If the slop tank decanting procedures described in section 2.12 have been followed, the slop tank will contain an amount of oil on top of an emulsion layer whose lower limit is 100 ppm of oil. If the next cargo is compatible with the slops generated from the last cargo, it is possible, with Charterer's approval, to load the next cargo on top of the slops. Load-on-top (LOT), procedures must be conducted in compliance with the requirements of MARPOL regulation 15 ('Retention of oil on board').
The LOT regulations and recommendations require that the oil residues from all tank washings be collected in one or two tanks, which are then topped off with the next cargo.
It should be made clear by the charterer's or owner's loading orders if the vessel is to LOT of the previous slops. If it is not clear, then the owner must be requested to clarify the point before arrival at the load port. If LOT procedures will not be used, then the slops must be discharged ashore. Alternatively, the slops may be retained on board and the slop tank not loaded with cargo.
Charterer's sometimes require the slops to be spread evenly around the vessel's tanks before loading. However, bearing in mind that the slops (after decanting the free water), may contain up to 30% water in suspension, a serious salt water contamination of the next cargo may result by doing this. Most refineries have equipment to remove salt from the crude oil before refining, however salt water can be a contaminant of all crude oils. In excessive amounts, it can cause serious damage to the refractory equipment in the refinery. More particularly, if the oil is to be used in the production of asphalt, it can cause 'loss of penetration' in the finished product.