Oil Washing

In the process of crude oil washing, gas is generated which will be vented from the tank so there is an increase in the loss of valuable light fractions.  However, because the washing moves the clingage and deposits to the tank bottoms from whence they can be stripped ashore, it increases the overall cargo outturned.  It leaves the tanks much cleaner so easing the problems of cleaning for ballast purposes and reduces the consumption of burners for subsequent tank washing.  This is the primary advantage to the shipowner except in those instances when he might otherwise face a claim for short delivery but for the extra oil recovered and put ashore as a result of crude oil washing.  The proportion of the tanks that must be crude oil washed in order to comply with the Marpol Convention is only that necessary for clean ballast. Except on the last discharge before drydocking, when it is to owner’s advantage to have all the tanks cleaned, the number that the shipowner will generally wish to crude oil wash is about one third.  By this means all the tanks are washed over a cycle of three voyages and this is sufficient to prevent the undue build up of residues in the tank bottoms. Crude oil washing increases the overall discharge time, for which the charterer may or may not have to pay depending on the type and terms of the charter, but any increase in the outturn quantity is to the cargo receiver’s advantage.  This is because it is better for them to receive the maximum cargo delivery, rather than have a potential claim for short delivery.  Many charterers therefore request that all tanks containing their cargo should be crude oil washed, which unfortunately increases the chance of a claim for short delivery on the subsequent voyage.