375 



Table 2. — Sources of oil pollution to the oceans (from Porricelli, Keith and 

 Storch, 1971; sec table 1 also) 



Percent 



Automobile crankcase oil disposal 29. 4 



Tankers 28. 4 



Other vessels than tankers and tank barges 17. 3 



Industrial machinery waste oil 15. 3 



Refinery Petrochemical plant disjwsal 6. 1 



Offshore production 2. 1 



Tank barges 1. 4 



Total 100. 00 



Table 3. — Relative magnitude of oil influx from various tanker sources {from 



Porricelli, Keith, and Storch, 1911 



Percent 



Tank cleaning operations 70 



Casualties 18 



Discharges 7 



Terminals 5 



Total 100 



After discharging cargo, a tanker without exclusive segregated ballast tanks 

 will take suflScient sea water aboard in her cargo tanks to insure proper pro- 

 peller immersion and to provide handling and sea-keeping characteristics. The 

 amount of ballast taken aboard dej^ends ui>on the anticipated weather con- 

 ditions, the distance and route of the ballast voyage, the vessel's lightship weight 

 displacement, length to depth ratio, and other vessel characteristics. The amount 

 of ballast taken aboard generally varies from 20 to 50 i^rcent of the vessel's 

 total cargo carrying capacity but may be greater during periods of adverse 

 weather. 



The ballast that is put directly into cargo tanks immediately after cargo 

 discharge comes into contact and "mixes" with the oil that adhered to the 

 tank surfaces or remained at the suction mouths after cargo discharge. This 

 oily ballast must be disposed of in some way prior to arrival at the loading 

 port unless the loading port has suitable reception capability. After disposal 

 of the oily ballast, clean ballast suitable for direct disposal into the harbor 

 at the loading port must be taken aboard. In the absence of segregated baUast 

 tanks, empty cargo tanks must be washed down to remove the residue oil 

 and provide space for the clean ballast. These tank washings are pumijed 

 overboard and the clean tanks are filled with .^ea water which can be dis- 

 posed of into the harbor at the loading port. The number of tanks washed 

 is a function of the i>articular vessel's proportions, the weather, the route, 

 the owner's desire to perio.lically clean tanks, etc. This generally amounts to 

 between one-third and one-half of the vessel's tanks per ballast voyage. 



This operation is referred to in this section as "uncontrolled ballast discharge." 

 It results in all of the oil residue from the cleaned tanks and approximately 15 

 percent of the oil residue (clingage; see below) from the tanks which were 

 initially ballasted being pumped overboard. The amount of oil influx that results 

 from this operation on any given voyage depends on the amount of oil that 

 remains in the tanks after discharge at the unloading port. This number is com- 

 monly referred to as clingage. Clinga'ge ranges from 0.1 percent to 0.9 percent 

 of the cargo capacity depending on the type of oil, the stripping capability of 

 the tanker, and the internal structure of the tanker ; it is considered to average 

 0.4 percent. 



All tankers do not pump the oil residue from their tank cleaning operations 

 directly overboard. With the "load on top" (LOT) system, the tank cleaning 

 residue (water and oil) is pumped into a holding tank. Here the mixture is 

 allowed to settle and the water drawn off the bottom so that only oil remains 

 in the tank. This oil is then combined with the next cargo ; hence, the term 

 "load on top." 



If all tanker.s employed a one hundred percent efficient LOT system one 

 hundred percent of the time, tank cleaning operations would not be a major 

 source of oil pollution ; however, all tankers are not capable of conducting 

 LOT operations. In addition, LOT operations are not 100 percent efl3cient and are 

 estimated to accommodate only 80 percent of the potential operational pollution 

 arising from tank washings. This is because : 



