3. The third method, not widely used, is to weld the pipe into 

 strands ashore, support these strands with floats, and 

 then tow the strands to location. On reaching location, 

 the pipe is flooded and welded onto the main pipeline. 



Vessels : Almost all offshore pipelines with the exception of 

 gathering lines between platforms, are constructed using specially built 

 pipe-laying barges and pipe-laying ships. Pipe-laying barges are of 

 numerous types. Traditionally, they have been conventional barges on 

 which a pipe-laying rig was built, but standard ship hulls and semi- 

 submersibles are both in use. In the last few years, as offshore 

 operations have pushed into hostile areas such as the North Sea, pipe- 

 laying barges have grown quite large. One of the more modern barges, 

 Semac , measures 180 feet by 433 feet. 



Along with the growth in the size of barges has been a trend toward 

 the construction of semi-submersible barges. Semi-submersibles can 

 better withstand heavy seas. Semi-submersibles can operate in seas 

 approaching 15 feet, whereas operations in a large conventional barge 

 must cease when seas reach 6 to 10 feet. Thus, semi-submersibles have a 

 considerably longer working season than conventional barges. 



Coated pipe is brought to the barge in supply boats from a pipe- 

 staging area onshore. Two to three supply boats may be needed to keep 

 the barge supplied with pipe. Under good conditions, over a mile of 

 pipeline can be laid in a day. This is approximately the amount of pipe 

 which can be kept on the deck of the lay barge. Thus, continual resupply 

 from shore must be maintained or pipe-laying operations will come to a 

 halt. This is extremely costly since a lay barge may rent for up to 

 $200,000 per day. 



The need for constant resupply means that a staging area will be 

 located as near as possible to the pipeline corridor with deepwater 

 access. Not only will transit distances and time be reduced , but more 

 importantly the weather window (required period of good weather) for 

 resupply may be greatly reduced. Short runs from the staging area to 

 the barge may even allow resupply during the lull in a storm. 



On standard pipe-laying barges, the precoated pipe is put aboard 

 the barge, stacked, and moved joint by joint to the bow of the barge as 

 it enters into the lay system. The pipe ends are inspected for damage, 

 the joints are prepared for welding, each section is aligned with the 

 previous section at the "line-up station." and finally the welds are 

 made. Each successive joint is tested (usually by X-ray); the weld 

 joint is coated with "mastic," synthetic compound or concrete; and then 

 the pipe is launched. 



All pipe-laying barges and ships are held in place and moved 

 forward with a multi -anchor mooring system. Most barges have from 12 to 

 14 anchors. Part of the anchors are being moved forward with anchor- 



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