handling tugs to new positions determined by utilizing the ship's 

 navigation system and its radar, while the remaining anchors hold the 

 barge onsite. Once the new anchors have been set and additional 

 sections of pipe have been welded to the pipeline, the barge is winched 

 forward. Two or three anchor-handling rigs are required to service a 

 pipelaying barge (Figure 19). 



The construction of a pipeline is significantly affected by weather 

 and sea conditions. A pipe-laying season may range from 220 to 270 days 

 for large lay barges; but heavy weather conditions may reduce work time 

 to about 40 percent of the laying season (e.g., in the North Sea, where 

 the most efficient barges lay approximately 37 to 50 miles of pipe per 

 year at a rate of 1.24 miles on a good working day [26]. 



Offshore pipelines are often buried for protection from mechanical 

 damage from currents and waves and from bottom fishing activity and 

 anchoring. A "bury barge" tows a sled which digs a trench by jetting 

 water at high pressure into the ocean bottom (Figure 20). Several 

 passes of the jet sled may be required in order to dig a trench of 

 appropriate depth, depending upon bottom conditions. Currently, 

 Department of Transportation regulations require offshore pipeline 

 burial of 3 feet in water depths less than 200 feet. Offshore gathering 

 lines, which come under the jurisdiction of the USGS do not presently 

 have burial requirements [26]. 



Construction procedures are different for "the shore approach," or 

 landfall, where neither barges and marine craft nor regular onshore 

 pipe-laying methods can be employed. Most of the generally used methods 

 include opening a trench from shore side to a water depth where barges 

 can operate, fabricating the pipeline string onshore or on the lay 

 barge, pulling the pipeline string into position, refilling and protecting 

 the ditch, and restoring the site. Heavy construction equipment, such 

 as trenchers and large winches, operates at the landfall site to pull 

 pipeline in ecologically fragile areas. Environmental damage from 

 pipeline construction can be partially mitigated by careful construction 

 and restoration techniques. 



Pipeline Construction in Wetlands : In the process of moving oil 

 and gas from offshore to upland, an offshore pipeline often must cross 

 through wetland areas. Severe environmental alterations and damage have 

 occurred in wetland crossings. The long canals and resulting berms of 

 spoil left behind have altered water and nutrient flows, thus lowering 

 natural productivity and causing salt water intrusion, loss of wetland 

 habitat, and other problems. 



Typical pipeline construction through wetlands is similar to offshore 

 pipe-laying with the exception that the barges are considerably smaller 

 and narrower and that a canal to allow passage of the barge is usually 

 dug using either a cutter head dredge or a dragline in place of the jet 

 sled. 



93 



