Tankers: Tank cleaning and deballasting operations are environ- 

 mentally harmful. Tank cleaning is required when: 



1. A cargo is to be carried which will not tolerate residues 

 from a previous cargo. 



2. A vessel is to undergo repair work which by its nature 

 requires gas free conditions. 



3. Clean ballast is to be taken on board. 



Previous to the development of the large super- tanker, the prevailing 

 custom was to clean all tanks, the object being to prepare the vessel to 

 carry a different cargo. A tanker will generally operate with as many 

 full tanks as possible, depending on the density of the oil, on one leg 

 of its voyage and will return with ballast water in certain tanks, that 

 if no commercial cargo is available, insure that the vessel is seaworthy 

 and capable of safe navigation [31]. 



The quantity of ballast water taken on is large, sometimes as much 

 as fifty percent of the loaded deadweight tonnage. The actual amount 

 and disposition of this ballast will depend upon the following factors 

 [31]: (1) stability and trim, (2) propeller immersion, (3) machinery 

 vibration avoidance, (4) length of voyage, (5) hull stresses, (6) steering 

 characteristics, and (7) sea state. 



A normal ballasting procedure is as follows: the vessel upon 

 discharge of its oil, takes on ballast water either at the dock or 

 immediately upon departure. This water is placed in uncleaned empty oil 

 tanks according to an optimum profile plan as indicated by the above 

 factors. Upon departure, the crew embarks on the tank cleaning operation 

 of the still empty tank in preparation for taking clean ballast on 

 board. After certain tanks are cleaned, they are filled with clean 

 ballast and the dirty ballast tanks are then emptied by pumping overboard. 

 The reason for this is that the discharge of dirty ballast water is 

 prohibited in coastal areas by either international or local pollution 

 laws, therefore if a vessel is to maintain its seaworthiness for the 

 entire length of the voyage, it must be in a position to de-ballast only 

 clean water while coming into port [31]. 



The actual mechanics of the tank cleaning operation are accomplished 

 by spraying cold or heated high pressure sea water into the cargo tanks 

 through tank cleaning heads. Upon completion of the operation, the 

 clean tanks are filled with sea water while the dirty tanks are pumped 

 dry. Cleaning water sprayed into the dirty tanks will dislodge 

 much of the oil adhering to structural members and, if directly 

 pumped overboard, will result in significant discharges of oil. 

 At present, this practice has been restricted by recent legislation 

 limiting the quantities of oil pumped overboard. Reliable sources 

 indicate that the amount of oil left as clingage in cargo tanks is 



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