by the seawater used as ballast. When the tanker arrives to obtain 

 another load, it discharges the oily ballast water. If the cargo is 

 different in composition than the oil previously carried, the tanks are 

 washed out with hot seawater, which is also dumped. The amount of oil 

 released to the sea in tank washings, ballast, and bilge waters is 

 estimated to be about 0.4 percent of the cargo carried [7]. Thus, a 

 100,000 ton tanker carrying a full load of 700,000 barrels would discharge 

 2,800 barrels of oil each round trip, or 135,000 gallons. In comparison, 

 1,000 gallons of oil is considered to be a major spill [107]. Table 30 

 summarizes the causes of oil pollution by tankers and other vessels, in 

 comparison with non-marine operations. 



Oil spills from tankers due to bilge pumping, leaks, and bunkering 

 operations occur as follows: 



1. Bilge pumping - The bilge is the lowest point of the ship's 

 inner bottom and is used to collect oily wastewater from the 

 machinery spaces. This wastewater is discharged at regular 

 intervals. 



2. Leaks - Leaks occur due to cracks in the hull. Welded 

 construction and double bottoms reduce leaks to a negli- 

 gible quantity. 



3. Bunkering operations - Discharges during refueling opera- 

 tions are similar to those that occur during cargo transfer 

 and have similar causes. 



Several improved design features and operational procedures can be 

 implemented to reduce oil pollution. Some are required in various s'tate 

 and Federal regulations. Those that are optional are used by industry 

 if seen to be cost effective. Most U.S. ships are now constructed with 

 double bottoms for additional safety in case of grounding. A double 

 bottom uses an outer wall for the hull of the tanker and an inner wall 

 for the structure of the oil tanks. The space in between can be used 

 for ballast. 



About 80 percent of tankers now use the Load On Top (LOT) procedure 

 that essentially allows for gravity separation of oil and ballast water 

 during the noncargo leg and the discharge of the bottom stratum of denser 

 seawater before entering port [171]. In this procedure, a new cargo of 

 oil is taken on and loaded on top of the retained oil. Segregated 

 ballast systems can also reduce oil discharge by providing separate oil 

 and ballast tanks, often designing wing tanks along the side as ballast 

 tanks since they are most likely to rupture from a collision. 



Fitting of ships with modern navigational systems could significantly 

 reduce accidents. Added safety could be provided by twin propellers and 

 twin rudders for added maneuverability and operational flexibility and by 



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