When cargo is loaded or trans-shipped, it is essential to consider the nature of previous cargoes. In some cases, it is virtually impossible during tank cleaning to remove all traces of previous cargo to a level which is not detectable by modern laboratory equipment. For this reason, restrictions are laid down in the contracts of sale, regarding the immediate previous cargo carried in each of the ship’s tanks. These restrictions are imposed within the industry by such bodies as FOSFA and NIOP. Their rules should always be consulted. They are constantly under review and may change in the future. Similar restrictions were imposed in the past concerning leaded petroleum or other leaded products. Shippers and charterers should be notified in good time of the nature of the three previous cargoes carried in each individual tank.
It is important that, before loading, every care and attention should be paid to the proper preparation of tanks, pumps and pipelines. It is very important that the tank coating is maintained to a high standard. The coating covering all sections of the tank must be sound. Where any breakdown of the coating takes place, particularly where epoxy and polyurethane coatings are concerned, there is a risk that the remains of previous cargoes may accumulate, creating a potential source of contamination. The breakdown of epoxy coating usually manifests itself in the form of blisters, open or closed, or in areas where the coating is detached, forming pockets which cannot be reached by cleaning water. In these areas, there is also a risk that rust may form, which is again likely to trap cargo residues and lead to contamination. It is not possible to properly clean tanks with damaged coatings. Cases have been recorded where traces of the third previous cargo have been found when samples of damaged coatings were tested.
Another possible source of contamination is the penetration and softening of epoxy and polyurethane coating by a previous cargo. This may find its way later into newly loaded products. Masters should always
consult the ‘cargo resistance’ list provided by the manufacturers of the tank coating. This will list those cargoes to which the tank coating is resistant. For cargoes not included in the list, or cargoes without resistance indicators, or when deviating from the maximum temperatures indicated on the list, the manufacturers should always be consulted.
Bearing in mind that even the most minute traces of previous cargoes may be discovered, (although this may not always lead to significant damage), it is evident that the washing of cargo tanks must be performed with the utmost care. The precise method of cleaning will depend on the previous cargo carried and the state of cleanliness required for the products to be loaded. The relevant tank cleaning guides should always be consulted. Generally, the most important part of the tank cleaning process is butterworthing with hot or cold sea water at sufficient pressure and at the appropriate tank levels. This should be followed by fresh water washing in order to remove sea water residues. Tanks which may have contained monomer or drying oils should first be washed with sufficient quantities of cold water to avoid polymerisation 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.
On completion, the tanks should be clean, dry and free from residual odours. It may also be desirable to take wallwash samples and have them analysed for traces of previous cargoes, but this requires skilled inspectors. The presence of an odour in a tank, which has been cleaned, indicates the presence of cargo residues and also indicates the need for further cleaning. It is advisable, when checking for residual odours, to make the test after the tank has been closed for a period.
Interior of tanks Below left: A newly coated tank. Below right: Coating starting to break down but clean.
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