burying procedures. Placement of supports for elevated flowlines is 

 accomplished by marsh buggy, boat, and hand operations. Site character- 

 istics such as water current, water depth, and water-course size 

 determine what field equipment is required. 



Where overhead electrical lines are utilized, these lines typically 

 parallel existing access corridors. Installation requires only minor 

 equipment for placing support poles and stringing lines. A tractor or 

 backhoe operating from a leveed road or access canal may perform these 

 activities. Holes are dug, and poles are placed and anchored. The site 

 is then abandoned. 



Installation of petroleum transport pipelines 20 to 150 cm (8 to 60 inches) 

 in diameter requires a more complex series of activities. Lines are 

 buried in coastal delta marshes using either the push method or the 

 flotation method. Both methods require a survey crew to partially clear 

 (seldom \jery extensive) and stake an alignment which typically flows the 

 shortest straight-line route between points. Access is provided by marsh 

 buggy or boat. Pipeline installation through delta marshes requires a 

 set of \/ery different techniques which are determined by site character- 

 istics. A barge-mounted hydraulic or bucket dredge may be used in 

 deepwater sites. In shallow-water areas with firm soils, a dragline or 

 marsh buggy dragline is utilized. Levees, ridges, and other elevated 

 formations may be bisected by canal construction, producing new flow 

 regimes. 



If the push method is utilized, which is normally the case where the 

 marsh substrate is firm enough to support the equipment, a dragline on 

 mats or a buggy-mounted backhoe follows the survey crew and excavates a 

 ditch 2 to 3 m (4 to 6 ft) deep and 3 m (8-10 ft) wide. Spoil is piled 

 continuously along one or both sides. A specialized stationary barge, 

 located in a dredged slip, assembles the pipe sections and pushes the 

 line down the ditch; the pipe is supported by floats and guided by a 

 marsh buggy. Once positioned, the line is sunk in place by removing the 

 floats. Typically, about one m (three to four ft) of water will 

 remain above the pipe. The ditch may be left open, but it is more 

 frequently backfilled by the dragline. If backfilling is not performed, 

 an open-water canal bordered by one or two continuous spoil levees 

 remains. Backfilled routes appear as straight shallow depressions due to 

 subsequent spoil shrinkage and subsidence. When small watercourses are 

 crossed, earthen plugs are placed parallel to the drainages to prevent 

 water flow down the ditch. Traversal of larger water bodies (canals, 

 streams, larger deltaic meanders) requires the use of dredges or jetting 

 equipment. Easement widths range from about 10 to 50 m (30 to 150 ft), 

 depending upon the line size. 



If the flotation method of pipe laying is utilized, which is often the 

 case in the delta where the substrate is too unstable to support instal- 

 lation equipment, a barge-mounted bucket or hydraulic dredge excavates 

 the ditch. A canal 12 to 15 m (40 to 50 ft) wide and 2 to 3 m (6 to 9 ft) 



278 



