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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.

Recommendations for completing the LOT procedure include:
1 Follow the LOT slop tank decanting procedure closely, Sections 2.12.4, 2.12.7 and 2.12.8).
2 If charterers insist that slops are to be spread in all vessel's tanks, the master should refuse to do so, responding that this procedure may contaminate the cargo.
3 If the charterer's intentions prevail and slops are distributed among the cargo tanks before loading, then when arriving at the discharge port suggest in writing (cable/telex/letter), that 'wet bottoms' and slops are discharged first to a single shore tank in order to minimise the danger of spreading 'wet bottoms' throughout the shore tank system.
4 The vessel's first duty is the 'proper care of the cargo'. If the charterers requirement for commingling of the slops in all cargo tanks is followed, a serious cargo claim may result, since very often the Bill of Lading has passed into the hands of a third party who is unaware of the charterer's request and to whom the vessel will be responsible. The vessel may be found unseaworthy with regard to the cargo and a possible regress by owners against the charterer's may turn out to be a doubtful venture (due to no charterer's assets). Therefore, proper care of the cargo takes priority over charterer's orders/requirements.

If the cargo is to consist of two or more 'parcels' of different crude oils, the largest parcel should be loaded on top of any retained slops, unless the smaller parcel is more compatible.

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