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Hazards of tank cleaning onboard chemical tankers - supervision and preparations

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.

The additional risk created by cargo gases expelled from the tanks cannot be overemphasised. Depending on the most recent cargo carried in tanks that are to be cleaned, vapours that are toxic, flammable and corrosive should be expected to be released onto and around the cargo deck area. It is therefore of utmost; importance that every possible care is exercised during all operations connected with tank cleaning and gas freeing, and that the operations are carried out using the approved procedures and arrangements for the ship.

chemical tanker navigation at sea
Personnel involved should be fully aware of the dangers and take necessary precautions, because the consequences of an inadvertent error can be very serious and far reaching.

All ships certified to carry noxious liquid substances in bulk must be provided with a Procedures and Arrangements (P&A) Manual, approved by the flag administration, which addresses the marine environmental aspects of removal and disposal of residues from cargo tanks, and describes how to perform those operations. The Manual should be adhered to in all respects, including the performance of mandatory pre-wash requirements in accordance with MARPOL 73/78 Annex II.

Preparatory guideline for tank cleaning operation

A responsible officer should supervise all tank cleaning and gas freeing operations. All stages of the operation should be performed in a safe manner appropriate to each individual chemical's physical characteristics, such as toxicity, corrosiveness and reactivity.

A pre-cleaning meeting under the leadership of the responsible officer should be held prior to any tank cleaning or gas freeing operation. Other crew members involved should be identified by the responsible officer, and their role explained. It is to clarify that all personnel involved fully understand their duties during the forthcoming tank cleaning operation.

The meeting should confirm:

i) The tanks to be cleaned, and the cleaning sequence.
ii) The type of cargo to be cleaned from each tank, and its characteristics. Cargo information sheets should be available so that personnel involved are familiar with the hazards.
iii) The major risks during cleaning, such as toxicity, flammability, corrosiveness and reactivity.
iv) The safety equipment and personal protective equipment to be available and ready for use throughout the operation and during connecting and disconnecting of hoses at the cargo manifold.
v) The cleaning instructions to be followed in each case.
vi) The means of disposal of any cargo, residues and the contaminated cleaning water. The relevant slop tank must be specified in each case.
vii) The precautions necessary to confirm that the cargo deck area is free from cargo vapours during tank washing and gas freeing operations.
viii) That at regular intervals throughout the operation, checks will be made to ensure that tank washings containing cargo are not inadvertently being discharged into the sea.

A written tank cleaning schedule should be drawn up and made available for reference by all personnel participating in the operations.

Chemical tanker tank cleaning machine

Fig:Chemical tanker tank cleaning machine


Before any tank cleaning or gas freeing operations begin, the responsible officer should confirm that all necessary equipment is available, and that adequate checks are made to establish that all equipment to be used is in good working condition. Both before and during tank cleaning and gas freeing operations, the responsible officer should be satisfied that the appropriate precautions are being observed. All personnel on board should be notified that tank cleaning or gas freeing is about to begin, and only those personnel involved in the operations should be allowed into the cargo tank area.

If other craft are alongside the tanker, their personnel should be notified that tank cleaning operations are about to commence, and their compliance with all appropriate safety measures should be confirmed.

When gas freeing or tank cleaning while alongside at a terminal, the precautions for cargo handling should be observed where appropriate. Before starting, the permission of the port authority and terminal operator should be obtained, and the appropriate personnel ashore should be consulted to confirm that conditions on the jetty do not present a hazard, and to obtain agreement that operations can start.

The following checks should be made before operations commence:

i) That essential protective clothing and respiratory protection equipment are being worn if so required.
ii) That fresh water shower and eyewash arrangements are ready for immediate use in the event of contamination of personnel.
iii) That work not related to cargo operations, and not otherwise essential, is avoided in the cargo area during tank cleaning operations.
iv) That cargo pipelines serving a set of cargo tanks are isolated from the tanks to be cleaned or gas freed, unless all tanks in that set are to be cleaned.
v) That tanks served by a common vent system are properly isolated.
vi) That cargo tank lids, tank washing openings, ullage openings and sighting ports in uncleaned tanks are kept closed until they are to be cleaned.
vii) That all sea and overboard discharge valves connected to the cargo and ballast systems are shut and secured when not in use.
viii) That pump-room precautions are being observed and will continue to be observed throughout tank cleaning and gas freeing operations.
ix) That fire fighting equipment is ready for immediate use.

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.

Following reference publications provide useful guidance and international regulations for carrying hazardous chemicals at sea.

Our detail pages contain somewhat larger lists of resources where you may find more useful information.

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