This would allow an existing pipeline to accommodate flow from a future 

 nearby find and thus limit additional pipeline laying. Although more 

 materials would be required to build a larger pipeline, less energy 

 would be expended in pumping oil or gas. 



An important design requirement is to incorporate suitable leak 

 detection sensors and line shut-off valves at key points in the pipeline 

 system. 



4.20.4 Construction 



Laying a marine pipeline is a major activity that has the potential 

 for environmental disturbance. A "lay barge spread", the unit which lays 

 the pipeline, consists of a lay barge, one to three tug boats, and pipe 

 supply vessels. In shallow situations where burial of the pipeline may 

 be required (less than 200 foot depth) a "bury barge", or "jet barge", is 

 added to the flotilla to do the work. Burial is effected by jetting 

 sediment from underneath the pipeline. As the jetting device ("jet 

 sled") is pulled forward along the bottom the pipeline settles into the 

 trench and is partially buried by the sediments as they settle back into 

 the depression. Complete burial can then be accomplished by either using 

 a drag sled which pushes sediment back into the trench or by additional 

 jetting from the side [107]. 



In all shipping fairways and anchorage areas, pipelines must be 

 buried at least 10 feet deep regardless of water depth. Only lines in the 

 gathering system between adjacent platforms may remain unburied in the 

 shallower areas. In some areas, such as the Southern California OCS, the 

 water depth requirement has been increased to 250 feet. In addition, 

 offshore pipelines are required to be coated for mechanical and corrosion 

 protection. Electrolytic protection against corrosion is also 

 required [139]. 



The use of high pressure water jets to bury pipe increases turbidity 

 which may affect benthic organisms. Disruption of sediments will have 

 greater adverse effects in populated and industrialized harbors where 

 sediments may be polluted. In shallow waters, experience has shown 

 that bottom restoration is rapid, whereas in deeper waters, more than 

 a year may be required. The selection of pipeline routes along bedrock 

 requires the use of underwater blasting which may kill marine animals 

 in the blasting area [7], 



The principal safeguard required over the whole pipeline run is 

 avoidance of vital areas (reefs, spawning areas, kelp beds, etc.) by 

 suitable alignment of the pipeline. Specific construction mitigation 

 measures are required in shallow areas, particularly in estuaries. 

 These include the use of special construction techniques, such as use of 

 sediment screens and other siltation prevention techniques during under- 

 water trenching, and scheduling jet-sled operations to avoid critically 

 sensitive periods, such as breeding and rearing. 



203 



