/reducers, gauging equipment, etc. Industry publications clearly highlight and caution regarding their use due to their flammability and toxicity hazards.


5. Pre-cleaning /washing of cargo tanks
   Washing between different grades of cargo is the most common reason for tank cleaning. In most cargo sequences on chemical tankers, this cleaning may consist of no more than a simple hot or cold seawater wash. A simple water wash will disperse many types of chemicals and has been found effective between clean petroleum products such as gas oil and kerosene.


6. Final cleaning of cargo tanks prior loading
   Method of final cleaning to be used depends on both previous cargo and cargo to be loaded. As a general rule the tanks and piping shall be completely drained of water or residues before loading. The bottom of the tanks may have to be dried up with rags.


7. Tank cleaning and posoning hazards
   Certain substances affect the tissues locally as an irritant (cashew nut shell oil) or cause grave damage to the eyes, skin or mucous membranes (e g strong acids and caustic). Other substances may be absorbed by contact to the skin without local effects (e g nitrobenzene, aniline).


8. Testing of tanks and cargoes
   Most common tests and checks for oil and chemical cargoes include testing tank walls for cleanliness. Testing is normally carried out by independent surveyors who, according to local practice or a written agreement in the charter party, are accepted by shipper, receiver and owner.


9. Practical tank cleaning methods for various noxious liquid cargo
   Tanks that may have contained monomer or drying oils should first be cleaned with sufficient cold water quantities to avoid polymerization of cargo residues. In some cases, it is necessary to employ tank cleaning chemicals, but their use is generally limited as it may be difficult to dispose of slops.


10. Special tank cleaning method
    If a special method involving cleaning agents is to be used, it may create an additional hazard for the crew. Shipboard procedures should ensure that personnel are familiar with, and protected from, the health hazards associated with such a method. The cleaning agents may be added to the wash water or used alone. The cleaning procedures adopted should not entail the need for personnel to enter the tank.


11. Determining proper tank cleaning by acid wash method
    The acid wash method is used if there is any suspicion that a cargo of aromatics may have been contaminated by a previous oil cargo. The method is also used as a check that a tank is sufficiently cleaned before loading aromatics.


12. Supervision of all tank cleaning and gas freeing operations
    Tank cleaning is essential on-a chemical tanker, but it must be recognised as a potentially hazardous operation, and rigorous precautions should be observed throughout the process. Together with gas freeing, it is probably the most hazardous operation routinely undertaken on a chemical tanker.


13. Disposal of tank washings, slops and dirty ballast - safe method
    During normal operations of a chemical carrier, the main need to dispose of chemical residues, slops or water contaminated with cargo will arise during or immediately after tank cleaning. Final disposal of slops or washwater should be in accordance with the ship's P&A Manual. Tank washings and slops may be retained on board in a slop tank, or discharged ashore or into barges.


14. Cargo tank damage during pigging operations
    Blowing and pigging of pipelines at terminals poses inherent risks for the terminal and a chemical tanker. Frequent damages to tanks have occurred. If there are doubts about the shore operation or signs of problems ashore the OOW must immediately request clarification.


15. Determining water contamination in chemical cargo
    Presence of free water in non water-soluble products can, very roughly be determined on board by warming a sample of the product in a test tube or in a bottle. Water will then collect at the bottom of the tube and can be seen after some time.


16. Determining Sulphur contamination (sulphides) in chemical cargo
    Certain products, in particular "virgin naphta feedstock'' (petroleum naphta) are severely contaminated by minor amounts of sulphides (and also lead compounds), which poison catalysts in further processing. Previous heavy oils or dirty harbour ballast water may have left traces of sulphides in the cargo tanks.


17. Chloride contamination in chemical cargo - how to resolve?
    Certain cargoes are very senstive to chloride contamination, in particular glycols, methanol, ethanol . The tanks should be finally washed with fresh water. However, chlorides (salt) may still be present and a check might be useful. The greatest risk for salt deposits is on horizontal surfaces.


18. APHA (Hazen) method for determining color of very light chemical products
    A method called APHA (Hazen) is often used for very light products, defined in ASTM D-1209, viz aromatics, ketones. This colour scale is defined with an origin in 100 cc distilled water (value 0) to succesively higher values (max 500) by adding APHA-solution (a platinum-cobolt salt solution).


19. Loading, discharging & care of Phenol - Safety guideline
    PHENOL is carried at sea in a generally pure state. As a result, it has a high freezing point of approximately 40~ 41deg C. PHENOL is also extremely dangerous when it comes into contact with the eyes or skin and can be fatal.


20. Hazards of Phenol - safe handling of Phenol on chemical tankers.
    Phenol must be carried at temperatures within charterers instructions, typically between +50 and + 60 degrees C. Heating instructions of the Shipper or Owners must be followed to avoid protests and delays in the port of discharge. Overheating can damage this cargo. A full cargo heating log must be maintained.


21. Handling benzene & methanol safety precautions
    Benzene is known as a strong carcinogen and known to cause leukaemia. When handling cargoes with more than Benzene concentration of 0.5%, the Master is to ensure that all personnel involved are aware of the long term hazards.


22. Personal protective equipments for carcinogens & cyanide-like cargoes onboard chemical tankers
    A carcinogen is a substance that may cause cancer by contact or by inhalation. It is therefore essential that the highest safety precautions are taken when handling these cargoes. Access to deck areas must be restricted to duty personnel only. All accommodation doors and ports must be closed and ventilation put on recirculation. Any member of crews involved in cargo operations must wear chemical protective suits and breathing apparatus


23. Handling ACRYLONITRILE safety precautions
    ACRYLONITRILE are high value and require sophisticated handling for safety, health and loss prevention reasons. They need careful consideration prior loading , tank coating compatibility, cross compatibility with other cargoes carried, environmental controls if required (inerting).


24. handling ISOCYANATES safety precautions
    Product safety data sheets may be available from various sources. For safety preparation, until the specific product safety data sheet can be obtained, Chemical Data Guide for Bulk Shipment by Water (U.S. DoT), should be used.


25. Loading, carrying & discharging of Sulphuric acid - regulatory requirements & special handling methods
    IBC code compatibility chart strictly prohibits water in adjacent compartment to Sulphuric acid as you are aware if both come in contact with each other will generate a violent reaction. It is therefore recommended that the during loading of sulphuric acid adjacent ballast tanks to be always stripped dry to the maximum efficiency of the deballasting equipment used.

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