Gaseous Crudes

For black products heating can be a major consideration. Heavy fuel oils may have to be kept at 1500F. or even more for otherwise their very high viscosity will cause protracted pumping time and increased outturn losses. With white products the requirements for the degree of cleanliness of the tanks, pumps and lines is likely to be much more critical and exacting.  For all white products the charterer will wish to know the previous (usually three previous) cargoes carried  in order to ensure that the tanks can be made suitable for the cargo to be loaded, for example that the previous cargoes were all lead free.  If heated cargo is carried in  tanks adjacent to light clean products (or gaseous crudes) there will be increased vapour losses and possibly vapour pressure problems when discharging, particularly if there is a high ambient temperature. The most common size of clean product carrier is about 30,000 tons deadweight while the average size dirty product carrier is larger, in the region of 50,000/70,000 tons.  Both clean and dirty products can and are carried in larger ships even VLPCs (very large product carriers) but few charterers can find regular employment for the movement of such large quantities of product.  Size of cargo tanks is also a consideration in this respect insofar as cargo receivers can only take their product parcels in quantities consistent with the size of their shore tanks.  Unless there is a big saving in shipping supplies in larger parcels the cargo receiver is not going to invest in larger shore tanks. Most of the large product carriers are designed to carry crude and/or black products so that they can be programmed more economically.  Their tank coatings, mainly zinc or epoxy based, enable them to be cleaned more quickly between cargoes, from crude oils to dirty products or even to clean products, with either a cold or hot wash depending on the cargo to be loaded.  For some products, including all clean products, it may also be necessary to flush the tank bottoms, gas free, lift scale and mop up.  For colour critical grades flushing the tanks and lines with suitable wash oil may be needed.  Without tank coatings much more cleaning is required and several suitable intermediate cargoes, in order to change from dirty to clean products.  As a consequence and because of the steel wastage in unprotected tanks in the clean oil trade, any ship without coatings is likely to carry only dirty cargoes except perhaps if employed clean on her initial voyages after delivery from the builder’s yard.