Tank cleaning may be required for one or more of the following reasons:
1. To carry clean ballast.
2. To gas free tanks for internal inspections, repairs or prior to entering dry dock.
3. To remove sediment from tank top plating. This may be required if the vessel is engaged in the repetitive carriage of fuel oil or similar sediment settling cargoes. Although washing may not be necessary between the consecutive voyages, assuming the cargoes are compatible, many Ship Owners have found it prudent to water wash a small group of tanks on a rotation basis between voyages, thus preventing any large accumulation of sediments.
4. To load a different and not compatible grade of cargo. Washing between different grades of cargo is the most common reason for tank cleaning. In most cargo sequences on product 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. However, it should be noted that there is a number of grade sequences, particularly in the petroleum products trade, where no washing at all needs to be carried out. Thus, the decision for necessary tank cleaning required in such trades is often made only when knowledge of the next grade to be loaded is obtained.
Washing machines, their water supply and even the washing method are usually described by the term “Butterworth”. The machines, either fixed or portable, consist of revolving nozzles, which are moved by water driven gearing to create a spherical wash pattern or “cycle”.
With portable machines, both the machine and its flexible water supply hose are placed into the top of the tank to be cleaned through an opening called the “Butterworth Port”. The machines are progressively lowered down the height of a tank in stages or “drops” each usually of 10-15 feet. Graduation marks every 5 feet on the water supply hose are a useful check on the depth of the machine inside the tank. The lowest “drop” is normally about 5 feet above the “bottom” of the tank where the machine is positioned for a “bottom wash”. The wash duration at each drop is usually for one cycle of the machine, the cycle time varying between 30-60 minutes according to the size of the machine and its pump pressure.
Throughout the washing operation, cargo residues mixed with washing water are continuously stripped from the cargo tanks by the vessel normal cargo pumps. These washings are directed through the cargo line system into reception tanks, a slop tank or in some cases to shore facilities.