Tank Cleaning

Until the introduction of Crude Oil Washing (COW) which we discuss in the next section, tanks had to be washed with water.  The method used employs high pressure sea water delivered to the tanks via rotating nozzles which can best be envisaged as gigantic lawn sprinklers. These are positioned in such a way that the high pressure jets reach every part of the tank.  The manufacturer’s name most often associated with this system is “Butterworth” although there are several other types.  The water used is usually at ambient temperature although before a dry-dock the water may be heated.  This is because it is vital to ensure all residues are completely removed in order to avoid the risk of asphyxiation or inflammable fumes being present. Washing a tank with cargo does not seem an obvious method of cleaning the tank and if a tank was left for say 24 hours in order to drain after discharge, much of the clingage on the tank sides, stiffeners and other surfaces in the tank would drain to the tank bottoms without any assistance.  Pumping crude oil through the tank washing machines as each tank is discharged, shifts much of the clingage from the tank sides and the residues and oily deposits from the horizontals and tank bottoms. Although carried out concurrent with discharge of other tanks, it reduces the overall rate of discharge because a pump has to be used to feed the washings machines but it loses much less time and is much more effective than just natural drainage.  It also shifts the solids and semi-solids which would otherwise remain where they have settled out on the horizontals and tank bottoms.  After each tank is washed the tank bottoms are stripped to a collecting tank from where the collected oil will be pumped ashore.