gastropods and bivalves, tend to live in fine sediments (silt and 

 clay). These animals ingest sediment particles, digest bacteria and 

 organic matter, and excrete the remainder in fecal pellets. Filter 

 feeders inhabit coarses sediments (sand and gravel) and filter their 

 food from the water. For both detritus and filter feeders, oil in 

 the sediments and water column presents other dangers, such as the 

 absorption of hydrocarbons through the digestive tract and the 

 destruction of food sources [173]. 



4.19.6 Liquefied Natural Gas (LNG) Carriers 



LNG carriers are in limited use in the United States, and have mini- 

 mal potential for ecological disturbance. Accidents involving LNG car- 

 riers are of great concern, although few major incidents have occurred. 

 In the event of an escape of LNG, there is very rapid formation of a 

 vapor plume, which its low temperature causes to hang close to the 

 water areas until its temperature increases to make the gas lighter 

 than the air. Unconfined, the vapor mixed with air is not explosive, 

 but in a mixture of 5 to 15 percent vapor and air it is highly flammable. 

 Within enclosed spaces, if mixed with air in the presence of an ignition 

 source, it can explode. The primary danger present in a large-scale LNG 

 spill is a very intense fire at the spill site. A more remote hazard 

 is that the vapor plume could drift into enclosed spaces adjacent to a 

 spill site and explode or catch fire [176]. 



For these reasons, all LNG facilities should be sited as far as 

 possible from inhabited areas. Furthermore, strict safeguards should be 

 applied to any such development, such as elimination of ignition sources, 

 and maintaining buffer zones around LNG tanks. 



4.20 SUBMERGED TRANSMISSION SYSTEMS - SUBPROJECT 20 



This discussion of transmission systems is limited to submerged oil 

 and gas pipelines. These pipelines transport 98 percent of the oil 

 produced on the United States OCS. While they have a lower potential 

 for major oil spills than tanker transport, stringent safeguards are in 

 order. Proper design, location, construction, and operation of pipe- 

 lines will minimize the risk of oil spills and the disruption of benthic 

 communities and offshore vital areas. 



4.20.1 Summary 



Submerged transmission of oil and gas is conventionally done by 

 marine pipelines. These serve as oil transport linkages between offshore 

 platforms, subsea wells or single-point mooring (SPM) systems and onshore 

 facilities. Generally, pipelines represent the most efficient, economical 

 and cleanest method of moving bulk liquids. Given suitable volume, terrain, 

 and design life, they are the optimal transfer system [9]. 



200 



