Great tanker spills, resulting from collisions or groundings, have 

 aroused public sentiment against oil carriers. Most tanker accidents 

 occur in heavily traveled or shoal water areas, which are usually near the 

 environmentally sensitive coastline. Oil spills from pipeline breaks are 

 sometimes caused by anchors snagging exposed pipelines. These events have 

 been less publicized, possibly because most of the breaks (as well as nearly 

 all of the drilling and production accidents) have occurred off remote, 

 undeveloped shores of Louisiana. On the other hand, tanker incidents have 

 often plagued heavily populated harbors and developed shorelines. 



The possibility of large oil spills first arises during exploratory 

 drilling. Oil and gas, held under high pressure in porous subsea rock 

 layers, may be vented by a well placed drillhole. If not controlled, the 

 well then would become a "gusher", once welcomed as a sign of success, but 

 now carefully avoided, especially in offshore oil development. Complex 

 blowout preventers incorporating many backup systems and steel well casings 

 firmly cemented in place are now employed to reduce the likelihood of a 

 blowout (USGS requires the use of such safety devices [3]. The proba- 

 bility of a blowout has also been reduced by improved drilling practices 

 such as using heavier drilling mud, and by calculating risks of blowouts 

 by conducting more detailed exploration of a field, using COST holes and 

 modern seismic techniques. Nonetheless, since 1956 the USGS has recorded 

 48 blowouts in the Gulf of flexico alone. 



An additional risk of blowouts is incurred during the "workover" 

 phase when wells are reentered to eliminate clogging or perhaps are 

 deepened to bring deeper formation layers into production. The use of 

 blowout prevention equipment during this phase of production is again 

 an important safeguard. Careful monitoring of well pressure is especi- 

 ally important during workover when it may not be possible to use heavy 

 drilling mud to control down-hole pressure sources. 



Subproject: Marine Transportation Systems (SP-19) 



Disturbing Activities: Oil transfer, oil transport 



More than 70 percent of all oil discharged or spilled from tankers is 

 during routine operations, particularly bilge washing [4]. A few tankers 

 have separate ballast tank systems, but most of them load the empty oil tanks 

 with seawater to maintain sea-worthiness on the return leg of the journey. 

 Oily ballast water is discharged (which may have been partially treated) 

 upon arrival. An additional source of pollution is spillage from tankers 

 during transfer operations. Mechanical failure, faulty design, and human 

 error account for most of these accidents. 



Offshore tanker mooring buoys, usually single-point moorings (SPM), 

 are either large buoys anchored to the sea bottom or towers fixed to 

 the bottom. A tanker is moored to the SPM and loading hoses are connected. 

 The mooring and hoses can rotate around the SPM to allow a tanker to head 

 into prevailing winds or tides. With this flexible mooring system, a 



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